Tuning FX Crown MK2 accuracy shift day to day

[bcsquared]

Has anyone seen changes in accuracy with the same ammo and same tune after a period of time? I'm thinking maybe the tune wasn't as good as I thought but not sure what's causing the accuracy to change from one day to the next.

I got an FX Crown MK2 Continuum a few weeks ago and test a few slugs. The best groups shot were tested a few slugs, the H&N Slug .218 23gr was best at the time. After tuning it to shoot at 862fps, I was getting consistent groups at 5mm; about 1/4" at 25 yds.

The issue started yesterday after I missed a few shots at some doves; I figured it was just me. Some NSA slugs I ordered came in the mail yesterday as well so I tested them out this morning and shot the H&N's to benchmark them. The groups with the H&N slugs had blown up to 20mm, more than 3/4" at 25yds and started to vertically string. Using the same tune, the NSA .217 20.2gr slugs came out on top, literally shooting a single ragged hole at 25 yds. We're talking less than 4mm at this point.

This is the set up and chrony numbers. FX Crown MK2 with 380mm barrel, standard liner and stock FX moderator. Tune is 155bar, transfer port high (.25/.30), internal hammer spring adjustment is 2mm of play, external hammer spring adjustment at 22.

H&N Slug HP .218 23gr. Avg 862 fps, ES 15, SD 4
NSA .217 20.2gr. Avg 884 fps, ES 9, SD 3

 

[madeintheuk]

First of all, the fx superior baffles tend to like .217 diameter more than .218 (in my opinion) and with the larger diameter, it’s easier to lead up the barrel, which will definitely cause accuracy issues.
the nsa 20.2 is the magic slug that seems to shoot lifts out in almost any barrel it’s put through. So no surprise you’re having those groups.

before changing anything about your tune (or blaming it), try cleaning your barrel first, and then shoot another string. See if those H&N’s tighten up again. If they do, they’re leading up your barrel too fast. Try going down a size. Or sticking with the nsa’s

 

[Long_Gun_Dallas]

Pretty much everything I was going to say and more ^

 

[Revoman]

The above, plus the possibility that at times, the first several shots fired after sitting for a long time, tend to not be great on some FX rifles (mostly Impacts)....but usually not the Crown's. Just something else to check.

 

[madeintheuk]

The above, plus the possibility that at times, the first several shots fired after sitting for a long time, tend to not be great on some FX rifles (mostly Impacts)....but usually not the Crown's. Just something else to check.

The 2 main causes of this on FX rifles (mostly impact) is reg creep and valve rod stiction. Reg creep is easy to check by looking at the gauge, stiction usually only effects the impact valve due to its stupidly long valve rod that travels through a bunch or sealing orings, all of which can cause stiction against the valve rod. @Bigragu had a great idea of putting a few drops of synthetic oil in the foster fitting every few fills that can work its way through the system and keep those orings lubed up. Ever since he told me this, I’ve been religiously doing it to ALL my pcp’s every 3-4 fills. It won’t hurt anything, and is more beneficial than anything for preventative maintenance.

 

[bcsquared]

The 2 main causes of this on FX rifles (mostly impact) is reg creep and valve rod stiction. Reg creep is easy to check by looking at the gauge, stiction usually only effects the impact valve due to its stupidly long valve rod that travels through a bunch or sealing orings, all of which can cause stiction against the valve rod. @Bigragu had a great idea of putting a few drops of synthetic oil in the foster fitting every few fills that can work its way through the system and keep those orings lubed up. Ever since he told me this, I’ve been religiously doing it to ALL my pcp’s every 3-4 fills. It won’t hurt anything, and is more beneficial than anything for preventative maintenance.

I do this with silicone lube on my other PCP's.

I had the same thoughts about reg creep as well but checked it and shot another group. At the time, the groups seemed to be getting worse on the H&N's. I'm going to clean the barrel and check again tomorrow. I have a feeling madeinuk is on the money.

 

[Chukar60]

How many shots are you running through the barrel before you shoot a group with a new slug?
When I was setting up my M3, it would take 5-10 shots to get the barrel to settle in to a different brand of slug.
Or, you may try cleaning the barrel every time you switch slug brands, then shoot a couple “fouling” shots before you shoot a group.

 

[bcsquared]

I didn't do any fouling shots between slugs yesterday. Going straight into the NSA's had no problem grouping with the .217 20.2gr slugs. I switched back to the H&N's to verify grouping after the NSA's. I shot a 15 round string of the H&N's in 3 groups but it got progressively worse. The chrony numbers were solid so that's why I got concerned.

I just found it odd that it was a tack driver one day and the next, I'm struggling to hit the broadside of a barn. I appreciate everyone's suggestions, definitely going to be testing and trying it out. However it's tempting just to stick with the NSA's since they've grouped the best out of all the slugs I've tried.

 

[Vetmx]

You are at the part of slug testing that is every bit as important as finding the right settings and slug that gets you all excited. You are at the consistency and what it takes to maintain consistency part of the process. Without going into major detail here, if the NSA behaves over the long haul and you like its accuracy, that’s what you use. With the H&N it could be a couple things. Most slug shooters have walked away from the bench very happy they finally found magic. Only to return and get a reality check. It happens way more than you think especially if the shooter posted bragging pictures. We don’t ever hear about day two. Day two could be the result of a good shooting slug that requires a more frequent cleaning interval. Or day ones good accuracy came at a certain point of your choke fouling and that’s where you stopped shooting. Sometimes you find a good shooter, clean the barrel and it never shoots again. Like Revoman said, it’s not unusual for any slug gun to not be exactly spot on after sitting. My best slug guns with spot on first shot velocity can still take a couple shots to start cutting the same hole. We can blame long valve rods or reg creep but I also believe it’s the condition of the bore of the barrel. It is what it is. And that’s why we test over long term. It helps us know what to expect when we grab our guns.

 

[bcsquared]

Closing the loop on this conversation. I appreciate everyone's feedback, there are a lot of nuances and testing to the tuning process besides finding the right FPS and intended grouping. The Crown is my first FX airgun so I'm still learning a lot.

On Monday, I cleaned the barrel and tested a few slugs; H&N .217 21gr, Varmint Knocker .217 22gr, NSA .217 20.2gr. I shot 5 of each slug to foul the barrel then moved into 3 groups of 5 shots. The H&N's were still horrible and couldn't get a group smaller than 15mm CTC. Varmint Knockers and NSA's both averaged 7.5mm CTC.

Yesterday, I wanted to see what would happen to the groups from a full bottle to empty (before reg pressure dropped). The Varmint Knockers started opening up quite a bit after 15 shots. The NSA's stayed tight (6-10mm CTC) for 40 shots before starting to open up. On the current tune, I'm getting 60 shots before the reg starts to drop. So looks like I'm staying with the NSA's for now.

 

[Airgunmann]

I didn't do any fouling shots between slugs yesterday. Going straight into the NSA's had no problem grouping with the .217 20.2gr slugs. I switched back to the H&N's to verify grouping after the NSA's. I shot a 15 round string of the H&N's in 3 groups but it got progressively worse. The chrony numbers were solid so that's why I got concerned.

I just found it odd that it was a tack driver one day and the next, I'm struggling to hit the broadside of a barn. I appreciate everyone's suggestions, definitely going to be testing and trying it out. However it's tempting just to stick with the NSA's since they've grouped the best out of all the slugs I've tried.

Like stated above if your Chrono numbers are staying the same and it was getting progressively worse with shooting h&n it sure sounds like a barrel leading problem

 

[Vetmx]

Your good slug starting to open up after 40 shots or basically when your tank pressure is under 190b is understandable. Even with a regulator, a guns harmonics can change as the pressure in the tank drops. You can try to tune around it but you may find they don’t shoot as great as they used to for those 40 shots. So this is another crossroads in slug city. I would stay with the NSA and the 40. You know when to refill from now on and can hopefully live happily ever after until another slug strikes your fancy.

 

[qball]

I had QC issues with H&N slugs, just not consistent enough for me to ever buy them again. NSA on the other hand is simply phenomenal.

 

[Bobbybeef]

I think the regs are crap i shoot at a 1/4 diameter spinner at 35 yards if i take my crown out and shoot i will miss for a bit if i cock and fire about 6 times blank i hit the spinner every shot for the rest of the mag 177 cal that tells me that the reg needs to settle i can shoot a shot or two for the rest of the day and its lights out next morning same thing i am just to lazy to replace the reg.

 

[Vetmx]

If a gun is harmonically sensitive, or at least a very important part of the gun is harmonically sensitive, the things in this discussion can happen. The guns I shoot slugs out of that are purpose built with a proper barrel, don’t experience any of this even if the reg creeps or I turn the hammer spring down or up a little. Slug still hits its mark. Ironically, I have a slug barrel on one of my M3’s and that gun is still a little more sensitive than a single reg, slug barreled bullpup I have. It’s just in the nature of a complex gun. Pellets? No problem. My one M3 pellet shooter is almost Taipan like. Because I don’t screw with it.

 

[bcsquared]

I had QC issues with H&N slugs, just not consistent enough for me to ever buy them again. NSA on the other hand is simply phenomenal.

I see that comment a lot here. The H&N .217 21gr is excellent out of my JTS Airacuda Max; I keep several tins for that. IME their diabolo shaped pellets are great but their hunting pellets don't shoot well in any of my airguns.