Sunday, August 24, 2008

On Controlled Feed.

Warning: dense, rambling gun technical talk by a rank amateur follows. Don't take this to be anything other than an essay sitting in the writer's backpack which said writer feels good about but which has yet to be tossed to the instructor's mighty Red Pen of Despair.

It's pretty well known that the TT-33 Tokarev pistol is essentially a 1911, down to the old-school swinging link locking system. After reading about the Controlled Feed principle, I decided to see how the feed worked on my NORINCO Model 213, which is a faithful Tokarev copy in 9mm.

(As an aside, there's a whole family of cartridges here - starting with 7.63 Borchardt, which is identical to 7.63 Mauser and 7.62x25 Tokarev except for loading; the 7.65 Parabellum, which is essentially the Borchardt shortened to 19mm for the Parabellum "Luger" pistol, the 9mm Parabellum, which is the 7.65 blown out to a straight-walled 9, and the rather more obscure 9mm Mauser, which at 9x25mm has the potential to be stout as hell and was chambered in some very nasty Hungarian sub-guns as well as the Mauser Broomhandle. Upshot is, the case web and rim are identical for all of them, except the extraction groove, which can cause some issues with certain extractors when switching. So a 7.62->9mm conversion is blisteringly simple, even more so since the muzzle energy is roughly the same, so the recoil spring works for both...the CZ52 is known for its easy conversion thusly, though the cartridge length leads to feed issues. Anyway...)

I loaded one magazine with six empty cases for space (didn't have enough dummies) and my single 9mm snap cap, and closed the slide slowly. Sure enough, it did a perfect controlled 1911-style feed - the round plunges down the feed ramp a few degrees before being lifted into the chamber - if it doesn't (at extremely slow slide speeds, mind you) the round log-jams between the feed lips and the feed ramp. This is why 1911s are rather finicky with regard to bullet shape compared to SIGs, Glocks, and other modern guns with their straight-in feed angle - the ogive guides the round through the dip, up the ramp, slipping the extractor groove under the extractor hook in the process, and generally maintaining control over the round at all times; one could cycle the slide at any orientation (upside down, in zero-gee, etc) at extremely slow speed and expect the round to slip in. The ramp holds the round solid in all three axes, maintaining tension with the magazine, and then guiding the round into the extractor to finish the feed. Chambering an empty case almost works; the round dips, but the bottom front edge of the case jams into the feed ramp. It actually might feed fine, if the ramp didn't have visible toolmarks on it. Feeding is the one kind of malfunction I don't usually experience with this gun, so with nicely rounded 9mm hardball (or Pow'R'Ball, for that matter) the finish of the ramp is inconsequential. It's actually a rather nasty lock-up, which clears rather similarly to a double-feed, as long as the round doesn't snap out fo the magazine and lodge between the top edge of the barrel and the bottom of the feed ramp. This is all academic, because we don't feed empties except for silly little exploratory tests like this. Actually, I can see why feeding empties makes sense as a function check on 1911s.

My SIG has a much simpler feed - the round gets pushed out of the magazine, guided somewhat by the barrel ramp, but the round simply pops out of the mag in time to slide behind the extractor hook and be pushed into the barrel, guided up by the ramp. Running it very slowly, there's a very noticeable 'snap' where the round escapes control of the magazine and pops up to chamber height in one motion. Since the ramp gently guides and assists rather than controls (and to wit there's about half as much ramp), the ogive (or lack thereof) of the round isn't too important. Overall cartridge length isn't too important either, since the moment the round snaps out of the magazine, it's (loosely) controlled by the extractor. There's a brief period of very loose control between the extractor catching the rim and the front of the case entering the chamber, so an empty can flop forward and stop the slide from closing; however, it's almost there, so it usually goes in by pushing the back of the slide (the rim might slip out of the extractor in some cases, at which point it logjams, with the rim below the extractor and the case angled up into the chamber).

It's all stated better elsewhere, but it's a cool bit of gun geekery, and explains the need to polish a 1911 ramp as opposed to changing the geometry of a 1911 ramp. Note the complete lack of control that gun now has - getting rounds into the chamber is a crap shoot.

Inspiration for this idle fiddling was this here fascinating comparison of 1911 magazine choices. Us tacticool SIG mall ninjas have it easy - OEM or crap. Hat tip to Xavier.


Tam said...

Good piece! Couple minor quibbles:

FWIW, the SIG and Glock (and all Browning tilting-barrel short-recoil pistols) are Controlled Round Feed designs.

The term "Controlled Round Feed" simply means that the round is captured (ie "controlled") by the extractor before it is fully chambered. The opposite is "push feed", where the breechface simply pushes the round into the chamber until the extractor hook pops over the rim of the already seated cartridge from behind.

Also, 7.65 Luger/9x19 Para are in no way related to 7.63 Mauser.

Bunnyman said...

Thanks for the props, and the clarification on feed types.

For curiosity's sake: 7.65 Luger not related to 7.65 Borchardt? I'm a bit surprised, since according to my sources Georg Luger morphed the Borchardt into the Parabellum pistol, so I can't see such similarity in the cartridges being coincidence. Of course, that's the pistols evolving, not the cartridge, and Cartridges of the World flatly states "The 7.65 Luger was designed by Georg Luger," so...