I tried this system before on my " push botton valve" experiment.It needed more flow..
Take a barrel, shove in a rubber " sabot" and put a reducer on the muzzle.
The sabot can now push an arrow, but it cannot exit the barrel.A drilled out center point helps to keep the arrow aligned and steady.
This causes the gas to be trapped.
Of course..I made it so that not doesn't seal 100% ..the air bleeds out slowly..resulting in a silent shot.
An arrow gives better penetration while the sabot gives it more force..

Unfortunately, threading a reducer into the muzzle made the barrel sleeve turn over it's foam tape insides and droop even more, but this is just a prototype,
I will replace it by a shiny new barrel all together
*( " I will replace...")*

I will also drill ports in front of the sabot to enable it to push out the air in front faster.Next try the system will be removable, so I can still fire marbles and 16 mm ball bearings if I choose to.
The arrow ( or airrow..) got launched in complete silence, apart from the impact, wich was not too loud and most importantly, doesn't even sound like a shot.
Damage was done at under 300 psi.The gun could take a lot more..
The target is 4cm thick quality hardwood, shot at a 90 degree angle to the fibers. ( hardest to do)
So much for reusable arrows..I can't get it out...