This is my private entry into NATO's 2003 PDW contest, and meets some requirements better than any others. I was looking at FN's Five-Seven pistol, and H&K's PDW. They have mags of 20 rounds - neither could accomplish 25 rounds in a compact mag. Then I wondered if I could do better in a pistol frame. YES.
My dimensions are based on a Glock grip for the 10mm/.45ACP in double column.
UPDATE: I just saw the altered dimensions for the .45GAP Glock pistol. Apparently, my life just became easier. Glock thinks a thinner pistol grip wall and mag wall is OK. I had intended to use the 'corner posts' of the mag at the bullet taper and rebated rim base for structure, while thinning the walls. Life just became easier! I am also exploring an unorthox 'puller sabot' with a slightly wider front bullet portion, and a very long but thinner rear portion. I need a combination of less pressure and more powder, and the best way to do this is more internal cartridge space not occupied by bullet. Good news indeed!
Since I want twice as many rounds across, this means each cartridge/bullet combo cannot exceed .240" wide by 1.26" long.
NOTE: I *think* I can expand the walls to .1" thickness by adding .01" thickness to them. This is still only 2/3 of the FN round, which makes me think I should keep the pressure down. (Can anyone find SAAMI specs on the HK 4.6mm?)
The round is based closely on the internal ballistics of the .22WMR rimfire. Of course, I'll want a centrefire version, and a shorter bullet / longer case.
The magazine holds c.40 rounds in a pistol grip handle due to the unorthodox design. (Can you say Robocop? <:) This design was built into the 1980's Italian Spectre M4 SMG, as well as the WWII Suomi. It is essentially a doubled double-column magazine.
Aside: the Russians have patented this there. It is called the 'quad stack mag' and the patent # is RU 2158890. See the AK74-M.
It holds almost twice the normal rounds of a double column mag, at the same height.
My round is based on a width of .240" - the .22WMR is .238", so I could even add 1/100" taper on each side.For testing, just adding .001" wall thickness is easier. I need to make the rounds from scratch since ammo companies do not help wildcatters. I settled on nearly straight-walled cartridge (perfectly straight-walled for blowback) and a rebated rim design. I use rebated to allow a blowback mechanism to include a supporting sleeve on the bolt, to provide full cartridge support, in order to avoid upsupported case head ruptures, should I choose to increase the pressure. I am tentatively making the rebated rim have a width of .220". However, I'm concerned that a rebated rim will experience blown out primer pockets. If that proves to be the case, then I'll try the highly unorthodox 'assault rifle M/2030' countersunk base design. I should
I obtain 220-260fpe with this shell. After all, the .22WMR was initially supposed to generate c. 213 fpe in a pistol barrel. Reloaders have successfully made that 330fpe.
This is accomplished with a very long cartridge length combined with a very short protruding bullet length - I added .06" to the case length to keep the pressure down; this leaves only .17" for a protruding bullet. The cartridge is specialized for a SLAP-style saboted subcalibre bullet, plus for a semi-wadcutter subsonic load. Both loads are optimized for external ballistics. The subsonic load has a rebated boat-tail base. A fast round needs a pointy tip, but a slow round needs a tapered BASE instead. The SLAP round should go 3000fps with a 11-13 grain payload. 1-2 of these grains is the sabot - parasitic mass. The subsonic load moves 1000fps with a 36 grain projectile. Note that both payloads have a nearly identical recoil impulse profile. This is very important, since blowback mechanisms are extremely finicky about impulse. Finding a rifling rate to not over or under-stabilize loads should be a bitch. The SLAP sabot load could optionally be clipped on using a crimp at the lip, to make an 'assault rifle lite' version - this frees up enough space to add extra powder. The PDW recoil should be about 10% less than a .22LR - really. I found load data for a 6.5" barrel for the .22WMR rated at 198fpe. I use the same .38-to-.357 trick with the longer shell to improve on this. Note that my round at 3000 fps with a great BCE should exhibit fragmentation effects at short range and tumbling on impact. Obviously, instability should occur at the 3" of the FN P90, rather than the 6" of the HK PDW.
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I did a quick back-of-the-envelope calculation, and dimensions should be 1 1/4" wide by 6 1/4" high by <12" long, all at c. 1 1/2 lbs empty. I think I can stuff about 3 clips, or 150 rounds on for a total of 3 lbs. I plan to have an extra mag in the front folding grip, plus another under the sight picture. That's 150 rounds and gun for c. 3 lbs.
I think the fat mag might make the sight picture very high. This is good for ergonomics while firing, but would be bad for compactness before firing. Even with just the fold-down front pistol grip holding a spare mag, that should still be about 80 rounds handy. The fat base on the mag may mean a very minimal pistol grip on the lower half, perhaps just on the front and back, to provide the structure to resist recoil.
