My Line SI. It's not pretty, but it solves some problems for someone on a low budget, like myself.
Instead of buying a stock and ASA, I put a tank buttstock I had on a 13 ci tank. The raised sight rail I already owned and the sight I picked up for almost nothing on eBay. The setup works perfectly with a mask, getting around the usual problem of mask-bumping while sighting down the barrel with a tank coming out the back.
For the feed tube, I took some old copper pipe that was lying around, cut, sanded, primed and painted it, and then took one of my wife's old hair bands to make a feedgate (that tape is the stick-to-itself army tape, I find it's stronger and lasts longer). I had to cut relief slits in the copper adapter on the other side to make it work as a feedneck. Then, I clamped it with a hose clamp.
While not gorgeous, it does look fairly balanced and it shoots darts. People at the local field seem drawn to it in a relic-that-can-do-damage sort of way. I'm extremely happy with it. Got some ideas from previous posts here, thanks much for the thread/sticky.