Higher velocity than published?


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sallymo
April 10, 2011, 10:55 AM
I am hoping someone can explain this to me. I bought my first chronograph and I am getting higher velocities than expected. Shooting a S&W M&Pc 4" barrel 45 acp. Load is 5.4 grns of AA #2, Fed 150 with a 200 grn Laser Cast SWC and RP cases. Very slight taper crimp. COL 1.220. No obvious signs of pressure on the spent cases. Chrono gave me an average of 940 fps. Accurate Arms load data indicates that velocity out of a 5" barrel with 5.8 grns of powder. The only difference in my load being AA data has a COL length of 1.190. Most posts are about velocity being too low but I am having the opposite problem here and I am worried that the load is too hot. BTW shot some factory ammo and velocities were what I expected so I don't think it is the chrono. Thanks

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rcmodel
April 10, 2011, 11:22 AM
My older Accurate powder guide lists 5.8 MAX AA#2 with a 200 lead bullet giving 939 FPS.

I doubt your load is too hot, just that your gun has a "faster" barrel then the one AA used in testing.

How far is it throwing the brass compared to factory loads?
If it's chucking them into the next county, maybe your scales are not weighing what you think they are.

Might be wise to get some scale check weights and see.

rc

sallymo
April 10, 2011, 12:48 PM
Thanks for the advice RC. I do have check weights and the brass is being thrown similar to factory loads. Seems peculiar to me to be getting these chrono readings but maybe as you say I have a "fast" barrel.

Peter M. Eick
April 17, 2011, 10:29 AM
Just as an example of fast barrels, my 4" diamondback is routinely faster then my 8 3/8" Pre-27 with the same load.

For example: 3.7 grns of AA2 with a 158 Lasercast SWC
2" detective special 720 fps
4" diamondback 809 fps
5" Heavy Duty 790 fps
6" Outdoorsman 795 fps
8 3/8" pre-27 771 fps


Go figure.

Walkalong
April 17, 2011, 10:33 AM
Yep.

I have a "slow" 6" barreled M-28. My 4" barreled .357s sometimes duplicate the velocity with the same load, and always come close.

I have a "fast" (Wilson) barreled Colt 1911. It routinely gives up to 50 FPS more than other 5" barreled 1911s.

Lots of variables out there.

45ACPUSER
April 17, 2011, 11:54 AM
Different platforms will give different velocities...
Testing done in non vented bbls will have higher velocites....Hell, you could compare tow similar guns and there would variances....You have to compare apples to apples same platform and same components......elevation, temperature, and other factors bring things into play....

Hell I have 150fps difference over expected velocity rather then real world velocity.

bds
April 17, 2011, 12:29 PM
I am finding that newer semi-auto pistol barrels have much lower "lands" in the rifling and may produce less gas leak around the land/groove/bullet voids that results in high chamber pressure/muzzle velocities than the testing fixture barrels.

Glock barrels with smoother hill/valley rifling have been producing higher than published load data velocities since they came out as they produce even less leakage of gas than land/groove rifling.

codefour
April 17, 2011, 12:49 PM
I agree with the others. it sunds like a barrel issue but is not necessarily a problem. For instance, my Sig 226 produces higher velocities thena my Glock 22 in .40 S&W with the exact same load. It was 54 fps higher on average. I compared my old Springield 1911 and my Sig 220 with the same .45 load. The Sig was again faster by a wide margin.

Chrographs are cool tools. I borrowed a friends last time at the range and plan on buying one next.

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