Velocity versus Accuracy

[JimD]

The subject of changing a PCPs velocity as a way to improve accuracy comes up fairly often. I've tried it occasionally without clear favorable results. But I decided to do a more systematic test. I have a NOE mold that makes 27 grain 25 caliber pellets. I've never gotten them to shoot small groups in either of my 25s. So I tested them in my 25 caliber Avenger today. I shot two 5 shot groups, at least, at 1200 psi, 1400 psi, 1600 psi, 1800 psi, 2000 psi, and 2200 psi. I included FX 25.4 grain but only at 1200, 2000, and 2200 psi regulator setting. I got concerned switching back and forth on the pellets was affecting the group size. I don't think that happened based upon my final group with the FX pellets at 2000 reg setting (same as my first groups before shooting any of the cast pellets).

I'll attach the target but my bottom line is the velocity did not significantly affect the accuracy. Doesn't mean that always happens but I think it is true for my guns. For the FX pellets, the 2200 setting gave me an average velocity of 886. 5 shot groups were .3 inches and .75 inches at 25 yards but in the big one 4 where in .25 inches. The 1200 reg setting gave me an average velocity of 729 fps and groups of .3 and .4 inches. I only shot one group at 2000 and the average velocity was 865 and the group was .3 inches. That is not a lot of different velocity data points but I have more for the cast.

The 1200 reg setting gave me an average velocity of 708 fps with the cast and groups of 1.1 and .4 inches. The bigger group had 4 in .6 inches. This was the best I have done with these pellets and might deserve more testing except I don't really want to shoot at 708 fps. At 1400 I got an average of 794 fps and 0.8 and 0.5 inch groups. At 1600 I got 814 fps and 1 and .9 inch groups. At 1800, 835 fps and 1.1 and .7 inch groups. At 2000 I got 849fps and .9 and .75 inch groups. I then tried a couple hammer spring settings at this regulator pressure and the +1.5 turn setting gave me the same velocity and .75 and 1.2 inch groups (4 in .75 for the bigger one). The 1200 and 1400 reg setting groups might be slightly better than the others but that could just be the normal variation in my shooting. I was not fine tuning the hammer spring for each regulator setting but that was true for both pellet types. I did try several settings for the 2000 regulator setting and I think the 1.5 turn setting was a little better but still within the range of the other results.

Still seems like a reasonable thing to check on PCPs but I will be surprised if I ever get much different results by changing velocity, I was shooting at 25 yards.

 

[Long_Gun_Dallas]

There are so, so many variables at play when it comes down to accuracy. Gun dynamics, barrel whip, turbulence, rifling to projectile interface, leade, grooves, crown, muzzle devices and their numerous affects on airflow and harmonics. Projectile stability and bc. Humidity, temperature, wind. And the idiot pulling the trigger (among the numerous other things I still failed to mention)

In other words, just off hand, adjusting pressure will alter shot cycle dynamics, turbulence, harmonics, pellet interaction with rifling, and pellet interaction with air resistance and cross winds. Possibly more. Some of these can be further tuned in conjunction to extract even more accuracy.... or do the opposite

 

[Hateful McNasty]

I hear some on finding that "sweet spot"

And apparently I'm no Sherlock Holmes Mr Watson

 

[JimD]

I think there are better and worse tunes for a given pellet. But changing the tune, in my experience, will not make a gun "like" a pellet it shoots poorly. If it shoots poorly with one tune, it will shoot poorly with other tunes. I see the path to airgun accuracy as finding a pellet that your gun shoots well and then trying different things to tweak it to be a bit better. In this test I was making fairly crude changes in velocity to see if it would change the fact that the gun doesn't like the cast pellets very much. There may be a promise of a small improvement at 700 fps or less but the velocity changes didn't make the gun start shooting the pellet well. I plan to keep the mold just in case I ever can't get pellets but as long as I can buy pellets pretty inexpensively and easily I plan to just do that.

 

[elh0102]

I think there are better and worse tunes for a given pellet. But changing the tune, in my experience, will not make a gun "like" a pellet it shoots poorly. If it shoots poorly with one tune, it will shoot poorly with other tunes. I see the path to airgun accuracy as finding a pellet that your gun shoots well and then trying different things to tweak it to be a bit better. In this test I was making fairly crude changes in velocity to see if it would change the fact that the gun doesn't like the cast pellets very much. There may be a promise of a small improvement at 700 fps or less but the velocity changes didn't make the gun start shooting the pellet well. I plan to keep the mold just in case I ever can't get pellets but as long as I can buy pellets pretty inexpensively and easily I plan to just do that.

I agree that a poor shooting pellet or slug will not be tuned into a star performer. But I think we're really concerned with the good-to-great projectiles. Given your purposes, endless testing may not be worth it, but I think you need to shoot a lot more groups to form conclusions. A specific slug/pellet, assuming a barrel of a given twist rate and profile, and being shot at a specific distance, will find a velocity range in which it is more accurate. Years ago when testing .223 bullets, I tried some long VLD bullets, and I could not shoot them fast enough for stability in a 14 twist barrel. Going to an 8 twist barrel resulted in very good accuracy. That's an extreme example, but I think the same variables exist in any similar testing.

 

[JimD]

I used to shoot powder burners some and handloaded for them to try and improve accuracy. I did not try a lot of projectiles I changed the powder charge and the type of powder primarily. That seemed to be typical from what I read others were doing and from the conversations I had with others at the range I belonged to at the time. I've thought some about PBs versus airguns and I think the huge difference in the gas pressure makes them pretty different. Centerfires are often 60,000 psi or more pressure in the chamber immediately behind the bullet. I left my Avenger at 2000 psi on the regulator and the pressure behind the pellet will be less because it has to go through the transfer port to the chamber area after the valve opens. I think both types have some harmonics set up by the pressure pulse but I think it has to be a lot milder in the airgun due to the hugely lower pressure.

I agree that my limited data set is not enough to reach any grand overall conclusions that would apply to all airguns and projectiles. There are random variables affecting the group size which include my limited shooting skills. But I think you have to agree that in this test, the "good" shooting FX projectile was pretty consistent in shooting much smaller groups at the three pressure/velocities I tried. The "bad" shooting cast pellet was also consistent in shooting larger groups at all velocities. So for this gun and these two projectiles I would argue that the velocity was not making a large difference. Even though the data set is admittedly small it is also consistent. I did not do any statistics on these results but from a statistical standpoint each time you have a data set that points in a particular direction the odds that the conclusion it indicates is false goes down dramatically. You need a lot less data to get to a high probability conclusion when it all points in the same direction.

This is just my guess but I think the harmonics of the pellet sliding through the barrel are more important in airguns because the pressure pulse is comparitively mild. In PBs, the characteristics of the pressure pulse (from the powder burning) seem to be more important. Using the right twist rate is going to overwhelm things due to stability, however. It sounds like you came to that conclusion. Twist rate seems to matter in airguns too but pellets seem to shoot well at relatively long twist rates.

 

[B_y_r_o_n]

Have you let anyone else shoot your cast pellets to see what type of results they get? Could be something with the cast pellet itself and velocity changes be damned!

 

[JimD]

Only my two 25s have shot the pellets I've cast but several others bought molds from NOE for the same pellet about the same time I bought mine. We thought there was an issue with the mold, there is a dimensional difference between the drawing NOE has for the pellets it makes and our cast pellets. I returned mine and got another mold from NOE but it didn't change things at all. The pellets look good and I compared the weight difference in my cast ones and some JSBs I had handy and found less variation in my cast ones. My belief is that guns just "like" some pellets and not others. My Avenger is pretty picky. I don't know why they like what they like but it seems to be the case. My Avenger will shoot at least OK with the JSB made pellets I've tried but seems to shoot best with the FX 25.4s. It hates everything made by H&N. My P35s prefer H&N although the 177 and 22 will shoot JSBs pretty well. I'd love to know why that is but I don't think it's a velocity thing.

Twist rate came up in this thread and I should have mentioned that my research indicates that our PCPs, if made recently, probably have a faster twist rate than we need for pellets. RTI put a special slow twist barrel on the Prophet in 22 initially and it was that slow twist barrel that did well at a 75 yard benchrest competition. The twist rate was slower than 1 in 20. FX used to use that slow a twist rate but I think all theirs are faster than this now. I measured my P35-177 at one in 18 if I remember right. I haven't measured the others but I assume they are either the same or at least similar. I see twists like that on other guns. I think we are getting barrels that are a little faster than we need for pellets because of the interest in slugs. You could argue that the slightly better results I saw at 1200 psi are because the faster than necessary twist in the Avenger barrel is over stabilizing the cast pellets at higher velocities. While that might be worth investigating I don't really want to shoot that gun with a muzzle velocity of 700 fps or less. The regulator might work at 1000 but I think 1200 is the recommended minimum. The results at 1200 were still not what I wanted to see. I think it is definitely true that we can drive pellets to speeds at which they are not accurate, especially if there is wind. But having to keep them under 700 fps seems too low to be useful. I think most pellets can safely go 850 fps and some are reportedly accurate over 900 fps. In warmer weather my Avenger shoots the FX 25.4s at about 930 fps and is accurate, at least on calm days, at that speed. I left the regulator a little lower than it was because I did not see great accuracy at 100 yards from it at that setting. I'm hoping a little slower will shoot better at longer range.

 

[EdinGa]

I'm a caster and a reloader and I can tell you that there is definitely nodes of accuracy around velocity, seating depth and other variables. Air rifles are a different animal in my experience. I really can't give a good explanation as to why but like you, I suspect it has more to do with the pressures involved. I'm perfectly happy with saying this pcp likes this pellet or slug and another likes something else.

Jim, I have 2 pcps that shoot the NOE 27 grain pellet well, but none of them like the pellets from the 34 grain mold.

 

[Long_Gun_Dallas]

I know the 22 cal fx barrels I have are 1:18 for pellet A, and 1:16 for superior.

As for harmonics, it does seem more pronounced in airguns. Why that is, I don't know. But small changes in power or pressure can have pretty large affects on accuracy. When tuning a gun, I try and tune the valve first. Find what pressure it and the hammer opperate best at then back off of that pressure by 5-10bar. I'll also back down the hammer force a smidge as well. What you're left with is a very quick, snappy, and powerful valve action that also generates 90-95% of max power output. While also wasting very little air. Valve dwell is very low, so pressure impulse is a very quick spike. I believe this cuts down on turbulence and harmonic vibration as well. The gun will also get significantly quieter in most cases. From here you can play with overall power output by whatever means you have at your disposal until you find that sweet spot.

Of course if the gun performs, and you are happy with it, then this is all unnecessary. But I do prefer doing this to cut down on noise and air useage. And also get a better idea of what the gun can do. As well as what it likes and does not like.

 

[Airgunmann]

I've found the 25 gr have been better at least in my guns at about 900 fps or a tad faster

 

[elh0102]

I admit, my experience in precision loading of CF ammunition far exceeds my tuning experience with air rifles. I guess that's why, for the most part, I so enjoy just loading and shooting pellets.

 

[JimD]

Ed, anything to distinguish the guns which shoot the NOE 27 grain hunter well? Velocity? Do you know if their twist rate is slower or faster?

I've calculated the "efficiency" of my three P35s. The 25 and 177 caliber were good in efficiency but the more accurate 22 caliber was less so. I recently had to reset the hammer spring in the 22 because a grub screw fell out of the cocking mechanism into the hammers path so I had to remove the adjuster to extract it and put it back where it goes (with a drop of blue locktite). When I reset the hammer spring I deliberately set the hammer spring a little lighter than would give me maximum velocity for the regulator setting. But that gave me a first shot that was 10-20 fps lower than the rest of the string. So I turned the hammer spring up a little and I think I have that fixed. My P35-25 did the same thing about a year ago. I don't know if this is unique to the regulator in the P35s or "normal". I will need to do a longer string to recalculate efficiency but my guess is I am back where I started. The regulator in the 177 and 22 are set where they came to me. The 25 is turned down a bit because the 20 grain FTTs it likes were going 900 fps or more and I didn't think I wanted that. Accuracy didn't change when I turned the regulator down. But I had to decrease the hammer spring to match the new regulator setting. Long way of saying I set my hammer spring pretty close to maximum velocity in my P35s because if I do not, the first shot velocity is low. I still get good efficiency in two of the 3 and it isn't bad in the third. I had to shorten the hammer spring of the 177 to be able to set the hammer spring low enough to match the regulator. It's probably the same spring used in the 22 and 25.