There are lots of manufacturers of M-16/AR-15 “pattern” guns, but only one that currently makes the M-4 (Colt) and one that makes the M-16 (FN). You can’t just take any AR-15 and call it “MILSPEC”.
Getting a government contract can make or break a company.
If you have the tooling, the process, the skill and the CAPITAL to front such a large operation, you could make a ton of cash.
If you’re a little guy, you may find that the “fine details” of design, quality control, warrenty, delivery and service will drag you to the brink of bankruptcy and beyond.
There are enough changes between the M16 and M4 to warrant the latter being deemed a separate product.