It depends on the production of the Bushmaster Rifle. The original Bushmasters were made by Rock River Arms. All were in the 20" barrel configuration. Both A-1 and A-4 models. And the new ones are made by DPMS since the Freedom Group (Cerberus) owns Remington, DPMS, Bushmaster, Marlin and several other companies. So if it is one of the new ones any Quad Rail that fits a DPMS 308 to my understanding should fit the Bushmaster 308. You might want to look at a two piece quad rail since you would not have to remove the Barrel to replace the barrel nut and etc. If it is the two piece all you will need to do is re-zero when completed. I would contact Daniels Defence, Fulton Armory (Clint), Yankee Hill or one of those companies for guidance.