Head Space Brain Work

floydthecat wrote: See where I am headed with this?

floydthecat wrote: I don’t think a carbine is going to explode just because a field gauge says it’s bad. Pretty sure a lot of barrels and bolts have been replaced, when there was no need. I think if one watches his brass, it will keep shooting as long as the firing pin can punch the primer.

Excessive head space will not necessarily lead to light primer strikes. The firing pin travels much farther than any potential excessive head space and can definitely reach the primer and discharge the round. What happens with excessive head space is the case will stretch to fit the chamber and possibly fail near the unsupported area near the rim. This will allow hot gasses at high pressure to blow back into the receiver possibly injuring the shooter. I guess someone has determined that .020" is the maximum amount of stretch that is safe before the case is likely to fail on a new case, thus the "field" gauge specification. This is particularly important for reloads as they become inherently weaker each time they're trimmed and reloaded.   

If the bolt is in battery, the firing pin can travel up to .060" beyond the bolt face regardless of whether or not the extractor snaps over the rim. The cartridge can only be "ahead" of the bolt face by the amount of the actual head space which, if .020", leaves up to .040" of firing pin travel past the cartridge base. This is plenty to discharge the primer. If your extractor has not snapped over the rim, I suspect your bolt was out-of-battery resulting in the safety feature of the receiver firing pin cam preventing sufficient movement of the firing pin to reach the primer thus failing to discharge the cartridge.       

I found this for carbine chamber specs from sami.

floydthecat wrote: Good stuff guys. The conversation is developing just as I hoped. My understanding is that head space gauges are engineered on the short side, conservative that is. SAAMI specs. usually include a chamber drawing and knowing what the max chamber dimensions are would be useful. The current SAAMI specs. don’t list the chamber dimensions for 7.62 x 33, at least I do not see any. Maybe they are in a publication that I have not seen? I have seen the aftermath of a ruptured case and it wasn’t pretty. Luckily, just blew out a magazine and burned the shooters hand. I was able to repair the gun and upon interrogation, the shooter admitted to firing some gun show reloads. Only the good lord and the reloader knows how they were loaded. I certainly do not want to be guilty of encouraging anyone to shoot a dangerous weapon. I just think in reality it’s not completely necessary to junk a carbine action based on head space gauges alone, especially for the casual shooter. I think it’s reasonable to expect you can take one a couple thousandths past 1.300 and remain completely safe. There is only .005 between a 1.295 no-go and a 1.300 field reject gauge and most carbines measure closer to 1.297 than a no-go of 1.295. Taking the max out to even 1.302 or 1.303 is another half-life left in the gun. Just watch your fired brass, use good quality ammo and avoid short-cased ammo. I am not a fan of having to shop ammo to find something that works, but I have never measured a commercial 30 carbine round off the shelf of less than 1.284 in length, but I do check it all to make sure I get no long or short cases. I think if the action locks-up properly and one maintains the .020-or-less space allowance between the cartridge base and the bolt-face and uses good quality ammo, he can safely shoot-on a bit longer. Defer that $400 (or whatever) barrel change and purchase more ammo.

Don, just curious, how did your excessive head space end up being .080"? Sounds like a lot for even a worn out barrel.

jackp1028 wrote: Don, just curious, how did your excessive head space end up being .080"? Sounds like a lot for even a worn out barrel.

Some semi-auto pistols head space of the extractor. Early Colt 38-Super enthusiast get real puffed and refuse to use that term, but Colt did it up into the middle 80’s I think, finally manufacturing barrels to head space on the case mouth.

03manV wrote: [QUOTE Floyd, I agree with your few thousandths not making any difference. If 1.295 is "optimum", giving 0.005" headspace on the go gage; then a minimum cartridge at 1.284" is then 0.011" in headspace, already 0.001" over the SAAMI target of 0.010"!