One for the think tank - Ron chime in please!

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nukemmc
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One for the think tank - Ron chime in please!

Post by nukemmc »

I have 3 rifles made to accept my .30 Ops Inc 3rd model:
- Rem 700 in 260 with a match chamber, Heavy Palma, 25" Bartlein 1:8" barrel.
- GAP10 Gen 2 in 260 with a 20" Bartlein 1:8.2" barrel
- Armalite AR10 in 308 with a 20" WOP 1:10" barrel

Both gas guns use adjustable gas blocks and are adjusted so that it reliable cycles to an open bolt with a magazine inserted.

Shooting the GAP10 today doing load development with 140SMKs and H4350 from 42.0-43.0 in .2gr increments. Without the can on, I had NO ejector marks on the brass ... at ANY load level. I had extra in 42.4, 42.6 and 42.8 and wanted to see POI shift and accuracy difference with the can on. EVERY round fired with the can had ejector marks, some even swiped enough to raise a pretty good burr. Since I FL sized all the brass to the same headspace dimension (1.629-1.631) and I load powder to +/-0.1gr, sort bullets by ogive dimensions so that my Bast-to-ogive dimension is +/- 0.002" I can discount the ammo being the cause.

Since the un-suppressed rounds show no signs of ejector marking, I assume the bolt is locked for the duration of the pressure in the barrel. I have to wonder, knowing a very little about the Ops Inc can design, if the first baffle and the large chamber that sits over the barrel are creating a secondary pressure wave that comes back down the bore either after the bolt has started to unlock or more likely when the brass case has "sprung" back to a somewhat smaller size, and thrusts the brass back into the bolt face.

I have been less-than directly behind enough muzzle brakes to be very familiar with the pressure wave from them. This is the only thing I can think of to explain the difference between unsuppressed and suppressed rounds. The pressure within the barrel cannot be significant enough of a difference as there is only a 15-30fps increase in velocity with the suppressed ammo.

Top-to-bottom rows:
42.6gr H4350 - no suppressor
42.4gr H4350 - suppressed (I rotated the raised ejector marks that I filed down to the 6 o'clock)
42.8gr H4350 - no suppressor
Image

OP Seberger patent drawing for Ops Inc can:
Image

What says you folks???
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fishman
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Re: One for the think tank - Ron chime in please!

Post by fishman »

You seem very firearm savvy. I'm surprised that you're not aware of the effects suppressors have on semi auto firearms.

Suppressors absolutely do increase backpressure. A suppressor is designed to not let the propellant gasses exit the barrel quickly. The peak pressure in the barrel isn't any higher, but the pressure vs time curve stays elevated far longer with a suppressor attached. The area underneath a pressure vs time curve is proportional the the amount of momentum (NOT energy) that is exerted by the pressure. If you increase the area under the curve you increase how much momentum is put into the bolt which increases the speed of it traveling back which increases the force exerted on the brass during extraction.

There isn't a secondary pressure wave bouncing back, the pressure just doesn't drop as quickly when you use a silencer.

All you need to do is readjust your gas block after you put the silencer on. This will delay the unlocking until the pressure falls to optimal levels, reducing the amount of momentum and energy delivered to the bolt. After you remove the silencer, the gun might not cycle until you readjust the ha block again. This is a big part of why adjustable gas blocks exist in the first place.
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T-Rex
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Re: One for the think tank - Ron chime in please!

Post by T-Rex »

Your saying the ejector made those marks (grooves)? Not the extractor?
If so, that is a common sign of over-pressure. Fishman's suggestion for an adj. gas block is spot on. You should have one w/ suppressor use, regardless.

If it is the ejector, take a look at it. Does it have a pronounced/sharp edge, where the face meets the cylinder profile? If so, there are 2 things you can do.
1. Put a slight chamfer around the entire ejector.
2. Put a larger chamfer on the side/edge closest to the firing pin.

If it is the extractor, and not the ejector, look for raised or sharp edges. Regardless of whether or not this is the issue, one should always check their extractor for these things. Sharp edges can cause rounds, from the right side of the magazine, to feed jam, when they're drawn from the mag and try to slide across the bolt face and are caught by the sharp edge.
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nukemmc
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Re: One for the think tank - Ron chime in please!

Post by nukemmc »

Thanks for the input.

Yes, it is the ejector leaving nice, round, raised marks on the case head. They are big enough (0.003-0.006") as to affect headspace dimension when sizing, hence me filing them down.

I do adjust the AGB with the can on such that the bolt just barely stays open on an empty mag then open it 1 more click (90°). With the can off it will eject the round but not lock back on the magazine.

Yep, checked for sharp ejector and ejector hole edges. Had to radius them on my MK12 for the same reason. I'll look again with my loupes though.

One other piece of data - same ammunition and can in my bolt rifle does not leave any ejector marks.
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