Acra Rail On This Bushmaster Help

I can’t find a picture of the bottom of the hand guard. The sides look like it does have some M-Loc slots. If there is one at the bottom you could buy an M-Loc arca rail and run off that.
 
That handguard isn't MLOK compatible. You can get a JP Enterprises ARCA rail that's rounded and then drill holes into the bottom of the tube to help mount it. You could also get a new free float rail that has the ARCA dovetails machined into it already or you could go with a FF MLOK rail and use a bolt on ARCA rail, for which there are many options.