bum

It's always a good idea to include pictures; help us help you.

I would open it back up (sucks I know) and remove just the spring. Then carefully prime it while it's opened up (make sure parts don't pop out and go flying on ya) andto see if you can figure out if something is not lined up or in the way. Move around any other parts too, blasters can be freakin temperamental.

You could also check how smoothly the plunger head fits inside the new spring. I tried adding some random spring to my Alpha Trooper; it would get all bunched up with the stock spring and was not smooth making priming the blaster suck. Could be similar for you if your spring doesn't fit well.

Doom did some studies based on flight drag. It's kind of interesting... link:

http://btrettel.nerfers.com/

Just to echo what's been said, as long as the velocity is enough to compensate for the increased dart size (and from the studies the dart diameter to length ratio has to be appropriate) then your larger darts should be better.

Another Bandit style Borderlands gun. I call it Irwin

This is amaze-balls; I want one. How much did all the materials cost and did it take a long time to make?