I am guessing that that is about the limit to the blowback method. I gather that Hi Point specializes in these types of guns and that the penalty for simplicity is weight.
Oh I'm sure there's a point of diminishing returns with weight you could push past to make a pure blow back 50BMG.I'm just not sure as to the safety of converting a slide/barrel/chamber made for 40 S&W or even 45ACP(in the slide's case) to something with that much more pressure.
I think I remember reading that you would need a two or three pound bolt to make a .30-06 blowback. In a way the old naval fortress disappearing cannon were blow back guns- they just took the whole barrel with them. And of course people had to reloud them before they released the spring. Those most have been fun herky-jerky guns to be around.
I'm not sure if the disappearing carriages count because they weren't cycling the weapon just moving it below cover.