so about.. 2 and a half months ago a buddy and i were shooting the garand at the range. we were shooting reloads that my grampa and i made, he is an experienced reloader and has been doing this since the 60s. i have a 1942 manufacture spiringfield garand. anyway, about 5 rounds in, i pulled the trigger and heard 2 rounds go off. there was only 7 cases on the ground, for a short time we couldnt even find the missing case or any evidence of it. when we did find it, the neck was all ballooned out, and the case looked like a giant .45. so we are assuming what had happened was the round went off as it was being chambered, and we have no idea why. luckily the bullet left the barrel and i only got minor powder burns on my neck..
An experienced reloader and an experienced gas gunner reloader are two different things.
What you had is obviously an out of battery slamfire. That cartridge likely had a sensitive primer and it is highly probable that Grandpa’s reloads were too long or too fat.
The Garand has a free floating firing pin, like this M1 carbine. The M1 Carbine and the M1 Garand have a firing pin retraction cam. This cam pulls the firing pin back during extraction. It also has limited utility as a safety device, but is easily defeated by tight or long rounds. If your round was smaller than the chamber the odds would have been in your favor to have an in battery slamfire.
Only at final cam down is the firing pin retracted. Up to then the firing pin is totally free floating and tapping the heck out of the primer.
This is a M1 Carbine firing pin retraction cam.
This is the M1 Garand firing pin retraction cam they are functionally identical, just the carbine is easier to visually understand.
These are M1 Garand receivers and the firing pin is fully forward and just touching the firing pin retraction cam. As you can see there is only thousand's of an inch of forward movement left in bolt cam down and yet the firing pin is out about 0.064" of the bolt face.
This is most likely where your sensitive primer went off. Before lug engagement.
If the bolt has to stop here to crunch fit a long case or a fat case that firing pin is rebounding off the back of the primer at its highest velocity in its forward travel.
That is why it is important to small base size cases used in these rifles and to set up the dies with a case gage and size to gage minimum. This will reduce the risk of an out of battery slamfire.
I am curious to know if Grandpa used federal primers. Federal primers are the most sensitive primer on the market and the most "slamfiring" primer in Garands.
For these rifles it is safety critical to ream primer pockets to depth, seat the primers by hand, and verify that all of the primers are below the primer pocket. There is a chance that a cocked primer, with the anvil firmly seated on something, will cause a primer initiated slamfire. A high primer can cause a slamfire but only if the anvil is firmly seated. High primers are one of the most common cause of misfires because the primer won't fire unless the anvil is seated and is pushed up into the primer cake. However, given a shallow pocket it is theoretically possible that high primers could have done this.
Just examine the back of the ammunition you have and see if there are high or cocked primers.
It is also safety critical to use the least sensitive primer around because these rifles will slamfire in battery or worse, out of battery, given a sensitive enough primer.
I recommend CCI #34's and Tula7.62 primers as they considered "Mil Spec" primers. Which means they are less sensitive than commercial primers, federal being the most sensitive commercial primer on the market.
It is my considered opinion that Granda Pa's reloads cracked your receiver. It is also my considered opinion that your receiver cannot be repaired.
You are very fortunate that you did not lose an eye, or have wood fragments injure you.
Would you please post clear pictures of the blown case and the primer? The slamfire cases I have seen had clear firing pin indentations in the primer.