[Dire_Wolf]

I don't think chopping BBs is a mag thing... that doesn't make sense. If you had just installed a new tightbore inner barrel, that would be the cause of it. As I recall, the tightbores cause the hopup to grip the BB more tightly, and the BB may not go into the barrel as easily, causing the bolt motion to chop the BB. Also, If you put the stock nozzle back on, instead of the NPAS nozzle that I assume you have installed, the chopping should go away. Fix is to use an emery cloth or fine file and work the end of the nozzle down slightly, to make the diameter smaller to fit into the hop and barrel assembly easier.

Stope, I'm not sure what you're asking, regarding the semi-only. There's a mod to the selector rod that will do that, but it's irreversable. Essentially you file off the nub that moves the disconnector out of the way for full-auto. Remove the full-auto sear as well, and you'll get the same operation in semi and auto.

Accuracy upgrades are similar to any airsoft gun. Tightbores and a properly dialed hopup will help. Gas guns, because of the inherent cooldown, will have consistency issues from shot to shot unless you wait for the mag to return to ambient temp. CO2 mags can help this a bit. Because the shots don't have perfectly consistent velocity based on the cooldown and slightly variable gas pressure, hopup effect will also be slightly variable.