FAS SP607 (and other) feeding

[Shooting Kiwi]

The feeding mechanism of my FAS seems to rely on luck or good timing, rather than strict guidance.

Why do I say this? Because, as the round is pushed towards the breech by the slide, the nose of the bullet starts to rotate, either by contact with the loading ramp or by the drag of the magazine lips on the top of the case rim. The dimensions are such that, when the bullet's nose is just at the mouth of the breech, its rim leaves the lips of the magazine. It is then more-or-less flying, momentarily in contact with nothing other than the advancing slide, rather than being guided. So the direction it is pointing depends on its rotational acceleration and the time it is allowed to rotate befor ith slide carries it into the breech.

The result of this is that if I remove an auto-fed round, there may be a flat on the bullet nose, where it has slid along the top of the chamber, or a small crescentic indentation where the underside of the bullet has been caught by the junction of loading ramp and bottom of breech opening. Twice, the bullet has been flicked upwards too much, missing the breech, and the advancing slide has then tried to guillotine the case.

The Unique DES 69 feeding mechanism looks similar.

I suppose it doesn't matter too much, as long as the round goes in, and is slightly deformed consistently.

Is this normal? Are there any target pistols using a feeding mechanism where the round is properly guided home, rather than being thrown in the general direction of the breech? Comments please.

[Shooting Kiwi]

Update to above...

Have just returned from trying Unique DES69 at the range.

Gun functioned well enough and was more accurate than its shooter, but looking at a sample of withdrawn chambered rounds, each time, the bullet was heavily deformed where it had been pushed against the corner of the loading ramp and the breech.

Closing the slide slowly, to feed a round, the round jams with nose against top of chamber, undersurface against corner of ramp and breech and rim still held by magazine lips. Of course, when auto-cycling, the much greater forces just ram the thing home - and deform the bullet as necessary. Bullet also noted to be loose in case after chambering, and a little bent, nose-down.

A little bending and easing of the magazine lips achieved considerable unreliability, but no feeding without bullet deformation.

This seems to be a design problem because of the steeply angled path the round has to take into the chamber. Do any target pistols have a straighter path?

Come on, people! No-one's replied to this yet. Is it really so uninteresting? Please pull out a chambered round (after an auto-cycle) and let me know which gun can chamber a round without deforming the projectile. Better still, tell me how to correct this problem. Please!

[David Levene]

I think the reason you have had no replies is because it realy doesn't matter.

Does the gun function reliably? Do the bullets go where you expect?

If the answer to both questions is "yes" then I don't really care how the gun does it.

You talk about problems you see when you close the slide slowly; guns are not designed to feed rounds slowly.

[Shooting Kiwi]

Ah, yes, I see - pragmatism! (Or is it free market politics?)

I was hoping for science, but, the more I get into shooting, the confusder I get!

[ben]

Shooting Kiwi
As a sometime gun designer perhaps I can cast some light. Your observations do not give me any surprise. Moving a cartridge from a magazine, or a belt, in any gun, is a difficult design problem because, for most of the travel, the cartridge is not constrained laterally. A few earlier guns did provide effective lateral constraint but at the expense of more costly machining. The common practice is that the magazine feed lips serve to aim the cartridge into the breech. Thereafter, the cartridge is brutally levered into line by the breech aperture. The inertia of the cartridge also has its effects.
David is only partially correct in declaring it doesn't matter. The timing of events in a feed mechanism are almost entirely geometric and if one has a defective feed the first thing I would do is to look at the timing when cycled slowly. Beyond that you are talking of high speed cine.
With the usual practice of making magazines out of pressed steel it follows that it is very easy for the feed lips to become deformed, with the consequence that feed failures follow. A topic which arises fairly often on this site.
The UK engineering literature contains an entertaining account of the feed difficulties encountered in the 1950's with an automatic 3.7 inch calibre anti-aircraft machine cannon.

[Fred]

When I purchased a used FAS 607, I tried many different brands and lots of ammunition in an effort to obtain at least semi-reliable performance. Nothing worked. I spent a great deal of time and effort on tuning 3 magazines, checking recoil spring strength, and examining extractor tips, and, again, no success.

Finally I contacted Rob Potter in Australia, via email, and asked for his advice. He suggested that the problem might lie in the profile of the feed ramp. It turned out that the feed ramp had a significant hump in the middle, which caused the tip of the incoming round to ride much too high, and catch on the top of the chamber opening.

I worried about modifying the feed ramp, but then figured I really had very little to lose, as the gun was unusable as is. After a very careful grinding down of the hump so that the feed ramp profile is closer to being straight, the gun is now semi-reliable. It might even be pretty good, but, at this point I am so tired of working on it that I set it aside for a while.

Anyway, the point of this lament is that the profile of the feed ramp seems to be very important on the 607 (and probably most other semi-autos that have feed ramps). Even though you would think the manufacturer would be on top of something like this, it's probably worth checking if you're having feeding problems.

HTH,
FredB

[Shooting Kiwi]

OK, I know these pistols are meant to feed when the slide is slamming to and fro at great speed, but, unless things are moving at relativistic speeds, dimensions and geometry still apply. Thus, I maintain that you can tell something about the mechanism by looking at it whilst moving the slide slowly.

For what it's worth, I dug deeper...

Another club member, a retired gun dealer, looked at the rounds mutilated by the Unique and shrugged. He thought most of the guns in the club would do something similar.

Local gunsmith, a previous national pistol record-holder, suggested the front of the rear magazine lips be trimmed, to release the cartridge rim earlier.

Today, played with a Hammerli 215, Hammerli 208 and Walther GSP. None of these caused the bullet waist to be rammed into the corner of the loading ramp and the breech. All released the cartridge rim just as the bullet nose entered the breech. All caused a small deformation on the underside of the bullet nose where it hit the loading ramp. All seemed to require the round to rise less from magazine to bore than the Unique does.

My conclusions? The Unique's design makes things difficult for itself. Its feeding can be improved by doing something to the mag. lips. Most (all?) semi-auto bullets get battered on their way to the chamber. Many types of bullet deformation, if consistent from shot to shot, doesn't matter too much, as shown by FW Mann, all those years ago. Semi-auto pistols are 'funny' engineering.

I offer these observations merely to add to the general confusion...