Not sure which ACM M500 you have... if it's the BE clone of the Marushin gas M500 SSB (3x8mm/5x6mm) the magazine tube is held by a ring which screw into the underside of the barrel (which also holds the locking mechanism for the BB loading tube): I'm guessing those are the screws you're talking about? Otherwise you've got some other ACM shotgun.
What's wrong with the screws anyway? They're stripped? Or the holes are stripped?
The clamp you're looking for is a magazine extension tube clamp. Something like this:
http://www.midwayusa.com/eproductpag...eitemid=643064
You can find it easily at any local sporting goods store that has firearms accessories: Mossberg 500/590s are very common hunting shotguns, probably second only to Rem 870s.
Coachster's suggestion about loctiting them is probably worth looking into as well.