I used to use CB caps on my trapline, for dispatching trapped critters. They worked quite well, but I eventually switched to air rifles for this task because the ammo was considerably cheaper. I have two RWS air rifles in .22 and .177, and both work well for what I ask them to do. They will dispatch any trapped critter, or take a rabbit or squirrel at better than 30 yards.

I haven't seen those SCUBA air rifles lately, but I haven't been particularly looking for them. A few years ago I met a fellow at the range who was using a custom SCUBA rifle to fire .54 slugs at a 100-yard target. I was impressed, but i don't know how practical it really is. I really can't see myself lugging around a tank and a foot pump while I'm out hunting.

If memory serves me right, Lewis and Clark took an air rifle with them on their expedition. The trouble is, these pneumatic rifles were pretty much a mature technology two hundred years ago. Different materials are used obviously, but their power, accuracy, and weight really hasn't improved.

So, yes, I've seen the cool-looking pneumatic air rifles, but I'll stick with the break-barrel spring air rifles. They're powerful enough for want I want them to do, and there's less things that can go wrong with them.

Onward and upward,
airforce