I never had but I'd think the mag would be the problem. Also, I don't know if the BAR has the same lever on the side of it that my 12 ga. has, but you can lock out the next shot. here's pics of what I have. The 1st one is in the normal shot position and the 2nd position is where I can lock out...
Here's one for cheap from Harbor Freight. A gunsmith will charge you double that at least to do it. When you're done sell it at half price on here. I'll bet someone will buy it.
https://www.harborfreight.com/6-in-buffer-61557.html
Brownell's would probably have that. This member has this also for sale. Looks similar to yours.
https://www.theoutdoorstrader.com/threads/fs-remington-700-factory-trigger-reduced.3003691/
You can't just give it to a machine shop or a guy with a Bridgeport, as simple as this job is. Well, I guess you could if you can find someone that wants to open himself up for the liability. You do need to have an FFL to do any machine work on a gun...
Try tapping the bolt head with the ball end of a ball peen hammer to get the metal where the socket is to tighten up. Then get a soldering iron and heat up the bolt as best you could and screw it out. Good luck.