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 ;)

stancrist said:

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

That looks like a more refined version of the Dagny Dagger.  (Or rather, the Dagny Dagger looks like a less refined version of this, as the Aeroshell appears to have been around longer, at least conceptually.)  The idea seems to work, at least in 9x19mm.  The DD uses what appears to be a NiCu penetrator, but that's only to get around US ammunition laws; as one of its creators said, there's no practical reason why someone couldn't use a different penetrator material.

I'd expect some Hague Convention related gripes from certain parties, especially if Elbonia optimized the sabot for destructive effect on soft targets - an easy thing to do, and one that wouldn't affect hard target penetration.

Mustrakrakis said:

I'd expect some Hague Convention related gripes from certain parties, especially if Elbonia optimized the sabot for destructive effect on soft targets - an easy thing to do, and one that wouldn't affect hard target penetration.

Not sure why? Not a hollow point and would it be more injuring than a regular FMJ?

Forget the polymer layer; tungsten rod will shrug off the aluminum shell shortly after penetration begins due to acoustic mismatch between the materials. A more serious problem is stabilizing the projectile even at PDW velocities and spin rates. I won''t make the calculations for you, but it won't be aerodynamically stable beyond a few dozen meters; lousy gyroscopic moment.