It's a good question, and not easy to answer. Taking 6" off the end will have a huge effect on timing and the gas impulse. Opens the door for short stroking so you may have to open up the gas port however the Bushmaster may already be overgassed. Too many unanswered questions but the bottom line - this could be a nightmare, I'd get the right length barrel in the first place.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek