Quote Originally Posted by DrkBlue View Post
Supposedly the 5.7 cartridge has roughly 30% less recoil force as the 9x19mm, so the basic math doesn’t change much.
This may be a little counter-intuitive, but let's be certain about two sources of recoil: the impulse the rest of the gun gets from sending a bullet downrange, and the impulse from the bolt slamming the rear of the receiver. The thing to understand there is that 5.7 has a significantly lower primary recoil impulse. It's basically the same as .22 WMR. However, the bolt mass for a blowback action in 5.7 is about 330 g minimum (usually about 600 g in order to improve the longevity of the weapon), which is the same as 9mm Luger. The latter is what I expect KelTec to deliver to your jaw. Unless they add a pneumatic buffer, people are going to be surprised.

Someone in this thread raised a question why P90 isn't beating its shooters, then. If you sum up the moving parts of its action, you get more than 600 g. I don't have a great answer. Perhaps actuating the Hall magazine allows the bolt to slow down, but that's only a possibility. I only shot PS90 once, and it was just as uncomfortable for me as a Subbie. My neck is too long, so I had to crane painfully in order to look in its weird AUG-like optic. The recoil was not entirely insignificant, but the whole thing was so awkward that I don't remember it in particular.