A few weeks ago my children proudly announced that they'd made an air
rifle from some plumbing parts and a plastic bottle and a Schrader
valve.

I was annoyed because they were supposed to be studying for their exams,
but then both impressed and a bit horrified to discover that this thing
can fire a triple-A battery about 100m.

"What you really need is some sort of metal tank instead of that bottle"
I pointed out to them, and someone must have heard, because yesterday
evening in the lane behind our house I found a 10-litre expansion tank.

So, a couple of purely theoretical questions - firstly, the tank has a
screw fitting - what could we^they use to attach the trigger -
http://cdn.toolstation.com/images/13...y/stock/webbig
/76890.jpg - to it?

Secondly, the water side of the expansion vessel of course has the
rubber membrane attached to it, which is going to need to be ruptured in
such a way that air pumped into the valve on the air side can exit
rapidly from the other end when required - any suggestions on how to do
that?

Thanks,

Daniele