rego

The bigger the Ah the better. Go with whatever is the biggest that fits in the battery tray and you can afford. Anything over 1 Ah should be plenty, especially being Lipo chemistry. It will easily provide the necessary current to fire darts.

I can second the Hakko FX888D. The thing has amazingly positive reviews and only costs $100 dollars. However, if you are only looking for something for occasional use, not regular hobby use, go with a moderately priced pencil iron. They're not perfect, but they'll work fine and dandy if you only need to use it once every six months. Just be sure to only use the good solder, not any of that lead free crap.

If I remember correctly, I recall reading a thread on spudfiles that simply capping off the return port of the cylinder with a plug worked perfectly. I would recommend to try out that route first, being the smallest, cheapest, and easiest. Otherwise the next easiest would be to use normal extension springs between the frame of the blaster and ram (the moving part). 

 

I would also recommend reading through all of captainslug's abp5k and arr threads. While most of it is not particularly useful, the design of the arr seems like it would help with component placement. He placed the qev in front of the breech and entered the barrel at 90 degrees, which keeps deadspace limited. The bolt is solid and rams darts in front of the qev port in the barrel.

 

This can also be seen in pvcarsenal17's RR Assisted Bolt action. Almost no information about it, but spudfiles has the thread with pictures and short descriptions. It seems similar to this, with almost entirely off the shelf parts

I believe the issue is the barrel. Buzz Bee darts are a bitch about barrels. They have to be just perfect. The dart peg can in some cases be essential to the firing of the dart. I've had a few blasters myself that turn to shit after removing barrels. That would be the first thing to check.

Unfortunately, if the problem doesn't lie with the barrel, then you may have disrupted the valve. I cut a rangemaster tank open before, and I really don't see what you could screw up, as long as none of the plastic is stuck in the tank. However, keep in mind that the rangemaster has an internal hammer setup that rapidly depresses the firing pin. If you are testing the tank outside of the gun, that may also be the issue.

I can definitely agree with roboman. Cad is great, but for most pvc blasters, its easier to just make them. I only cad blasters that are plusbow style, with polycarbonate and such. You can never beat just sketching out most blasters on graph paper. I usually sketch up most blasters I cad on paper beforehand anyways.

Unfortunately, I only own a copy of Solidworks 2004, which is way past outdated at this point.