Small powder charge in a big case. The powder may have been far enough forward of the primer/flash that it didn't get a good ignition start (small amount of powder initially lit.
Seems I remember reading, years ago that the guys who run lead bullets and pistol/shotgun powders in their .30 caliber rifles use a "filler" on top of the powder to insure the powder stays on the primer end of the large, mostly empty case. The filler was usually some kind of fiber material that would burn or mostly burn up on ignition of the powder charge. Never tried it myself.
That's also one reason I used to think those compressed IMR4350 loads in .30-06 and .308 were so accurate. The powder was always pushed right up against the flash hole, there were no non uniform voids in the case that might affect burn rates. That old Rem. 03A4 would bust 3 out of 5 one gallon milk jugs of water at 500 yds. with that IMR4350 and the Sierra 180 grain Matchkings. No chance, with that powder and the amount in the case that it was a high pressure load either. Just uniform.
Do not use IMR4350 in your Garand when you get it. Too slow of a burn rate. Chamber pressures are fine. Gas port pressure is way too high. An adjustable gas plug properly adjusted for the load would work fine to keep gas port pressures in the design range for the M1 Garand. I've got a twice busted M1A op rod to show you how hard that powder/bullet are on M1A rifles.
