Muzzle Velocity Variance critical to subsonic precision

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pman5kmo
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Re: Muzzle Velocity Variance critical to subsonic precision

Post by pman5kmo » Mon Jan 13, 2014 11:14 am

Dolomite_Supafly wrote:This is why case fill is so important. I have taken loads where the powder did not come close to filling the case and tested them. I would chamber on round with the muzzle pointed at the ground so the powder would be concentrated at the bullet. I would do the same except point the muzzle towards the sky and this would concentrate the powder against the primer. I would slower bring them to horizontal and fire them across a chronograph. The difference, even though the loads were identical was pretty amazing. The loads where the powder was against the primer resulted in a higher velocity.

I try to make sure the powders I use have very little, if any, space in the case. Even a slightly compressed load is better than a load with a lot of space.

Then there is the chance of SEE. Although I don't believe it I also do not want to have that one in a million chance blow my hands off.
http://africanxmag.com/secondary_explosion_effect.htm
http://www.reloadammo.com/liteload.htm
http://members.shaw.ca/cronhelm/DevelopSubsonic.html
Long range shooters strive for 90% minimum bulk density. In 300 BLK this is easily attained with almost all suitable powders.

Also why the use of magnum primers in .357 mag is highly encouraged. The chances of SE are almost nill unless your loading some stupid lite loads. It would be like loading a 148 HBWC .38 load with 1 grain BE powder... and even the is unlikely unless you over crimp and create conditions for something that requires such exact conditions to happen.

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dbooksta
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Re: Muzzle Velocity Variance critical to subsonic precision

Post by dbooksta » Tue Jan 14, 2014 10:30 am

300Blk wrote:Are you going to repeat it with something like 50 rounds of coated vs 50 rounds of uncoated to have some more confidence that it worked?
I tried a 100-round test with the standard A1680 load on 220gr SMKs. Unfortunately there was no difference in muzzle velocity variance between coated and uncoated bullets. Using my 16" barrel, no muzzle device:
  • Uncoated average = 923fps, stdev = 25fps
  • hBN coated average = 941fps, stdev = 27fps
Here's a photogenic sequence of photos showing the load process: I am not ready to extend the conclusions of this large test to my earlier observations on the effects of coating with my IMR4227 load because the 10.4gr A1680 load is not as efficient: QuickLOAD shows the pressure peak under 35kpsi and less than 85% of powder burn before uncorking. I also measured slightly higher average velocities on the coated rounds than the uncoated, which is the opposite of what should occur. This suggests the bullet lubricity (at least between copper and hBN) was not a significant factor with this load.

I finally secured some IMR4227 so when I get time I'll go back and try a larger A/B test with that. I also got some N110, which would be even more efficient but I haven't yet determined if that can cycle my action while subsonic.

dbooksta
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Re: Muzzle Velocity Variance critical to subsonic precision

Post by dbooksta » Tue Jan 14, 2014 10:34 am

Is anyone running moly who'd be willing to do an A/B test? I know it's a pain since you'd have to clean the barrel during the test, but if moly solves this problem I'd be inclined to switch to it!

ncorry
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Re: Muzzle Velocity Variance critical to subsonic precision

Post by ncorry » Tue Jan 14, 2014 11:20 am

dbooksta wrote:
I finally secured some IMR4227 so when I get time I'll go back and try a larger A/B test with that. I also got some N110, which would be even more efficient but I haven't yet determined if that can cycle my action while subsonic.
I've tried the IMR4227 with 16" carbine length system in a CMMG. Would cycle with suppressor about 1/2 the time and would not cycle at all without suppressor. Well, at least at velocities of less than 1100 fps. I've since swapped that barrel out to a 16" CMMG with a pistol length system. Haven't gone back with the IMR4227 to see.

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