Developing new firearms is surprisingly difficult and expensive. If there is a large, free, and robust civilian market for them, there is a significant place to

Developing new firearms is surprisingly difficult and expensive. If there is a large, free, and robust civilian market for them, there is a significant place to try out new designs, tweak them, and generally let the public pay for & field test new designs. With the 1934 & 1986 machinegun bans in place, the only customers are national militaries, which tend to want _one_ design, want it to last for decades, and while willing to pay a lot for R&D are incapable of the orders-of-magnitude more $$$ and design cycles that the civilian market is capable of. As a consequence, there are effectively no new designs available, meaning US national security (as far as small arms go) is stuck with a 50-year-old design improved only by minor tweaks (and most of those, to make the point, are the result of the civilian market for the closely related AR15 rifle). The US military keeps trying to improve over the M16/M4, but can't because there's nothing out there so deeply tested & known, and nothing with a comparably large infrastructure supporting it.