When I was a kid, I had a Remington model 514 which was my Christmas Present at age 9. My brother had a far, far better Remington model 513.
I have no idea why but the model 513 which had dual rear locking lugs and was a sweet shooter, was always quieter than my model 514.
I don’t think it had anything to do with the sound but the model 514 just used the bolt handle for lockup. It was never real accurate and was twice as loud as my brother’s gun.
not sure, aside that loud sonic bullets tend to tumble when transitioning to subsonic.