[Kos-Mos]

For your use, I would go with a Crossman C11 Airmag.

It runs on CO2, cost around 100$ most of the time (on sale from time to time) at Canadian Tire and is accurate and reliable enough to field if you ever want to play outside. Keep in mind that this shoots hot (close to 400 fps). But it you want to do target shooting, you need to have more power to keep a decent accuracy at close range.

DO NOT BUY BBS AT CAN TIRE/LARGE SURFACE STORE. The only acceptable options from my point of view are Madbull's IPSC grade BBs and BBBastard's. It will keep accuracy good and not damage the insides of the gun, especially since you cannot find replacement parts for this cheap gun.

As for your trap, the best I have seen it a 2' tall 1' square wooden box, with sand in the bottom (say 2" thick). The top is angled about 45deg and the shooter fires at an opening on the side near the top. There are no bounce backs, and you can attach a shooting target in the opening. You can also add a sheet metal plating inside the top (air duct end cap, 5$ at hardware stores). It will protect the wood and give an audible feedback. Plus you could also use the box to shoot pellets. The one I saw had a drawer in the bottom to clean/change the sand once in a while.