I'm not familiar with the forend mounting system used on a commercial semi-auto BAR rifle, but it appears that there must be some recess or forend hanger at the rear where it meets the receiver, and it is secured by the sling swivel stud near the front.
Doing a quick online search for Loose BAR Forend gave some results that suggested the problem is likely due to compressed or damaged wood at the rear. One guy said he tightened his up by inserting a shim made out of a piece of wide rubber band between the rear of the wood and the receiver. That might also be a temporary fix for John's friends' rifle. But a proper repair or glass bedding of the damaged or missing wood would naturally be better. It wouldn't surprise me if that looseness wasn't what caused the forend wood to become cracked in the first place. It may affect accuracy too.
I'm surprised Argo44 didn't know an individual could own a full auto BAR... same as any other legal machine gun. People can be seen shooting them in YouTube videos of the large machine gun shoots at Knob Creek, Ky., Big Sandy, Ariz., etc. I'm also surprised he didn't work in some pictures of his Jeep or Rolex watch.
