Btw: You never said it was going to be gas, and, goooood luck making a hicap GBB mag.
Anyone have any idea on how to build custom guns? I've gotten somewhat bored of airsoft, and building a gun would be a very fun way to get back into it. I want to make a semi-auto "rifle" with a large clip. I want it to have the accuracy of a sniper (more or less, I really just want it to shoot far without curving too badly), the clip of an assault rifle (At least 200 bb's), and powered by some form of gas.

LOL YOU'RE GOING TO MAKE A GAS GUN? YEAH RIGHT LOL.
Good luck with that, it's not going to happen.
A semi-automatic gas rifle with a 200rnd mag. lol
You don't know much about airsoft, do you?
I'm going to draw out a diagram because I don't feel like explaining my whole idea word by word:

Blue = A basic pump shotgun I will be using as a starting point.
Red = The 200 round extended magazine that came with the shotgun.
Yellow = A longer barrel for more accuracy.
Green = A scope mounted on the rail built into the shotgun.
Purple = A CO2 tank (Normally used for paintball) that I could use to power the gun.
I'd take a basic pump shotgun (Metal so I can modify it easier without braking it) and swap out the barrel with a longer one. I will then use the extended mag that comes with most pump shotguns (200-300 bb's). As for the gas, I'll use the parts from a CO2 pistol such as the Crosman C11:

The C11 has the clips separate from the CO2 cartridge so I don't have to build a clip completely from scratch (Which is possible as my friend has done it). I will use a paintball CO2 tank as both a stock and the main power for the gun. The tank will be connected to the CO2 cartridge valve via a hose. There might also be a second valve so I can limit how much CO2 goes through the hose. Plus, the tank will balence out the gun.
Because the shotgun already has a rail, I can attach a scope to it no problem.