The thread on headspace gauges prompted me to relate my experience with an M1 Garand I purchased from the DCM (predecessor to the CMP) back in 1980. Since I'd read that headspace is checked on these rifles prior to release, I didn't bother to check it myself. For the next 13 years I used that rifle (accurized) for NRA Hi-power match shooting, putting about 3,000 rounds through it in that time. practically all of this ammunition was a handload consisting of 46 grains of IMR 4895 with a 168-gr. sierra or Hornady match bullet in Gov't match cases.

A couple of years ago I went through a process of checking headspace on all my rifles, and to my surprise I found that the bolt of that M1 would close not only on a No-Go (.004" over minimum) but also on a Field (.010" over min.). In spite of that, I had never had a problem with a separated case head or unduly short case life (other than from the battering they receive fom the violent cycling). Replacing the bolt brought headspace back within limits. I don't think the load mentioned above is anywhere near hot enough to cause lug or recess setback. Rather, I think Uncle Sam let one slip through and that rifle had long headspace from day one.

I resized cases only enough to ensure reliable chambering, rather than screw the sizing die hard down on the shellholder, which might explain why many of the loads were compatible enough with the chamber to avoid problems. however, every 10 or so loadings I retired the cases I'd been using and introduced new ones. These new cases would have had head-to-cone dimensions based on an in-spec chamber, but even then no problems showed up.

It seems that one of two causes lies behind the lack of problem: Either the extractor does a consistently good job of holdong the case heads back against the bolt face and lets the shoulders fire-form (which seems somewhat unlikely given the slam-bang feeding and a spring-loaded ejector), or it takes more than ten thousandths plus over minimum to get into the problem zone.

The situation sorta raises the question of whether absolute minimum headspace is really as necessary as often claimed. I'd appreciate comments from those with wider experience on this matter.

Bob

My experience is limited to bolt actions.

With bench style guns in 270, 7mm Rem, 300 Win, accuracy "usually" falls off from about .008" headspace, but they are better at around .003". For whatever reason, 375 H&H on a bench gun will shoot real nice at .010" to .013"

The 375 H&H has a similar case capacity in relation to the bore size as a 30/06 so.......

Mike

Bob A---

I've seen factory Remington M-700s with .012 excess headspace that caused no trouble with factory ammo. That doesn't mean they're safe.

Your Garand showed what most M-1s do. At about 5,000 rounds the bolt is worn to excess headspace. You saved it a lot of damage by fireforming longer cases for it and so what you were shooting (after the first round) didn't have excess headspace. They were "30-06+.010" caliber.

Excess headspace causes trouble when short ammo is shot in them. When the short case is shot in a sloppy action it hammers the lugs and recesses...that causes more headspace. More headspace means less support for the case. The cartridge case is the weak link.

Old brass or overworked brass is much harder than fresh cases. Hard brass is brittle brass. If brittle brass turns loose, what Ken Howell calls the "Genie", at the peak of pressure the action WILL shed parts.....sometimes major parts.

Many times the shooter wrecks his shorts at the same instant. Hopefully there's no blood and pieces of eye balls to complicate clean up.

Jack, this may seem like a dumb question but if you shoot ammunition that is short on a action that is barreled for a belted magnum would that be any cause for concern?

-Mike

Alvinmack5--

I'm not sure what you're asking. Let me go through the several miliseconds of firing and maybe answer your question in the process.

When the firing pin hits the primer it seats the case firmly in the chamber before igniting the primer. (That's why fireformed ammo is more accurate)

When the primer fires, the first action is against the case around the flash hole....which forces the case even further forward. (That's why belts were invented for long, tapered cases.)

The primer cup and anvil is blown backwards, away from the case, and stops against the bolt face when it ignites. If it's going fast enough it blows the firing pin backwards against the main spring and the portion of the primer cup not supported by the bolt face keeps going into the firing pin hole. (Puckered or pierced primer)

As the powder charge ignites it blows the case out, against the chamber walls, starting at the neck and shoulder, and then backwards.

The brass stretches from what it *was*, minus the amount the primer drove it into the chamber.... until it reaches the length of the bolt face to the shoulder.

IE: The headspace is supposed to be 2.000" ----just to pick numbers, but the cartridge case actually measures 1.996. The primer drives the case .005 further into the chamber, and the gun has .010 excess headspace. That means the case stretches by .019" when it fires. ....
THEN the bullet starts down the bore.

If, during this fireforming the case fails, you have a very large amount of melted brass and hot gas loose near your face.

Belted mags hold the case between the case head and the front of the belt for firing. The rest of the case can do whatever it wants to. A 7 Rem. Mag shot in a 7 Weatherby chamber might leak a little gas but the gun or shooter is usually not hurt.

A 270 Winchester in a 7 Rem Mag can get VERY exciting, depending on at what part of the pressure curve the case lets go. Most of the time the stock is broken......... and ruined laundry, at least.

ONE good thing about push-feed actions is that many times a drastically short case (308 in on '06 chamber) will be herded into the chamber without the extractor snapping over the rim and so the gun doesn't fire because the firing pin doesn't reach the primer.

I once examined a push feed M-70 30-06 that had shot a military 7.62 Nato. The shooter couldn't get it to go off and surmised the firing pin wasn't reaching so he strung a faucet washer on the cartridge neck and forced the bolt closed.

He lost the gun, scope, and right eye in a flash of fire, smoke, and man-made Hell.

[ 10-18-2002, 01:09: Message edited by: JBelk ]

I should have clarified. I was talking about shooting 30-338 ammunition that was chambered for 308 Norma Mag. Where the difference in length is about .020

Since both cases headspace on the belt, you would not have an "excess headspace" condition. You would have a bullet seated farther from the lands than is normal, and accuracy would probably, though not necessarily, suffer.

The case would simply fireform to the chamber, leaving the neck somewhat short.