Why not use a straight case for a PDW cartridge?

[cluttonfred]

I'm putting this in Rifle Country since the most successful PDW, the FNH P90, is a shoulder arm. This picture got me thinking about small-caliber, high-velocity rounds for PDWs or other uses (left to right: 4.6x30mm, 5.7x28mm, .30 Carbine).



Wouldn't a straight rimless case like a smaller caliber .30 carbine (or a rimless, centerfire .22 magnum) be a better way to pack a lot of rounds into a small space? Yes the rounds would be a little longer, but not prohibitively so.

I have visions of a return to a Mauser C96 layout for a PDW with a quick-folding, lightweight stock and a generous magazine capacity (50 rounds) that could still be holster carried comfortably.

Thoughts anyone?

 

[rfurtkamp]

I have visions of a return to a Mauser C96 layout for a PDW with a quick-folding, lightweight stock and a generous magazine capacity (50 rounds) that could still be holster carried comfortably.

Where exactly on a C96-style weapon is 50 rounds of anything with enough oomph to matter going to fit, while still remaining holsterable? The P90's magazine is large.

 

[KenWP]

Why not think about caseless ammo as a way to conserve space also.

 

[Wes Janson]

Wouldn't a straight rimless case like a smaller caliber .30 carbine (or a rimless, centerfire .22 magnum) be a better way to pack a lot of rounds into a small space? Yes the rounds would be a little longer, but not prohibitively so.

That's very nearly exactly what 5.7x28mm is, so I'm not really clear what you're looking to accomplish.

 

[cluttonfred]

Where exactly on a C96-style weapon is 50 rounds of anything with enough oomph to matter going to fit, while still remaining holsterable? The P90's magazine is large.

50 rounds may have been a bit optimistic, I admit, but it's not impossible. By going to a straight case with the same bullet diameter, the cartridge diameter drops substantially. Take a look at that first photo again.

Taking the .22 Winchester Magnum Rimfire as a model, its base diameter is .241 inches. Calling it .25, a quarter inch, to make things easy, then 25 cartridges in a straight line would take up 6.25 inches. A second column, slightly staggered, would bring it to about 6.5 inches, so call it about 9 inches for a double-column magazine to leave room for the spring, follower, feed lips, etc.

Going to a four-column magazine like the Suomi "coffin" magazines would be even more compact and little more than one inch thick.

 

[GunTech]

To obtain the velocities needed, you need case volume. That means either a bottle neck case, or a long straight case. Shorter bottle neck cases feed better, and powder combusts better in a short fat case with a shoulder. Hence almost every high velocity cartridge is a bottleneck case.

It's also far easier to headspace off a shoulder than a case mouth, particularly when a crimp is involved.

 

[cluttonfred]

The headspacing I hadn't considered...presumably untapered rimless cartridges like the common autopistol cartridges and the .30 carbine are headspaced off the case mouth? Are there any significant disadvantages to that in terms of reliability? As far as I know, the M1 Carbine and better SMGs or pistol-caliber carbines do not have excessive problems due to headspacing. The feed issue and the powder would be trade-offs, to be sure, but not insurmountable obstacles, I imagine. Thanks for the comments, keep them coming, please!

 

[Dr. Tad Hussein Winslow]

You're gonna increase recoil with a larger bullet, and thus impede your ability to stay focused on the most teen-inesy of targets in full-auto fire. Also losing the ability to bust right through SBA, which is a main point of the 5.7x28, IIRC.

What about going back to the Ppsh 41?

And how, EXACTLY, are you defining "PDW"? Must it be full-auto, or are you including "civilian" weapons in this definition?

 

[DMK]

One thing to keep in mind is the 5.7 was designed to pierce body armor with it's small diameter and high velocity. The full auto capability and high capacity were bonuses.

The M1 carbine was a good PDW of it's day, but was replaced by compact 9mm submachine guns such as the MP5 before the P90 came along. However, without the capability to use AP ammo legally, and without full auto capability and the short barrel, a lot of the functionality of the P90 is lost to civilian owners.

For civilains, something like an M1 carbine with good 30rd mags makes a lot more sense as a compact and light weight PDW. If you can get an SBR and full auto, the MP5 type weapons make more sense. To pierce body armor, something like a compact 5.56x45 or 5.45x39 carbine loaded with FMJ should work at close range.

 

[highorder]

To obtain the velocities needed, you need case volume. That means either a bottle neck case, or a long straight case. Shorter bottle neck cases feed better, and powder combusts better in a short fat case with a shoulder. Hence almost every high velocity cartridge is a bottleneck case.

It's also far easier to headspace off a shoulder than a case mouth, particularly when a crimp is involved.

I was going to say all this stuff!

 

[GunTech]

The problem with ANY PDW using a specialized case is that you've just added another round to the inventory. It is perfectly possible to build very compact 5.56x45 weapons, and no one will deny that 5.56 beats 5.7x28 in every possible way.

Here's the Magpul PDW in 5.56x45

 

[Dr. Tad Hussein Winslow]

Here's the Magpul PDW in 5.56x45

Dude! Must....have.....one.....

 

[highorder]

vaporware?...

That thing WILL need a can.

 

[cluttonfred]

I think folks are missing the point here. You don't need to convince me about the pros of the .30 Carbine, my M1 Carbine is my favorite rifle and the gun I would own if I had to own only one.

I am wondering why, in the intererest of high capacity, no one has promoted a .22 caliber, straight-cased, rimless round for PDWs. For comparison, here are the specs on comparable 5.7x28mm and .22 WMR rounds, both firing 40g jacketed hollow point bullets.

5.7x28mm
40 gr (2.6 g) SS197SR JHP 594 m/s (1,950 ft/s) 461 J (340 ft·lbf)

.22 WMR
40 gr (2.6 g) JHP 1,910 ft/s (580 m/s) 324 ft·lbf (439 J)

You can see that they are very close and it wouldn't be hard to create a straight-cased, rimless, centerfire cartridge, call it .22 PDW to duplicate or better the 5.7mm.

If the powder was optimized for pistol-length barrels, it would even make for a heck of an autopistol round (admittedly trading effectiveness for magazine capacity...maybe 30 rounds in a full-size service auto pistol).

 

[briansmithwins]

It is perfectly possible to build very compact 5.56x45 weapons, and no one will deny that 5.56 beats 5.7x28 in every possible way.

Build me a blowback operated 5.56 weapon.

That would be one application where 5.7 is better than 5.56. Also, 5.56 is optimized for a 16-20" barrel. Using it in a 10" barrel is going to smoke your eyebrows off.

I'd be a lot more excited about the Magpul PDW is the Masada was in stores right now. How is that one coming along? BSW

 

[GunTech]

My understanding is that Bushmaster is was so busy turning out ARs that the ACR got pushed to the back burner.

Certainly a short barreled 5.56 is going to be loud, but there are compact solutions. Is it going to be any worse than a Krinkov?

Why is blowback operation important? It's not even used in service pistols any more, and just limits the operating pressure of you round. It certainly doesn't make for a cheaper weapon. I see no advantage whatsoever in having a blowback operated PDW.

I see a huge advantage in having a PDW that uses the same magazine and ammunition as the standard military rifle. So what if it's loud. The theory is that a PDW is an emergency weapon anyway, for non-combat troops. It's not like it's going to be getting the same amount of use as a standard combat rifle in the hands of regular infantry.

Understand the TO&E

 

[GunTech]

5.7x28mm
40 gr (2.6 g) SS197SR JHP 594 m/s (1,950 ft/s) 461 J (340 ft·lbf)

.22 WMR
40 gr (2.6 g) JHP 1,910 ft/s (580 m/s) 324 ft·lbf (439 J)

You can see that they are very close and it wouldn't be hard to create a straight-cased, rimless, centerfire cartridge, call it .22 PDW to duplicate or better the 5.7mm.

If the powder was optimized for pistol-length barrels, it would even make for a heck of an autopistol round (admittedly trading effectiveness for magazine capacity...maybe 30 rounds in a full-size service auto pistol).

Minimum case diameter is going to be dictated by primer size. I'm not sure you can get much smaller than 25 acp, so a straigh walled rimless case might be a problem. A pistol primer is 0.175" in diameter, so maybe it can be done.

BTW, There's nothing particularly unique about the 5.7x28. Colt had a nearly identical round for the 1969 vintage Colt SCAMP. The 5.56x29mm SCAMP delivered a 40gn projectile at 2100 fps from a pistol. The Colt MARS, a 5.56x30mm round managed 2600 fps with a 55 gn bullet

Interdynamics experimented with a 4.5mm rimfire, and of course Grendel built their P-30 in 22 magnum, with a 30 round capacity.

Personally, I like the looks of the Colt SCAMP. The FiveseveN lacks the happy switch.

 

[GunTech]

For those interesed in the MagPul PDR (PDW) here's their brochure.

http://www.magpul.com/pdfs/PDRtech_PR.pdf