Barrel Cleaning - What Actually Happens?

[SMH77]

Anyone that has been following me knows that I've spent a fair amount of time performing BC tests on a couple of my guns testing a variety of different things, such as BC's of different pellets shot through the same setup at the same temps through my RAW HM1000x, andmy son's FX Wildcat Mk II, BC's of the same pellet through different barrels to show the barrel effects on BC, BC's of the same pellet, through the same barrel across different temperatures, testing the BC characteristics of various slugs vs. JSB pellets, BC variability due to velocity changes, etc. I've done this testing primarily for my own learning, but I've shared it to help others here on the forum learn and see real world data with a controlled setup between different shoots to allow direct comparison betwen tests and I've done this for both .25 cal and .30 cal barrels on my primary setup.

Recently I put my .25 cal polygon barrel back on my RAW to get the setup ready to do some fall small game hunting. While tuning it in, I noticed that my BC was lower than I typically have seen (normally it sit at about 0.0555 - 0.0568)-coming in at slightly below the 0.050 mark (unusually low for this setup!). Knowing how much testing I've run through the gun in the past several months, including increasing the hammer spring tension, shooting slugs, etc I strongly suspected that my barrel was getting pretty dirty and was in need of a good cleaning. I've witnessed (and heard others talk about) a barrel getting dirty and having accuracy suffer as a results, but could some of that accuracy loss also show up in BC shifts that could actually be measured? Was this phenomenon what I was actually 'seeing' in the dirty barrel BC data? These were the questions swirling around in my mind...

So I did what I do, and set out to test the barrel before and after cleaning to see if I indeed could really see a difference in the BC / velocity numbers simply as a result of cleaning a dirty barrel. Since I found the results most interesting, I've decided to share them here with the rest of the community...

Objective:

To compare the external ballistics results the same setup with a dirty barrel, then clean the barrel and retest to see if I could see a change in performance and perhaps even see the infamous 'releading' taking place before my eyes...

Equipment:

RAW HM1000x in .25 cal with factory polygonal barrel (made by LW). Hammer spring basically set to its lowest position (shooting ~ 60 fpe), and left completely alone for this comparison. Testing done with this gun's favorite food: JSB Heavy, Mk I pellets (33.95 grain), all testing done with pellets single shot loaded into the gun.

Method: Same as Usual...

Measure the velocity of two common pellets at 1 yard and 50 yards, averaged over a string of 12 shots, compile the results and calculate the BC of each combination. I use Chairgun Pro (desktop version) for the calculation of BC and use the GA form factor for diabolo pellets. I capture the data using two Caldwell chronographs (that have been calibrated to each other) and then compile the results in Excel. I record the local weather conditions using my Accuweather app-but only as a double check of the weather conditions the Caldwell app pulls in from its source (not sure where-but they match pretty closely). I do use the average humidity over the shooting session and look for any changes in pressure or temperature during the session-and then use the averge value of those numbers (from the Caldwell numbers).

At yard: (Distance is set from the end of the moderator to the center of the rearmost sensor window)

At 50 yards: (distance is set via Nikon laser rangefinder to target)

Summary of Results:

All data shown below represents the same gun configuration for all testing tonight: meaning the hammer spring and regulator settings unchanged from setup to setup.

Order of data acquisition:

RAW with polygon barrel, shooting JSB MK I pellets first, then MK II pellets

RAW with LW rifled barrel, shooting JSB MK I pellets first, then MK II pellets

Wildcat Mk II with ST barrel, shooting JSB MK II pellets first (mag were already loaded), then Mk I pellets

Data:

Plots of the data:

Conclusion(s):

As it turns out, I was indeed NOT imagining things when I noticed what I had suspected to be my barrel leading up-the data clearly shows the results and, interestingly, the process of re-leading the barrel after a good cleaning. The column furthest to the left shows the 'dirty' barrel's 12-shot group performance, and the 3 columns to the right show three 12-shot groups, after cleaning the barrel (after waiting for the gun to come back down to ambient temperature again). Based on what I see above, and at the target, I would say the barrel isn't fully settled back down again after the 36 shots as the BC was still climbing back to it's more typical 0.056 reading-having said that, it's nearly there with the 36 shots taken. Had I known that there was still more data to be taken to show it settling out fully, I would have shot another 2-3 12-shot groups to show it settle out.

Notes:

• A single shot tray was used for all RAW shots
• Testing conditions were all held constant for the 3 groups shot after cleaning. Wind was very calm for both 'dirty' and 'clean' groups-however, the temp did rise by about 5 degrees as I went to clean my barrel. Having said that, from my other data: 5 degrees won't have much of an impact on the velocities and BC when the pressure and humidity are both held constant, so the comparison is still valid.

Hopefully you find this information interesting like I did when I acquired it and reviewed what is was showing me. I've heard about the accuracy going south in dirty barrels, but I didn't think there would be a way to see the impact of a dirty barrel via data-sure enough, the data shows it pretty clearly...

Please ask if you have any questions and feel free to point out anything I missed.



Sean

I hope you find this info helpful? Is so, please consider taking a second and simply leaving a '+' with a nice comment for me-it let's me know my time and effort is appreciated and keeps me going with this kind of work for the community. I've spent a bunch of money on Chronographs now, lead and time so it's a pretty small way of saying 'thanks for the efforts'. Enough said- Enjoy!

(Edit: Added underlines to the links above to make it more obvious that those are links to specific threads with other test results.)

 

[Smitty911]

Very interesting data. A couple additional shot strings would have been interesting as well.

It appears to be validation that if your point of impact is changing it's time to clean your barrel. You would probably also need to shoot enough pellets to "season" the barrel again prior to shooting for score.



Smitty 

 

[TheIceman]

Awesome job of identifying a problem, forming a hypothesis, developing a means of testing your hypothesis, then carrying out the testing to verify or disprove your hypothesis. Many of us glean bits of knowledge by reading this forum, and sometimes we just take it on faith when somebody says "it has been my experience." Nothing wrong with that. If it works, no explanation is needed for it to work for me like it does for you. But some of us are a little more thirsty for the whys and hows of it all, and airgunners like you are a Godsend for us. Thanks for putting forth the time and effort to test your theory, and for sharing it with us. I still have so much to learn, but what a fun time I am having while learning! A big +1 from me to you!

 

[CampFussell]

Awesome job of identifying a problem, forming a hypothesis, developing a means of testing your hypothesis, then carrying out the testing to verify or disprove your hypothesis. Many of us glean bits of knowledge by reading this forum, and sometimes we just take it on faith when somebody says "it has been my experience." Nothing wrong with that. If it works, no explanation is needed for it to work for me like it does for you. But some of us are a little more thirsty for the whys and hows of it all, and airgunners like you are a Godsend for us. Thanks for putting forth the time and effort to test your theory, and for sharing it with us. I still have so much to learn, but what a fun time I am having while learning! A big +1 from me to you!

+1

Amen....what he said!!!!!! Hard to put it any better than James said it! We have some awesome folks on this forum!

Jimmy

 

[SMH77]

So after posting the data, I did run out and shoot two more shot strings-despite the temp having warmed up further from this morning's testing-just to see if the BC's had stabilized. Bottom line: It looks like somewhere around the 4th dozen shots (call it 50) is where the barrel seems to have settled in, since the BC on the 4th string came right back to where they it normally is for this combination: 0.0555. This afternoon was a little trickier because the wind is blowing lightly (cross wind) by the time I was shooting the 5th group, causing a bit more standard deviation at the 50 yard chrony, and dropping the BC down slightly to 0.0526. In any case, it's pretty clear that the BC's have stabilized by about the 4th dozen-or, call it 50th, shot.

Note: there's a lot going on with that last group and the BC lowering back down below what I would expect. I have seen it before and it usually happens in conditions where the wind pushes the pellet laterally as they near the 50 yard chrony (think of it as a slightly 'angled' path between sensors-and therefore being slightly 'longer', resulting in the shot showing a lower velocity). I had a few of the shots show low velocities at the 50 yard chrony-and that dropped the average 50 yard velocity down enough to drop the BC in the calculation. I'll post the actual shot strings so you can see the data directly to see what I'm referring to (I've color coded the cells that I'm referring to). Anyway, I'm stating that only to say that I'm confident that the BC is stabilized back where I've seen it routinely in the past: ~ 0.0555 for this combination.

Here's the updated data with the 4th and 5th 12-shot groups added. I've also added the RAW data down below for each shot, and highlighted the cells where the 50 yard vel dropped below the min velocity of the entire 12-shot group that was shot just moments earlier. Note: date and time stamps are shown for each shot string also to show when they were shot relative to one another.

Sean

 

[flynviking]

A big +1. Thanks for sharing your data. I have two FX guns with the original ST barrell. Like many folks I clean when I notice my accuracy diminish, usually in the ballpark of 300-500 pellets using various JSBs. Like your data suggests there is a distinct period of about 40-50 shots after cleaning before the airguns settle back down with the JSBs. There is one notable exception, out of my guns they only shoot H&N Baracuda Match pellets well with a clean bore.

 

[SMH77]

Awesome job of identifying a problem, forming a hypothesis, developing a means of testing your hypothesis, then carrying out the testing to verify or disprove your hypothesis. Many of us glean bits of knowledge by reading this forum, and sometimes we just take it on faith when somebody says "it has been my experience." Nothing wrong with that. If it works, no explanation is needed for it to work for me like it does for you. But some of us are a little more thirsty for the whys and hows of it all, and airgunners like you are a Godsend for us. Thanks for putting forth the time and effort to test your theory, and for sharing it with us. I still have so much to learn, but what a fun time I am having while learning! A big +1 from me to you!

Thanks-I really appreciate the feedback and comments! It wasn't as obvious in my original post, but the references at the beginning are all different links back to specific posts with data covering what I stated in the intro. I've now edited the text to have underlines below each link so its a little more apparent. I have other links to other data as well-look in my profile, under 'thread started' and you can dig up more BC data as well as scope comparisons (and soon a bipod comparison as well: Accu-Tac, Atlas BT-10, Atlas PSR, and 'Fake' Atlas-but that's a bit 'off topic' for this thread...)



Sean

 

[jwrabbit123]

Accuracy , BC and POI shifts are related to everything including deposits , While PCP use lead only here is some data that applies to any gun .

Inside of any barrel is imperfections , from tooling or manufacturing , burrs and tiny cracks , While a match grade barrel is honed and or Lapped to minimize this , There is a scientific term used called

Copper Equilibrium , this is when I bullet is shot to coat the inside of barrel and fill in the VOIDS we just discussed , when it is their it is called , SEASONED , this means your gun will have 150 to say 200 good shots before it would need a cleaning and a few rounds to re season , On a copper removal strip , you may need to put 50 rounds or so to get it back seasoned , I never strip my guns .

Air guns are basically same as we use lead and lubricant or dry and we need to fill in voids to season our PCP guns .While some need cleaning alot and some not much depends on the lubricants , Also it is dependent on type of barre/ ST , unchoked or choked , Poly ,and so on .

If you test for POI , BC , and FPS , you will also see dry and different lubes also change BC and temps also change it as well . not a large amount but everything affects POI especially at long ranges for 1 shot one Kill

 

[AirSupply]

Really interesting. In my experience the King Heavies have a fairly narrow window where they will remain stable. If you watch some of Teds old video's you can see even when he is shooting at 100y sub Moa 1 or 2 of the pellets will have a tiny wobble. I think when the barrel gets to dirty you start to get a little more instability. 

A pellet that is slightly unstable has to travel slightly further to the target hence the velocity difference and of course the accuracy goes.That would explain the bc The dirty barrel must slow the pellet inside the barrel also as your 1y velocity are lower

I shoot at night with a torch occasionally and it makes it easier to see the pellet. When the barrel starts to get to the point it needs cleaning sure enough you start to see the little wobble. When you see it you would think the pellet could end up anywhere but they still group reasonably well. Clean the barrel and pellets are stable and accuracy is back to what it should be. 

Thanks for sharing all your hard work. +1

Michael

 

[bobgm3]

Great work! I've seen this type of info gathered before for testing powder-burners (Precision Shooting Magazine & Tactical Shooter ). It's interesting to see how similar overall the conclusions are for that that discipline and for airguns. Thank you for your efforts!

Bob

 

[Michael]

Good stuff!

 

[chohan]

This is very good info so thanks for the time/effort involved +1. That is one beautiful rifle.

 

[jking]

Nice work Sean and very interesting. Question, how clean did you go and what's your thoughts on maybe not cleaning as deep with the thought that maybe less cleaning your BC would be back and not take as many shots to get back to your optimum BC. I always wonder if I'm cleaning deeper and more often than needed. I probably tend to clean too often. And I also wonder how much of the lead flakes I'm pulling out are actually benefitting the barrel. My Crown always "seems" to shoot better after a fresh cleaning though but then again maybe it was just one of those days.

Thanks again for putting in the great work

jk

 

[FukoChan]

Wow fascinating data. Now you know when to clean your barrel when your bc drops. This might also mean there can be a correlation between bc and accuracy 

 

[LDP]

Theres no correlation to BC and accuracy or I should say accuracy potential. The BC is how easily an object passes thru the air. If higher BC numbers related to higher accuracy then people would not have as good or better results with lighter weight pellets. You wouldnt need to try multiple types and weights of pellets you would simply grab the pellet that would give you the highest BC and be done. 

With a higher BC you have less impact from the environment. That will make it easier to get consistent accuracy since you get less wind drift. So you could say less wind drift will make it easier for the shooter to do their part helping the shooter to be more accurate but the higher BC pellet is not more inherently accurate. 

Pellets have such dismal BC numbers it really does not have a huge impact on wind drift. I have posted the numbers before using the JSB Kings and JSB King heavies in .25 and theres not allot of difference in drop and wind drift between the two. They both drift allot when you see 5 mph of wind and more. If you compare a .25 to a .177 then yes theres more of a change but .25 to .25 not so much and really not enough to choose one over the other based on BC instead of just accuracy.

Good work on collecting the data for your rifle. You will be able to use it to keep your rifle shooting at its peak potential.

 

[SMH77]

Nice work Sean and very interesting. Question, how clean did you go and what's your thoughts on maybe not cleaning as deep with the thought that maybe less cleaning your BC would be back and not take as many shots to get back to your optimum BC. I always wonder if I'm cleaning deeper and more often than needed. I probably tend to clean too often. And I also wonder how much of the lead flakes I'm pulling out are actually benefitting the barrel. My Crown always "seems" to shoot better after a fresh cleaning though but then again maybe it was just one of those days.

Thanks again for putting in the great work

jk

Thanks for the nice comments and feedback-it is really appreciated. I'm just glad to help contribute original content to the forum and help drive common learning and discussion amongst the community.

You bring up some good questions / comments regarding cleaning - some of which I should have posted in my original post, so I'll cover them here.

First-regarding the 'dirty' state of the barrel and cleaning frequency: I had run a LOT of pellets down the barrel of my polygon barrel (probably 1-2k pellet, would be my guess) doing normal shooting, hunting, and all the BC work that's linked above and located in my profile. Accuracy seemed to hold well with no noted degradation (until the end). Likewise, the BC numbers I would calculate would reliably come in at the 0.055 - 0.057 range. This was after testing several types of pellets from JSB, H&N as well as testing Ratsniper slugs-none of them seemed to 'dirty' up my barrel. Key point though: my hammer spring tension was always at (or near) the lowest tension setting-I had never really played with it. Then I finally decided to change the HS tension when I put my .30 cal barrel back onto the gun to see what range of power I could get from the powerplant. Once finished with that, I decided to see just how hot the polygon barrel was shooting (turned out to be about 950 fps-which should be a good thing, right? Perhaps not...). I shot several magazines full of pellets with the .25 cal polygon barrel with the HST set to max. That's when I noticed my BC was calculating lower than I expected. At first I concluded that the pellets were simply getting too close to the sound barrel and shedding speed too rapidly-but then I recalled what Martin had told me about nearly 18 months ago (which seemed to coincide with my test results in this thread): the polygon barrels seem to lead very quickly when the JSB heavies are moving at or near the 950 fps mark.

Side note: originally Martin had shared results with me that had the polygon barrel pushing heavies at 950 fps-I was immediately lusting after that combo, but couldn't get my financial to allow an order right away. I kept emailing him to get additional information about the setup, options, etc until I was finally ready to place an order. When I was finally ready to order the gun, I told Martin I wanted him to set it up like he originally told me (near 68 fpe): shooting heavies at 950 fps through the polygon barrel. Initially he acted as though that wasn't an option-until I sent him his emails back. It was then that he realized he had actually shared that testing with someone (me) and explained that he wasn't setting guns up that way for customers. When I pressed him as to the 'why' behind that, he explained it was due to the super fast leading of the barrels that seemed to take place with that setup. When asked about 'how fast', his reply was something like "around 50 shots or so" (going by memory-but is was a shockingly low number of rounds).

Resuming my previous comments: I probably put 2-3 dozen rounds through the barrel shooting at about 945-950 fps at the muzzle. That was, of course, AFTER having shot 1-2k rounds through the barrel with little to no change in measured BC (aside from environmental effects). So it seems to me now, in retrospect, that what I may have noted as the BC dropping at high velocities with the polygon/JSB heavy combo may have actually just been documenting via BC data the observation that Martin had made to me via email: the JSB heavies seem to lead up the polygon barrels very, very quickly shooting that hot. [Side note: this has me scratching my head a bit as to whether the .22 cal Red Wolfs are noticing the same thing in their setups since they tend to run the 25 grain redesigned heavies near the same speed of 950 fps. I suspect there's something different though, because I believe Martin now has a similar configuration he's making available in .22 cal-so that is something still to be explored (by me). End side note.] I've said all that to say this: I'm not sure I'm convinced that the BC is actually dropping when shooting the Heavies (.25 cal) at ~ 950 fps or if that was just the extremely fast leading showing up in the data, hiding the real BC of that combination. In either case, I have two thoughts: 1) I don't intend to clean and remeasure the heavies shooting that hot again, because I don't want to keep my HS tension maxed out for very long so I don't damage the spring, plus I don't like cleaning my barrel a whole lot. 2) It leads up so fast it's really an impractical combination so who cares anyway as I'm not going to clean my barrel every 30-40 shots.

The other point I wanted to address was cleaning method I used today: I pulled one wet flannel patch (2") through my barrel (guided in through a straw, using a crown saver pull through), then I doubled two patches up, wet them and pulled that through-it was very tight and left a puddle of cleaning fluid (the old Beeman cleaner/degreaser) in my breech-ugh. From there, I pulled two dry patches through, then added a 1" 'Outers' (brand) square inside the 2" flannel to get the dry patch a little more tight to the barrel. I ended with putting two 1" patches inside the single flannel 2" patch on the last pull-and that seemed to work pretty well. I was still getting some grayish dirty spots on the last dry patch (total of 4 dry pulls through), but (like you suggested) I decided to not go nuts and 'deep clean' things as I wanted to leave the 'good leading' in place and just remove the excess material.

Anyway, hope that helps explain a bit more. Keep the comments and discussion coming-it's great to sort through this and learn together as a community.



Sean

 

[SMH77]

Theres no correlation to BC and accuracy or I should say accuracy potential. The BC is how easily an object passes thru the air. If higher BC numbers related to higher accuracy then people would not have as good or better results with lighter weight pellets. You wouldnt need to try multiple types and weights of pellets you would simply grab the pellet that would give you the highest BC and be done. 

With a higher BC you have less impact from the environment. That will make it easier to get consistent accuracy since you get less wind drift. So you could say less wind drift will make it easier for the shooter to do their part helping the shooter to be more accurate but the higher BC pellet is not more inherently accurate. 

Pellets have such dismal BC numbers it really does not have a huge impact on wind drift. I have posted the numbers before using the JSB Kings and JSB King heavies in .25 and theres not allot of difference in drop and wind drift between the two. They both drift allot when you see 5 mph of wind and more. If you compare a .25 to a .177 then yes theres more of a change but .25 to .25 not so much and really not enough to choose one over the other based on BC instead of just accuracy.

Good work on collecting the data for your rifle. You will be able to use it to keep your rifle shooting at its peak potential.

Thanks for the feedback and the comments. I hear what you are saying, but I've noticed a very significant difference in resistance to wind drift with the JSB heavies vs the JSB lite pellet. The caveat though (and it's a big one): the powerplant remained unchanged, so shooting the heavies at around 900 fps resulted in pushing the lites to the 'near 1000 fps' range and accuracy of the lites was very poor. As stated above, that setting was the min HS tension on my gun and I wasn't about to go messing with the reg setting to drop it down lower. However, while shooting in the wind, the lower BC of the lites really did seem to correspond to a much, much greater effect of the wind pushing the pellets off target through my setup, whereas in the same wind, the heavies were only slightly affected by the wind. Thus I have tended to conclude that the BC differences do impact the resistance to wind drift. I would also say that, in the case I'm describing in this thread where a barrel leads up very quickly (like 50 shots quickly), then effectively it does show up at your 50 yard velocity (just look at the data above) to the tune of about 20 fps slower with the change in BC going from 0.0555 down to 0.0498. That change in BC will cause both a slower pellet over distance, more susceptibility to wind, and (because its going slower) more time for wind to act on the pellet. Each of these resulting changes from the BC dropping (in the same exact pellet type, mind you) tend to 'appear' as accuracy 'opening up'-or accuracy loss. I'm guessing this is what's going on when people have noticed their barrels 'getting dirty' and needing to be cleaned. Once cleaned, and re-leaded, accuracy returns to the expected level once again (and so does the BC).

Anyway, as such, these are my current thoughts on how accuracy loss may actually be correlated to a drop in a given project type's (like a JSB Heavy, Mk I in a 'freshly leaded' barrel (BC = 0.0555) vs. a JSB Heavy Mk II in a 'excessively leaded' barrel (BC = 0.0498)). I'm open to discussion on it, but the data does seem to tie together the conventional 'wisdom' about knowing it's time to 'clean your barrel' when accuracy falls off...

Good discussion for sure-thank you for the comments!



Sean

 

[SMH77]

Great work! I've seen this type of info gathered before for testing powder-burners (Precision Shooting Magazine & Tactical Shooter ). It's interesting to see how similar overall the conclusions are for that that discipline and for airguns. Thank you for your efforts!

Bob

Thanks Bob! I had never seen this type of data before (I don't really shoot powder burners though)-thanks for pointing that out. Do you happen to know if there are online copies of the articles you could link in this thread (or send to me via PM if you don't wish to redirect AGN traffic)? I'd be curious to read their comments on the subject as well.



Sean

 

[SMH77]

Really interesting. In my experience the King Heavies have a fairly narrow window where they will remain stable. If you watch some of Teds old video's you can see even when he is shooting at 100y sub Moa 1 or 2 of the pellets will have a tiny wobble. I think when the barrel gets to dirty you start to get a little more instability. 

A pellet that is slightly unstable has to travel slightly further to the target hence the velocity difference and of course the accuracy goes.That would explain the bc The dirty barrel must slow the pellet inside the barrel also as your 1y velocity are lower

I shoot at night with a torch occasionally and it makes it easier to see the pellet. When the barrel starts to get to the point it needs cleaning sure enough you start to see the little wobble. When you see it you would think the pellet could end up anywhere but they still group reasonably well. Clean the barrel and pellets are stable and accuracy is back to what it should be. 

Thanks for sharing all your hard work. +1

Michael

Those are some fascinating observations Michael-thanks for sharing them! I will have to go look for some of Ted's videos-I do think I recall seeing some of his pellets 'corkscrew' or wobble a little in flight, but I really haven't watched much of his videos.

Also to your point, if the pellet literally does travel further, then it also implies it take more time to get there-and that is more time for the pellet to be pushed by the wind, or to continue falling to the earth. Both would tend to open up otherwise tight groups...



Sean

 

[FukoChan]

Theres no correlation to BC and accuracy or I should say accuracy potential. The BC is how easily an object passes thru the air. If higher BC numbers related to higher accuracy then people would not have as good or better results with lighter weight pellets. You wouldnt need to try multiple types and weights of pellets you would simply grab the pellet that would give you the highest BC and be done. 

With a higher BC you have less impact from the environment. That will make it easier to get consistent accuracy since you get less wind drift. So you could say less wind drift will make it easier for the shooter to do their part helping the shooter to be more accurate but the higher BC pellet is not more inherently accurate. 

Pellets have such dismal BC numbers it really does not have a huge impact on wind drift. I have posted the numbers before using the JSB Kings and JSB King heavies in .25 and theres not allot of difference in drop and wind drift between the two. They both drift allot when you see 5 mph of wind and more. If you compare a .25 to a .177 then yes theres more of a change but .25 to .25 not so much and really not enough to choose one over the other based on BC instead of just accuracy.

Good work on collecting the data for your rifle. You will be able to use it to keep your rifle shooting at its peak potential.

Well im just thinking about it like this: a dirty barrel doesnt give you maximum accuracy. Cleaning the barrel increases accuracy. However, it also increases bc so an increase in bc can increase accuracy?