Is that from firing out of battery or an issue with the brass? I'm not up on the 350 legend.
OOB ? Is something making the firing pin stick?
My first M1A experience...Delta Peach Bottom a guy was testing his reloads, let the bolt go on a single round in a mag and BOOM. Hot primers he said. Scared the shit out of me not expecting it.
I just happened to see this thread. The only way I can see this happening is if you have a short chamber and excessive headspace. Pretty much of a stretch I agree.
Did you follow up with CMMG?