I have a M&P 15 that im changing the barrel on, im prob gonna buy a BCM barrel but use the M&P bolt has anyone had experince changing out barrels on the M&P with using the old S&W bolt. My bolt seems to be MPI so headspace should be ok but just wanted some opinions thanks,

Buy the headspace gauges and be sure. Blown cartridges are no fun.

I always have my barrel guy fit a new bolt with each barrel. No worries, no problems. Nice tight headspace.

Why are you changing your barrel?

I am changing my barrel because ig going to a mid length gas system, i just bought a supressor, the M&Ps from whatvive read and heard are already overgassed i want to try and eliminate as much blow back as possible not to mention I have a ML gas system my other AR and love it

I'm not trying to argue or get you to change your mind but what if you built a new upper with that barrel, complete with it's own bcg? I must admit I'm biased against taking off existing barrels. It has always seemed so drastic to me when you can just change the upper out. But whatever works for you.

OK, rule of thumb: you can get away with using a new bolt on a used barrel, but there is no guarantee that you can use an old bolt on a new barrel. Get the headspace gauges.