So, normal small arms ammo is limited in pressure by the brass of the cartridges, I understand. The limit being around 440 MPa. Do the new technologies (hybrid, polymer, cta) allow for higher pressures? And if so, do we know how much?

Yes they do. 

Depending on which one you're talking about and the exact implementation you're talking about anywhere from 70k psi to 80k to knocking on right about 100k psi.

Shellshock triple nickel pistol cases for example have shown that they can be ran up to 70k and still be reloaded multiple times.

Sig's hybrid case is currently running close to 80k according to reports 

And true velocity can run up well over what traditional brass can do but may be limited to a bit less than sig.

And then there's straight up cased telescoped Textron which MAY be capable of higher than any of the others.

What you get for this in conjunction with better powder chemistries etc is the ability to replicate the performance of much larger cartridges fired out of longer barrels to varying degrees.

Honestly, we should pursue all of them because they each have some advantages that could be harnessed depending on your application

roguetechie said:

Yes they do. Depending on which one you're talking about and the exact implementation you're talking about anywhere from 70k psi to 80k to knocking on right about 100k psi. Shellshock triple nickel pistol cases for example have shown that they can be ran up to 70k and still be reloaded multiple times. Sig's hybrid case is currently running close to 80k according to reports And true velocity can run up well over what traditional brass can do but may be limited to a bit less than sig. And then there's straight up cased telescoped Textron which MAY be capable of higher than any of the others. What you get for this in conjunction with better powder chemistries etc is the ability to replicate the performance of much larger cartridges fired out of longer barrels to varying degrees. Honestly, we should pursue all of them because they each have some advantages that could be harnessed depending on your application

I did a bit of simulating and with 650 MPa I can get ~1200 J out of a .357 SIG case. Give it a decent bullet and we have the PDW cartridge.

Edit: some more tweaking and I am at 500 MPa and ~1500J

Very interesting... However joules isn't necessarily the whole story. 

There's definitely a basis for the quality of the joules delivered having a noticeable and repeatable effect on the amount and type of damage you can inflict.

It's both the entertaining and maddening part of the pdw solution space unfortunately.

I have some definite and strong opinions about what I believe a potentially good solution path could be, but they're just opinions. (Albeit reasonably well informed ones but still just opinions nonetheless)

Speaking for myself, it seems obvious that to get at least most of the critical requirements met we need to think quite a bit about the platform we launch the projectiles from so we can cheat like hell on some of the unfortunate constraints.

I'll think a bit about exactly what I'm trying to say and get back to you on that part though.

In the mean time, yeah being able to ramp up pressures (and to an extent being able to fill cases until they're packed tight) definitely opens up the potential options though. 

One of my thoughts on that note is that something like a thin wall 70k psi compatible shellshock style case short length 7.92 vbr might be worth taking a look at. Assuming you could pack enough powder in and seat the projectiles deep enough you probably have something pretty magical without reinventing the wheel.