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Old April 23, 2000, 09:04 AM   #1
k in AR
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Join Date: October 12, 1999
Location: Arkansas USA
Posts: 435
I have a Ruger M77 MKII in 7.62x39 & have a question about "primer problems" & chamber size.
I have noticed that with SOME reloads & even factory "Federal Classic" ammo, the primer tends to back out .003 to .006 inch.

There are no signs of High pressure & accuracy is surprisingly good. I know factory barrels are not know for real tight dimensions, but is this "too loose"?

ALSO: I checked some old post & found this:
Cheapo wrote<< "Cases are sized a bit on the short side, the fired in a chamber a bit on the long side. Initial pressure in the case sidewalls grips the sides of the chamber and the primer backs out a bit. Then pressure builds more and the case either slips back a bit (okay IMO if it's only a small distance) or stretches back (bad if the distance is long). The backed-out primer then gets smashed into the outer corners of the primer pocket.
Run a patched rod down the bore to near the throat and fire a primed case (check, double-check, and re-check that it has no powder!). The distance the primer backs out can give you a clue.">>

My SPEER reloading manual #12 isn't much help, in one paragraph is indicates that this isn't a problem & then in another warns that "excessive headspace" is dangerous???

Now my Question: How Much Primer "BACKOUT" is normal (&/or safe) in a Factory Barrel????
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Old April 23, 2000, 02:41 PM   #2
Join Date: April 8, 2000
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Old April 24, 2000, 04:21 PM   #3
Jeff, CA
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Maybe this is a sign of low pressure?

[This message has been edited by Jeff, CA (edited April 24, 2000).]
Old April 24, 2000, 07:15 PM   #4
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Join Date: June 7, 1999
Posts: 3,844
Without regard to caliber or rifle, what you describe sounds like one of two things, maybe three.

1. Overly long chamber, in-so-far as headspace dimension is concerned (distance from bolt face to datum line on cartridge case). In itself, likely not to be dangerous, though it will lead to shortened case life, and could screw up your rifles chamber, in case of head separation. The dimensions of chambers in new rifles are USUALLY within SAAMI specs, though "stuff happens"

2. Shoulder pushed back to far in reloading or loading at factory, thereby creating an "excess length condition". Factory ammunition is usually dimensionally correct, handloads or reloads are another thing entoirely, depends on who did the reloading.

3. Chamber pressure to low, not enough powder and or the wrong powder, to slow burning, though come to think about it, this would more likely create what looks like a "flattened" primer. This is usually a sign of high pressure, though you could see it with low pressure too. Not likely dangerous, unless you end up with a bullet stuck, unnoticed, in the barrel, and fire another round.

What you describe as "primer backout", so far as I know, is NOT normal.

[This message has been edited by alan (edited April 24, 2000).]
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