Not sure how much that would actually help.  With so little bullet extension past the case mouth, caliber would have to be reduced to about 5mm in order to use a bullet with a halfway decent ogive.

The short ogive length situation is exactly why I still take the Caleb crye pdw design seriously even though it's really out there and would be extremely hard to pull off effectively.

Several of my friends, including ones whose opinions I respect immensely because they're actually working as designers in the field, have up until pretty recently told me "or you can just accept that pistols have their limitations and not try to turn them into mini rifles" when I'd bring up pdw's and how to make them more effective.

You'll note that until pretty recently part.

Even for civilians here in certain parts of the US the threat dynamics and likelihood of gun fights that don't end in 7 rounds at less than 7 yards with only one hostile shooter has dramatically increased which has changed their attitudes substantially.

I along with them personally think that the future demands deep magazine capacities as well as the ability to mount a red dot on a non reciprocating top surface of your gun.

This also sorta x's out something like 7.5 fk by default because you're not going to get the required magazine capacities.

I think what's really called for is something akin to a 5.7x28 super with as much ogive space as can be begged stolen or borrowed.

Form factor wise I do agree that we really need it to feed through the grip...

But as Stan keeps pointing out, ogive space is mandatory!

The one place 7.5fk got it right was in designing for lead free from the outset.

Also if we're gonna go for something non small diameter (up to 7-8mm no more)  we definitely want it's standard loading to be a pseudo EPR for a variety of reasons.

Can we jam all that into a single platform and caliber?

Probably but it's gonna require genuine ground up development which is almost unheard of in the gun sphere currently.

This isn't something we can do by mixing and matching already existing stuff in different ways.

roguetechie said:

The one place 7.5fk got it right was in designing for lead free from the outset.

The 7.5 FK was developed around lightweight bullets, lead free and lead core.  See Msg 863.

There is nothing magical about lead free bullets that requires a special cartridge case design.

Lead free bullets can be used in cartridges which were originally developed and loaded with:

     Lead bullet

     Lead core, jacketed bullet

Squeeze bore!  Ok, ok, I have no idea how to do a squeeze bore PDW, but that is the only way that I can think of having a large diameter base (to get high velocity from a very short barrel) and reasonably decent BC.  That is, of course, someone figures out a new sabot design that shreds into tiny particles after leaving the barrel.

roguetechie said:

This also sorta x's out something like 7.5 fk by default because you're not going to get the required magazine capacities.

going full circle in this thread, what about a P90 style mag?

I think 7.5FK is a solid PDW caliber option if more power is desired; I doubt theres much room for improvement within the magazine-in-grip format for creating a cartridge of comparable power. 

The downside is that the power of 7.5 is likely to make FA dubiously controllable out of a 3lb MP9. And also the very real risk that the power ~2x that of 9mm will result in a much heavier MP9 style weapon. 

Still, its on the short list. Of existing calibers for a mag in grip PDW, I like the following.

-5.7x28: maximum capacity, minimum recoil 

-30 Super Carry: maximum capacity, better lethality/energy then 5.7

-9mm: easy option

-7.5fk: maximum power 

....

A 30 Super Carry 'long' PDW variant I think could be promising, if given an EPR style projectile modified to have a lower frag velocity. Similar to the 9mm developed by Federal for MHS:

Shitty sketch:

Just a guess, but I imagine it as a 90gr EPR @ 1500fps from a 5" barrel, modded to allow fragmentation at 1000fps. However without quickload I don't know whether that velocity is possible, and I'm not sure how low EPR can be modded to frag to. 

gatnerd said:

However without quickload I don't know whether that velocity is possible, and I'm not sure how low EPR can be modded to frag to. 

I am using https://www.grtools.de/doku.php

So, from what we know about super carry it's going to make a much better pcc or even long barrel pistol round than 9 para could ever dream of being. Every inch past the 3.1-4" of current barrel lengths will give you very non trivial jumps in velocity while it's incredibly high specific energy for a pistol round that's not 10mm will assist it in blowing clean through intermediate barriers and will basically treat thick layered clothing like it's not even there from a terminal effects standpoint.

It's one of the better machine pistol or smg round candidates we've seen in a long time.

My suspicion is if we go full liberty civil defense with a very light projectile, it's not gonna have any problems busting 3a without going to exotic projies or materials as well as being freakishly light if we use the same lightweight case tech. You'll also not suffer from the extreme velocity dropoff of current 9mm etc liberty because of reduced diameter allowing you to punch 3a further.

Step that up beyond Liberty civil defense and pistol epr levels to full on dagny dagger open source us code compliant pistol ap level, and it's going to seriously threaten even some level 3 uhmwpe only plates. Even to extended ranges from a 5-7 inch barrel.

For any of you not familiar with the dagny dagger project, Google dagny dagger crowd funding... It's a very interesting and relevant to our discussion project.

Tbh I wasn't thinking squeeze bore at all just moreof adequate length to diameter ratio to get a decent sized penetrator to fit without eating all our powder capacity.

I also wasn't worried about getting enough swept volume to get a good enough push for decent velocity.

My thinking was that we sorta take the logic we now know works for sure because of 30 super carry, ie that you can pretty easily make even a standard browning link less tilt barrel locked action and fire 50 60 even 70k psi loads out of it without having to super beef a pistol up until it's too large and chunky to be useful!

Granted my ideal pdw/combat pistol isn't Browning link less tilt barrel anyway but we now know for sure you absolutely could do it if you wanted to.

If you look at my response to gatnerd just before this one you'll see me talk about the dagny dagger project...

So what if we used the same lightweight and ultra high strength triple nickel cases liberty civil defense already uses across it's product line only instead in a straight wall 5.7 or 30 carbine case firing a mix of full caliber defensive loads and dagny dagger specialist loads essentially bringing back 7mm penna or 7.92 vbr in pissing hot high pressure variants?

With the powders we have available and the ability to load past 100% charge density we definitely have the needed pressures and gas generation to get absolutely ludicrous velocities in comparison to the current crop of legacy pistol rounds.

We gain the capacity to actually make muzzle brakes work with enough efficiency to be worth doing unlike current legacy rounds.

By reducing diameter and projectile weights we can push to velocities basically unseen outside of 5.7 if we really want to without blowing up guns assuming we keep current pistol barrel outside diameters. Likewise we can probably avoid destroying guns if we don't try to reduce locking surface sizes etc too which we wouldn't really want or need to do anyways.

We claw back projectile length to diameter ratios from both reducing projectile diameter AND from going to greater than 100% charge densities meaning we can bury the projectiles a little deeper in the case allowing us to get ballistically better projectiles with better ogive lengths without compromising feeding through the pistol grip. Likewise by reducing the diameter of both projectile and case, and going to all metal mags, we bump up standard flush fit with base of grip magazine capacities a very non trivial amount. (The 5.7x28 PSA rock holds 23 and a 7 penna redux full size would hold about the same with a 7.92 vbr redux likely bring more like 21-22)

Now there's also some less obvious benefits here too.

Projectile design and cost: with better ogive space higher velocity and etc your round is basically gonna be supersonic to the entire distance and then some your average shooter is going to be able to generate hits.

This makes making projectiles that expand etc reliably much simpler since you don't have to engineer for such a wide velocity range.

From a military issue and use standpoint all of the above also gives you a couple more nontrivial benefits.

1. Carrying the same number of magazines, you're going to have a much higher round count available for a given soldier.

2. By using both lightweight triple nickel cases and reduced diameter lighter weight likely entirely lead free projectiles, there's a good chance that this much higher total round count worth of ammo would actually weigh LESS than your conventional sig p320 mags loaded with current issue 9mm ammo in total and we all know cutting weight wherever possible is important 

To a large degree this is my preferred solution because while it would require SOME development, it's mostly benign and could be ported into existing platforms for development etc hell even fielding with not much beyond a mag and bbl swap.

gatnerd said:

Of existing calibers for a mag in grip PDW, I like the following.

-30 Super Carry

The 30 Super Carry really can't be a good choice.  It was not designed to use lead free bullets.