6mm BR & switchbarrel bolt gun

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mtngun
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Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Sat Jul 16, 2016 12:13 pm

Is it common for crimp-on checks to come off in flight? Today I attempted to answer that question.

5 shots were fired from each firearm with its usual load. No aiming, just pointed at the target from 30 feet. None of the gas checks had been annealed or given any special treatment whatsoever.

Result: there were no "extra" holes in the target with any of today's firearms.

5 shots is not enough to prove that gas checks never come off in flight, but it's enough to prove that gas checks do not typically come off in flight. Therefore I conclude that there must have been something amiss with my 6BR or its bullets that was making it fling gas checks prior to chamfering the entrance to its throat.

Loads tested today:
-- Contender Carbine 6x45, 55 gr. GC, ~2750 fps
-- Contender Carbine 357 mag, 180 gr. GC, ~ 1800 fps
-- M700 22 BR, 54 gr. GC, ~ 2750 fps
-- M700 7 BR, 100 gr. GC, ~ 2750 fps
-- M700 30 BR, 130 gr. GC, ~2700 fps
Image

Let me know if you can't see the photo above? I'm using a new-to-me photo server and I'm not positive that I have the sharing options set up correctly.

Conclusions:
-- it's not normal for crimp-on gas checks to come off in flight.
-- there must have been something wrong with either my 6BR chamber, or else something wrong with the bore-riding bullets that it was shooting at the time the problem reared its head.
-- chamfering the entrance to the 6BR throat greatly reduced, but did not completely eliminate, the errant gas checks.
-- I had shot quite a few tapered Loverins in the original throat without hitting the chronograph, so I'm not positive if the errant gas checks were common to all GC bullets in the original throat, or only with the EXP bore-riding bullets?

Where Do We Go From Here?
-- I'm inclined to set the barrel back and rechamber, but I haven't decided exactly what I should do differently other than to avoid a sharp corner at the entrance to the throat?
-- I do plan to try annealed checks and a snugger fit, but as mentioned previously, none of today's loads used annealed checks, and some of them were a "relaxed" fit, yet the gas checks stayed on.
-- I've been busy lately so it may be a while before I can schedule time to do the rechamber. In the meantime I may continue shooting the 6BR with plain base bullets, but will hold off on GC bullets.

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mtngun
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Location: Where the Salmon joins the Snake

Re: 6mm BR & switchbarrel bolt gun

Postby mtngun » Sun Nov 20, 2016 6:26 pm

I haven't had time to play with any of the 6mm's lately, but I've been doing some upgrades to my 100 yard range, including redoing the box that the chronograph sits in. I found a bunch of 6mm gas checks embedded in the old OSB chrono box. Also there were many 6mm indentations in the OSB where a gas check had struck, but didn't embed. Only 6mm checks, no 22, no 7mm, no 30, no 35, etc..
Image

Apparently the 6BR had been slinging checks for quite a while and I wasn't aware of it until a check zapped the chronograph. :roll: I never did fully understand what caused the errant checks, but suspect there must have been some sort of issue at the entrance to the throat.


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