A M-4 profile is going to be more than a pound lighter than a HBAR. The outside profile or length of a barrel have very little to do with accuracy. Barrel lenght effects velocity which effects the lenght of time the bullet is in the air before it hits the target which allows mother nature to act on it longer with gravity and wind drift (run-on sentence on purpose).

You'll need something welded to the end of that 14.5" barrel to make it 16" total unless you're building an SBR and have the permit.

I have a pair of shorties - a pre-ban with a 14.5 inch M-4 barrel and a welded Phantom flash suppressor and the other is a 14.5 inch M-4 barrel with a welded AK-47 brake.

The difference between a 14.5 inch and 16 inch isn't very much - compared to the 20 inch to either of them.