P = pressure.

V = volume.

So...

V1 = 11 gal = 2541 ci

V2 = 270 ci

P1 = 125+15 = 140 psia (note that I'm rounding 1 atm to 15 psi)

P2 = 15 psia

So...

P3 = (140*2541 + 15*270) / (2541+270) = 128 psia = 113 psig.

So your first shot will be at 113 psi.

For your second shot....

P1 = 128 psia

P2 = 15 psia

P3 = (128*2541 + 15*270) / (2541+270) = 117 psia = 102 psig.

Looks like you're going to get 2 shots out of that tank before the pressure falls below 100 psi (3rd shot is going to be on the order of 90 psi though, so not tooooo bad).

Seriously though, for what you're trying to do you're going about this all wrong. Ball valves? Pre-filled tanks? You need to do your homework. May I suggest you start

HERE.