I found this gun on another website, not sure if it would be breaking the rules posting the like due to the nature of the website, but if you want the URL pm me
This is a Pre-Charged Pneumatic that I built about 3 years ago called the BazookAir.
The 1.5" titanium tube holds 550cc's of 4,000 PSI compressed air. A small tube goes through the 10 oz striker to connect the air in the foregrip with the air held be in the stock.
The Delrin valve is fed by a relatively small pre-chamber that, combined with the large air volume, keeps the shot string much more consistent than most big-bore PCP's.
The trigger pulls a bar that rotates the sear. This bar can be lengthened for use in a bull-pup.
The striker is pulled back to cock before each shot.
This is the only safety. It can be un-cocked if a shot is not taken.
The breech holds 3 rounds stacked in-line with the air entering just behind the front bullet. After it is fired the spring-loaded bolt handle is pulled out about .100 and pushed forward about 1" to move bullet #2 into position.
I got this idea from the Benjamin 422 of the 1950's.
The length requires the use of bullets that are 200g to 250g so I built the valve/striker/transfer port for consistency rather than the higher FPE that a 400g-500g bullet can make.