Hey guys, I've had an  S&W MP15 for about ten years now. Bought it as a standard M4gery model and have made a few changes such as stock, grip, trigger guard, trigger, VTAC rail, low-profile gas block, etc. but nothing major. I'd like to put the current 16" 1:9 twist upper back to original configuration for general plinking/target purposes and build a 18" upper with 1:7 or 1:8 twist for hunting. The rifle currently has a standard carbine buffer tube, spring, and buffer. I've noticed that 18" barrels have either mid-length or rifle gas systems. Can I use my existing carbine buffer tube, spring, and buffer with a mid-length or rifle gas system or will I need to change them out?

Thanks for the help.

They will work just fine.

The gas port size changes based on gas system length to essentially make them very similar.

A lot of people like to add mass, and end up with an H2 buffer and blue Sprinco spring on midlength systems, but that might not run with a rifle length gas on an 18" barrel.  The standard carbine buffer and spring absolutely will.

I default to H buffers/Damage Ind enhanced springs on almost all mine now, zero issues.

That being said, as previously stated, the standard carbine configuration will most likely run issue free.