Cool. I have a work truck with a big tool bed, crane and all. Guy that owned it had it set up for a scuba tank to air tires up, etc. I was supposed to get them with but did not... He used all electric impacts, drills, etc., so no need in an air compressor.
That is a Caldwell Lead Sled. It has a bag full of lead shot... 25 pounds or so, that goes in the front for the kickers. I use it to sight on all my rifles. You can put a .300 mag in it and it doesn't hurt.
Funny though, the brake barrel pellet rifles kick forwards (that is why you have to get special scopes, BC they will kill a regular rifle scope), so you have to place the butt and inch or so from the rear rest, other wise you'll never get it sighted.
I have one of these to bore sight. End of the barrel has to be ferrous though:
Professional Laser Bore Sighter | Green Laser
The end of my pellet rifles aren't ferrous, go figure. Neither is my custom suppressed .22. So I just set up a box with a target on it at about 20', and work out from there.
I love the Lead Sled though! You can tell your what your rifle will really group. My billet AR is sub MOA at 200 yds. And that is cheap ammo, not precision.
Rob, any question you have, I'll do my best to answer.