Looks like the breech screws together. Looks like the bottom of the three parts of the bolt group is backwards to the way it installs. I don't think there's a trigger, it has a catch that flips like a pen flare gun, or a zip gun.
A .50 Barrett weighs 30 lbs. That thing probably only weighs five, after you've installed the buttstock. If you shot it, it would knock you down and stop you for falling.
The barrel is wrapped with reinforcement around the "firing chamber," and the breech screws shut.
Notice the screw-adjustable peep sight. Since this was a rifled gun, it was capable of better precision of fire than the usual smooth bores, and the peep sight helped exploit that superiority.
This was designed by the same guy who did the Whitworth rifle (small arm) the Confederates used as a sharpshooter's (sniper's) rifle. The field piece and the small arm shared the same polygonal rifling, not unlike a modern Glock pistol.
Calling artillery with a rifled bore "a rifle" was common at the time, such as the much more common Parrott rifle (designed by Robert Parker Parrott).
The D-I-Y .50 BMG has two layers of reinforcement around the firing chamber but considering what the screw-on breech does to rate of fire, I'm not sure why he thought it needed "cooling slits," but those would seem to me to have created points for stress to concentrate on.