I'm working on a modified bayonet that will have 3 weaver bases built onto it. I thought about mounting a laser sight to one of the bases and a tactical light on the other. I tried to order a laser sight with a wireless pressure switch, but Natchez was out of stock.
I do like the idea of the laser mounted in the cap though. Maybe all you need is a sleeve thats about the diameter of a 12 gauge shell. Just stick the laser sight in the sleeve and stick the whole assembly into the tube after the spring. Spin the cap on to lock the whole thing in place.
Of course a laser sight on a firearm that deploys multiple projectiles would be kind of a waste. (Unless you plan on mounting 9 to 100 laser sights!) Of course it would probably come in useful for shooting slugs, but I don't think the endcap setup would hold its zero too well.
"Ray guns don't vaporize Zorbonians, Zorbonians vaporize Zorbonians" Excerpt from The Far Side