M1 Garand blow up

[DocAV]

Two factors here: Steel case and Unlocking speed.

Steel cases in Rifles designed for Brass ( as was the original .30/06 Garand) don't go very well ( I found this with French Grey steel cases (.30cal) and extraction problems.

But the major cause of this .308 ( for want of a better term) problem is in the Conversion from Normal "Garand" to Short Barrelled "Tanker" (Misnomer) in .308.

The Garand works at a certain Port Pressure ( not to exceed 15,000 PSI); the Longer Garand Barrel and the Progressive 4895 type powder of the 30/06 gives this Optimum operating Port Pressure. The delay in opening and extraction time allows the Brass case to retract from the Chamber walls, and be extracted.

With a conversion to shorter barrel, probably with the Same Gas Port diameter, the Port Pressure is much higher ( given the same or higher chamber pressure of the .308 cartridge)
and so the Case (Brass or steel) is being "extracted" whilst the cases is still "stuck" to the case walls. Hence the Partial Head separation and eventual Destructive Gas escape, splitting the stock. Effectively, the Increased velocity of opening, allowed the Bolt's extractor to over-ride the rim, and lose Bolt head support of the case whilst the Case was still stuck to the chamber walls. With steel cases, this is very Problematical.

Your Flattened Primers are not a sign of "Bad Loads" but of Incorrect (too high) residual pressures due to Short Barrel and early Opening of the action.

The Problem is how the rifle was converted, and using "Non-complying" ammo case material.

If you read all the Books on the Development of both a Full Auto .30 cal Garand, and subsequent development of the M14 rifle, you will see that balancing Gas pressure curves and Bolt opening velocities is not an easy Matter. SA/FA rifles which have "gas regulators" or "Gas expansion cutoff systems" will always work better with Both Brass and steel ammo, if properly adjusted. Those which don't have these Gas variability mechanisms, will fail (Case or Mechanism) if the "Wrong" ammo is used (or a Barrel length is changed, without changing the Gas Port diameter).

any further discussion as to the technicalities of this problem can be done directly, at
info@avballistics.com.au ( personal experience with both .30/06 and .308 shorty Garands, and other "shorties" as well (SKS, FN-FAL, etc) where position of Gas Port is changed, or calibre is changed ( ie, a .308 x 1,6" in a SKS).

Another example is Swedish Ag42B, if the Gas port gets burnt out, the action will tear case rims off Brass cases ( ie, opening cycle too fast for Brass retraction).

Doc AV

Maybe the Cabellas Herter ammo was not at fault after all, except for being Steel, rather than Brass.