Case Doming

G

GrouseHiker

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This is my first post here, and I was going to do a bit of research to avoid asking the same old questions, but I didn't get any hits on "case doming." I'm trying to understand the dynamics of the rimfire cartridge (22WMR is my immediate project) to help me understand some observations I made shooting a new rifle yesterday.

The rifle is a Remington 597 semi-auto, and I noticed that consistently, base-domed cases were ejecting from just off the edge of the bench out to 6 ft or so. Flat-base cases (no doming) were ejecting 10 to 15 feet. I've also got suspicion the domed cases are also the culprits when the rifle experiences intermittent lack of cocking. The only thing I can come up with is the bolt may be moving back slower with the domed cases. Any other ideas? Headspace is 0.053 (SAAMI 0.050 to 0.056).
 
Are you trying to say that the case head area that meets the bolt is balloning in a convex fashion towards the bolt? If so I would suspect that the ammo is not right as you have stated others do not do that in your rifle. Rear end of rim fire should be flat before and after firing not blown out like excess pressure, in my humble opinion
 
Recalled

I'm pretty sure Remington has recalled (sorta buying back) all the 597's in .22 mag. You get a coupon toward another purchase.
 
Looks to me like headspace is excessive. This allows some cases to blow out to a "dome" shape.
It could also be that the bolt is moving back slightly (because of "doming") before case moves off chamber wall.
You might also look for high friction in bolt at rearward portion of it's stroke, but low friction or spring load at start of stroke. This could account for failure to cock. Clean and well lubricated parts can correct a lot of problems .
 
pacecil said:
Looks to me like headspace is excessive. This allows some cases to blow out to a "dome" shape.
It could also be that the bolt is moving back slightly (because of "doming") before case moves off chamber wall.
You might also look for high friction in bolt at rearward portion of it's stroke, but low friction or spring load at start of stroke. This could account for failure to cock. Clean and well lubricated parts can correct a lot of problems .
Click to expand...

This is getting into the area of interior ballistics that is not intuitive for me. I'm guessing that at ignition, the chamber pressure spikes and there is probably no way for the case to move due to friction between the case and the chamber wall. I'm also thinking that the case will stay locked against the chamber wall until the pressure drops enough for the case to release from the chamber wall (bullet leaving the barrel?). It seems the doming would have to occur while the bullet is still in the barrel. This line of thought tends to lead to variability in the ammunition.

My other quandry is that if the cartridge is riding back against the bolt due to excessive headspace, the firing pin will push it forward.

The lack of cocking issue is a whole other subject I think I have a handle on, but the doming (and possibly related slow bolt speed) creates unreliable bolt movement.

Added: I haven't yet done extensive measurements, but based on a few, the amount of doming far exceeds the "extra" headspace.
 
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i'd stop shooting it and contact remington, it sounds like the 597 has problems
under high pressure calibers.
 
These guns, especially in other than LR chamberings, seem to have a rich history involving feeding problems, slam fires, and less than return to full battery. It was never actually designed with anything other than LR in mind. I think the boys in marketing made an executive decision. There have been blown rims, etc. I'd get it fixed or sold.
 
tim said:
These guns, especially in other than LR chamberings, seem to have a rich history involving feeding problems, slam fires, and less than return to full battery. It was never actually designed with anything other than LR in mind. I think the boys in marketing made an executive decision. There have been blown rims, etc. I'd get it fixed or sold.
Click to expand...

Interesting - you're right on. This rifle was brand new in December - experienced a blowout on the 31st round on the Sunday after Christmas. I sent it back to Remington and it came back with new bolt and new barrel (despite a number of complications). I do believe I have the lack of cocking problem pinned down. I'll briefly cover it:

I think I've figured out why the 597 action is set up to give such a marginal touch on the trigger linkage "break" lever (or maybe it could be called the sear release lever) - it's about decocking ability. If the reset lever is not touched during hand cycling of the bolt back against the buffer, it's possible to de-cock the rifle in this manner. Conversely, when fired, the bolt is slammed against the buffer (compressing it) and trips the lever. This is where I think the case doming is coming in. I'm guessing the bolt is not coming back as hard when the cases are domed (evidenced by case ejection distance).

I don't have a rim thickness device and didn't think measuring case length would be very accurate. Anyway, I'm moving away from the idea that headspace is the culprit. If you look at the bottom of the bolt with a case in place, there is a lot of brass showing (bottom part of the rim and a portion of the base). "Support" from the bolt doesn't seem to be a consideration. I'm thinking that if the pressure spike is high enough to create case doming, the bolt is simply going to be pushed back.

Thinking out loud, maybe a modification that would reduce chamber pressure would help - maybe bore lapping or bore coating?
 
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