A picture is worth a thousand words and an example is likewise valuable. What really helps is having an actual 450 Bushmaster factory mag to compare your donor mag to. With a live model, it's not hard to experiment with your tweaking technique to form the lips of the donor mag to match the factory one, or even improve upon it. I ordered a couple of AR Stoner 20 round straight mags that are on sale today. They are my mag of choice since I'm not using my unit in a war zone. When they arrive, I will try to make an image rich, how-to article on tweaking your mag lips to reliably feed, but the first step for anyone considering making their own mags is to try your donor mag as it comes and see if it works un-tweaked. Some do. Some do, but unreliably and some just plain don't, without tweaking the lips, as MarkCo has already reported. My experience with the reasonably priced SS AR Stoner mags has been that their lips are too wide and some narrowing is in order. The half-moon cut at the front is assumed to be a necessity since the factory mag has it.
Hoot