graylion said:

what rate of fire? 800 rpm? Slower? Faster?

Yes, definitely one of those.   ;^)

stancrist said:

graylion said: what rate of fire? 800 rpm? Slower? Faster? Yes, definitely one of those.   ;^)

766 it is so ;o)

graylion said:

looks like a giant pistol Magazine is P90 style and on top telescoping shoulder stock Boberg "bullpup" operating mechanism closed bolt (open bolt is not really feasible with Boberg operation) roller delayed blowback red dot sight skeleton holster or sling (I am trying to imagine the commentary if somebody chooses to carry it in an appendix carry) 4 way selector for burst mode

The Boberg mechanism increases the barrel length by a cartridge length, so we are looking at a 11" barrel length

And I just had another idea: metal stamped gun or plastic? Same for mags? Metal mags have thinner walls I believe?

Plastics are the trend nowadays. Even for mags.

Metals are thinner but it has some problems like it could be easily dent and will have some reliability issues.

So next is the cartridge. I am thinking a polymer cartridge, running @ around 500 MPa - ca 72kPsI. The bullet I am thinking a spitzer. tungsten core with aluminium body, and a polymer film in between for easier separation. Bear with me a bit while I work out which calibre would appear to work best

graylion said:

The bullet I am thinking a spitzer. tungsten core with aluminium body, and a polymer film in between for easier separation.

If "body" refers to the jacket, I doubt that aluminum is a viable option.

And wouldn't polymer film violate international law requiring bullet components be visible on x-rays?

no, not the jacket. Imagine a pointy cylindrical tungsten penetrator with a diameter of ca 3.5mm and going the length of the projectile. Outside of that I want aluminium, keeping the bullet as light as possible, since speed is what penetrates armour. Between these two, a very thin layer of polymer as a lubricant to ease separation when hitting armour. Possibly drive bands made out of copper alloy since aluminium oxidises nearly instantaneously and aluminium oxide is very hard.

I am very much looking at a bullet with drive bands for reasons of efficiency

Hmm.  Won't a pointy tungsten penetrator + polymer film + aluminum body + copper drive bands = a very expensive and difficult to manufacture bullet?

What you have described sounds like basically a much more complex and costly version of the Aeroshell projectile.

SOCOM wants a new armor piercing sniper bullet. Here’s one option engineers are developing (militarytimes.com)

let me read up on that, but the basic inspiration is 7N31. Or 7N21? One of dem Russian bullets anyway ;)