Another headspace question......


PDA






119er
November 30, 2012, 11:04 PM
I purchased some pull down LC 12 .308 brass and the headspace measurement via Hornady comparator is 1.622". The tag attached to the SAI M1A I purchased says headspace was set to 1.632". (Fired measures 1.637") I am resizing them with some RCBS small base .308 dies and besides resizing the neck, it only barely touches the case near the head.

The stripped bolt closes on the case with no problem whatsoever and the case head has a very slight side to side movement perceptible only by feel. The way I see it, upon chambering, the cartridge will be held to the bolt face by the extractor but with pressure on the case head from the ejector. When the firing pin contacts the primer it will probably cause some forward moment to the case before ignition as there is somewhere around a .010" gap forward of the shoulder.

My questions are:
Is this condition more likely to cause stretching in the body due to the case possibly being shoved forwardbefore ignition and stretching rearward under pressure?

Does this dimension (.010") seem excessive? It seems to me that when I resize the 1.637" fired case I should at push the shoulder back to 1.630ish.

Am I reading too much into this?

If you enjoyed reading about "Another headspace question......" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
918v
November 30, 2012, 11:31 PM
.010" is normal. You may want to keep it at .005" to prolong case life while maintaining reliable chambering.

119er
November 30, 2012, 11:52 PM
Just to clarify, go .005" under the chamber dimension? Because .005" off of the fired case is right back at chamber dimension. I'm am 99% sure you meant the the first case I presented but wanted to be sure.

The rule I seem to have heard was to bump back .003" to .005" for semi-autos from the fired case dimension. If that is bad info I don't want to repeat it or follow it for that matter.

gamestalker
December 1, 2012, 12:09 AM
The standard maximum acceptable head space for most any firearm is .006". However, this is not what we are discussing here, specifically, what will function best in your auto loading action reliability, and not trash your brass in one of two reloadings.

I would agree that .003" should be plenty for your purpose, but tighter would be better in terms of brass life span, if it will function reliabily in your action and magazine. But as for .010", that seems a bit much to me. And since you are using small base dies, the .003" head space should function well for you since small base dies keep the body sized down a little bit more toward the head.

GS

918v
December 1, 2012, 10:37 AM
.005" off chamber dimension.

Factory chambers are set up as loose as .010". The reason I say .005" is due to springback. There is no way to get .005" with every single case. Some will spring back a little more than others. .003" is prolly too close for an auto if you account for fouling.

If you enjoyed reading about "Another headspace question......" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!