1st design advice (scroll to bottom) and noob help

[killshot11]

and for example on that gun, what does this mean : This gun utilises a screw in rubber washer compression fitting to adjust the amount of pressure required to burst a bb through the washer.

what does that mean and where is that located

[jackssmirkingrevenge]

what does this mean : This gun utilises a screw in rubber washer compression fitting to adjust the amount of pressure required to burst a bb through the washer.

I really must be in a Friday mood

This should be enough explanation. Note that the more you compress the rubber washer, the more it protrudes into the barrel and the stronger it hold the projectile, meaning it will need more pressure to pop the projectile and you get a more powerful shot.

[3VIL G3NIUS]

Not to be rude, but I'm kinda surprised that you don't seem to know much about BB guns considering you have built air cannons before. There are quite a few similarities and there is a lot of info in the different threads here.
I would just build the cloud or vortex as already mentioned, they chew through BBs and need quite a bit of air, but are very simple.

[pneumaticcannons]

With the design jsr gave you, you will have to quickly release the trigger to refrain from wasting air. But if you just add something like this
then were talking