Hornady Match ammo

[browningautorifle]

A round with more powder?

Sounds like a higher pressure to me. You'll never know. Without proper pressure testing you can't tell. Take it what it was, an anomaly and just move on. If you get more then change ammo and move on. Some ammo shouldn't be used in M1 rifles though because the pressures can be hard on the op rod and gas affected parts... You can also get a gas bleed off gas cylinder lock screw relief valve and then set up so it just cycles this rifle with this ammo and be gentle on your gass affected parts. It allows all the excess gasses out the front of the valve at the front.