Do you guys think something like this would work?
Its a portable car tyre compressor.

http://www.ebay.com.au/itm/12V-Car-Auto ... 19c0c26cd5

I have used these magnets:

http://www.ebay.com.au/itm/350500863888?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

To hold this ammo:

http://www.ebay.com.au/itm/Replacement-Parts-5mm-Dia-80PCS-Bike-Steel-Ball-Bearing-/280759246684?pt=AU_Sport_Cycling_Parts&hash=item415e8e1f5c


The hard part ...

finally my search for a homemade semi is over! very impressive Ant.

I am curious though. Does the valve spring compress against the elbow joint?

Also, is it only the barrels that slides when the barrel is hammering the valve and returning to position? I think I understand. will have to give it a ...