The Firing Line Forums - View Single Post

Grump · September 20, 2013, 10:24 AM

Different type of match than usual for HP Rifle if you're mag loading for the standing position.

In general, listen to what Bart B posts.

There's a difference between a slam fire and the various types of doubles that can happen. From the original post + additional information I would remotely diagnose (all dangers and disclaimers included!) just a bump-fire. You can possibly eliminate this with various semiautos by including holding your trigger finger back somewhat firmly as part of your follow-through. OTOH, the trigger group should probably be looked at for adequate engagement. A trigger job that leaves inadequate "overlap" between safety sear let-off and the primary sear catching the other hammer hooks can occasionally make those triggers act more like 8 ounces than 4.5 lbs.

Yeah, I used to size my semiauto brass to about midway between the Wilson min and max on those gages...until I discovered that as-fired, my cases were well above the max *ammo* headspace spec. The gunsmith "splitting the difference" between SAAMI and military headspace because of the 100% certainty of using DCM-issued USGI Match ammo at Leg Matches was why, but sizing that far for those chambers led to case life of only 4-5 reloads.

Completely unrelated to the doubling issue.

Excessive head clearance is also why your primers are showing a bit of mushrooming. Non-crimped primers back out on ignition, and then for full-power loads the case eventually slides back in the chamber and resumes contact with the boltface. The primer has time to expand radially before, essentially, being re-seated. This leads to false-positive "pressure signs", sometimes of alarming appearance. Lighter loads, especially with dirty chamber and/or not-polished brass, can leave high fired primers that weren't high when the rounds were loaded. Measure their protrusion and you can get an idea of how far back to unscrew your sizing die to give your ammo something more like .004 clearance instead of what you have now.

I predict that in YOUR chamber, min-headspace ammo has probably .010 head clearance.

For your continued education, there are two types of slam-fires: locked-bolt and "out of battery", which from the evidence I've seen is almost always a half-locked bolt. The second is the dangerous rifle-destroying type. There is a *belief* arising from the incidence of OOB slam-fires with Service Rifles and even at DCM and NRA HP matches with USGI Match ammo, that the minutely-longer ammo combined with a tight "match" chamber in those rifles with aftermarket or replacement barrels makes an OOB event more likely. I'm not convinced of that.

My belief now is that OOB firings with USGI ammo have been most likely caused by firing pin bounce in rifles with either worn firing pin tails, receiver bridges, or both, which allows the FP to contact the primers early in the bolt closing rotation rather than only the last little bit. Wear in this critical FP-retraction function (not only on bolt opening but especially as the bolt stops forward motion just before rotating closed) can even let a hammer jar-off fire the round, despite the hammer's own bolt-rotating camming surface.

There remains some suspicion that OOB firings with a worn parts situation* can even result in a round firing from only the firing pin bounce in the feeding cycle. This is partially because otherwise unexplained grenade (term from Wolf Publishing's writers) events were far more common with Garands than with M1As.

Sadly, the only PROVEN way to make an OOB slam-fire on demand is to load a round with spherical powder AND combine that with a primer seated high enough for the bolt face to light it up on bolt closing--IF enough powder has moved under the primer to hold it back for that impact to make the primer act as if it were at the bottom of the primer pocket. This is why anyone loading for a semiauto should NEVER, EVER use a case which accepts primer seating with less than the usual resistance. I have personally seen and inspected ammo that looked just fine, but the primer would back out dangerously when the case head was tilted 30° and tapped against a tabletop. Believe me, the feeding cycle when the bolt strips the round out is more vigorous than that.

Anyway, be careful and scrap out your brass when it gets those internal rings. My bench has a couple of dedicated spring-steel tools for such inspections.

Be safe.

*And especially with the unknown number of aftermarket receivers with receiver bridges placed or machined too close to the chamber--I had one with it actually too far back, which pulled the tail of the FP too far on opening and again on feeding--broke the FP after only 3,000 rounds or less--AND the whole bolt path was almost 1/8-inch too low to the magazine, preventing a full mag from fitting under a closed bolt.

September 20, 2013, 10:24 AM	#9
Grump Member Join Date: October 22, 2008 Posts: 73	Different type of match than usual for HP Rifle if you're mag loading for the standing position. In general, listen to what Bart B posts. There's a difference between a slam fire and the various types of doubles that can happen. From the original post + additional information I would remotely diagnose (all dangers and disclaimers included!) just a bump-fire. You can possibly eliminate this with various semiautos by including holding your trigger finger back somewhat firmly as part of your follow-through. OTOH, the trigger group should probably be looked at for adequate engagement. A trigger job that leaves inadequate "overlap" between safety sear let-off and the primary sear catching the other hammer hooks can occasionally make those triggers act more like 8 ounces than 4.5 lbs. Yeah, I used to size my semiauto brass to about midway between the Wilson min and max on those gages...until I discovered that as-fired, my cases were well above the max ammo headspace spec. The gunsmith "splitting the difference" between SAAMI and military headspace because of the 100% certainty of using DCM-issued USGI Match ammo at Leg Matches was why, but sizing that far for those chambers led to case life of only 4-5 reloads. Completely unrelated to the doubling issue. Excessive head clearance is also why your primers are showing a bit of mushrooming. Non-crimped primers back out on ignition, and then for full-power loads the case eventually slides back in the chamber and resumes contact with the boltface. The primer has time to expand radially before, essentially, being re-seated. This leads to false-positive "pressure signs", sometimes of alarming appearance. Lighter loads, especially with dirty chamber and/or not-polished brass, can leave high fired primers that weren't high when the rounds were loaded. Measure their protrusion and you can get an idea of how far back to unscrew your sizing die to give your ammo something more like .004 clearance instead of what you have now. I predict that in YOUR chamber, min-headspace ammo has probably .010 head clearance. For your continued education, there are two types of slam-fires: locked-bolt and "out of battery", which from the evidence I've seen is almost always a half-locked bolt. The second is the dangerous rifle-destroying type. There is a belief arising from the incidence of OOB slam-fires with Service Rifles and even at DCM and NRA HP matches with USGI Match ammo, that the minutely-longer ammo combined with a tight "match" chamber in those rifles with aftermarket or replacement barrels makes an OOB event more likely. I'm not convinced of that. My belief now is that OOB firings with USGI ammo have been most likely caused by firing pin bounce in rifles with either worn firing pin tails, receiver bridges, or both, which allows the FP to contact the primers early in the bolt closing rotation rather than only the last little bit. Wear in this critical FP-retraction function (not only on bolt opening but especially as the bolt stops forward motion just before rotating closed) can even let a hammer jar-off fire the round, despite the hammer's own bolt-rotating camming surface. There remains some suspicion that OOB firings with a worn parts situation* can even result in a round firing from only the firing pin bounce in the feeding cycle. This is partially because otherwise unexplained grenade (term from Wolf Publishing's writers) events were far more common with Garands than with M1As. Sadly, the only PROVEN way to make an OOB slam-fire on demand is to load a round with spherical powder AND combine that with a primer seated high enough for the bolt face to light it up on bolt closing--IF enough powder has moved under the primer to hold it back for that impact to make the primer act as if it were at the bottom of the primer pocket. This is why anyone loading for a semiauto should NEVER, EVER use a case which accepts primer seating with less than the usual resistance. I have personally seen and inspected ammo that looked just fine, but the primer would back out dangerously when the case head was tilted 30° and tapped against a tabletop. Believe me, the feeding cycle when the bolt strips the round out is more vigorous than that. Anyway, be careful and scrap out your brass when it gets those internal rings. My bench has a couple of dedicated spring-steel tools for such inspections. Be safe. *And especially with the unknown number of aftermarket receivers with receiver bridges placed or machined too close to the chamber--I had one with it actually too far back, which pulled the tail of the FP too far on opening and again on feeding--broke the FP after only 3,000 rounds or less--AND the whole bolt path was almost 1/8-inch too low to the magazine, preventing a full mag from fitting under a closed bolt.