My friend and his father always used to do that sort of thing. They altered weapons in a whole host of ways, usually to make weapons fully automatic, but sometimes for other purposes (sawing off the barrel of the shotgun being the most crude). At the same time, they were the most respectful people about guns that I knew (fireworks were a different story - I will still never forget jumping over the hood of their car to get some cover when the bag in the driveway was hit).

While a few of the shooters in recent years might have profited from a knowledge of gun repair and alteration, making it more difficult to just buy a semi-automatic or automatic weapon in a store would still help. It's like putting a lock on your door - it's not going to stop someone who knows how to pick it, but it will keep the "honest person honest".