Well you would certainly know as you stated you’re a Marine.
My question would be, keeping in mind the two calibers and the grains in each cartridge would there be an echo effect perhaps making one shot sound like two?
That I cannot say for certain but I can say the when I pulled butts in Camp Pendleton there was a low cinder block building where the Range SNCO and radio operator stayed and you could hear an echo off the cinder block when the rounds cracked over head. I am assuming you're asking because of claims there was another shooter. People should be concerned about the bullshitte rules of engagement that give the assassin the first shot. Having been to an event where PM John Majors spoke, I can assure you the UK snipers do not give you first bite of the apple. Had a rifle aimed at my head while I had my camera inspected! Once cleared, they lowered the rifle and called the sniper watching me. All I could think was for Gods sake don't sneeze.
For the record, I served two enlistments on active duty as a Marine, one enlistment in the Marine Reserves and spent the remainder of my career with the Navy.