Blast from the past
This sporting gun is gas operated with a rotating bolt head that engages the shoulders of the barrel...sorta like the M16. And like the M16, the BAR's bolt head is attached to a carrier. Unlike the M16 (which is gas impingement operated as opposed to the gas operation of the BAR), the BAR's carrier rides on two parallel action bars which are attached to an intertia piece found underneath the barrel.
Upon firing, gases enter the gas port, filling the gas cylinder. Pressure drives the gas piston against the intertia piece which begins travelling rearward. This compresses the action spring and causes the action bars to travel rearward along the action bar (for want of a better term) path in the receiver. Because the bolt carrier is attached to the action bars, the bolt carrier also travels rearward and in doing so, cams the bolt, unlocking the bolt's multiple heads from the shoulders of the barrel.
As the bolt travels rearward, the extractor pulls the spent case from the chamber. Pressure acting on the base of the case from the ejector ejects the case from the ejection port. The bolt's rearward travel also causes the hammer to rotate back past the cocked position.
Now, pressure from the compressed action spring forces the inertia piece forward. This causes the action bars to begin their forward movement and with it, the bolt carrier assembly.
As the bolt travels forward, the hammer follows until it is arrested by the sear. Pressure from the magazine spring forces the magazine follower up and pushes a cartridge into the feed lips of the magazine. (Funny, but doesn't the rearward travel of the bolt actually pushes the cartridges into the magazine ever so slightly?) The bolt strips the awaiting cartridge from the magazine. As the cartridge rides up and along the breech face of the bolt, it slides beneath the extractor.
The bolt head then reengages the shoulders and the bolt's camming action locks the action; completing the chambering process and the firing cycle.
It is quite unlike the fixed stock FN-FAL or the M1918 BAR or M-16/AR-15 where there is a recoil spring or buffer in the stock.
That said and done, I guess the commercial BAR can be made into a bull pup hunting rifle.