Not sure how big you are, but if you can comfortably shoot a Super X Model 1 they are one of, if not the finest gas operated semi auto shotgun ever produced. But it's heavy. I love, love, love mine. I just can't seem to miss with it. Not sure how hard/easy they are to find as they stopped making them in the mid 80's.
If still looking at pump guns, look at the mossy 590a1 if you're concerned about durability, They will survive a nuclear armageddon.
Depends on what kind of gun you are building. Some are so easy that a 5th grader could do it (AR's) but others are much more difficult. How the barrel attaches to to the receiver is probably the biggest issue you'll face. You should probably get some headspace gauges and figure out what you're doing before you build one and have it blow up in your hands.
Not really concerned about increased accuracy (math is math, for the most part) but rather features and a variety of output. But thanks, I'll check it out on my test android VM.
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