Double Feed... again

[JulChris]

This is the sequel of a previous thread (www.thehighroad.org/showthread.php?t=145490 ).
On the last trip to the range, I had a new stoppage.
After the shooting of over 700 rounds with no problems, on the last but one….. double feed!
To sum up:
Pistol: Kimber Custom II 5”, well tuned (after too many problems….), no buffer, no frills;
Over 4.000 rounds with no problems, except four random double feed on the last but one round;
The last three stoppages with the same magazine (perhaps also the first);
This pistol is used only for recreational shooting, but obviously the magazine will be rejected.
But…I’m just curious to know how this type of jam can occur!
I can’t understand why (in rare occasions, but it happens) the last but one cartridge become loose and jam the pistol while the very last no: a weak magazine spring would give also last round stoppage, as the magazine has not the dimple.
Here you find a pic of a (exactly recreated) double feed.
Help….

 

[1911Tuner]

Double Feed

Hi Chris,

In your picture...If the round in the chamber is an empty case, it's the extractor. If it's a live round, it's the magazine...or more specifically...
a weak spring. Have you noticed any live rounds among your empty brass?
How about the slide locking open with the last round laying loose on top of the mag? That'll occur too...it if hasn't already.

 

[JulChris]

Hi Tuner,
In the photo you see two live rounds.
As previously explained in another thread (see link above) my Kimber is utterly reliable (now) with exceptions of these (rare) double feeds on the last but one round.
I wonder how this type of jam can occur: no last round problems, even with magazines with no dimple, only on the sixth round out of seven.
Have you an explanation of these jams?

 

[1911Tuner]

Double, Double...Toil and Trouble

Hey Chris...Here's a cut and paste of an explanation that I wrote up about 3 years ago. Yours is a magazine problem, pure and simple. Inertia is the culprit, and it can show up in a couple of different ways. In your case, the 6th round is getting loose from the mag, and instead of being kicked out, it finds its way into the chamber while the slide is heading for the next round in the mag. With use, that might change to another equally vexing malfunction. Read on...

*******************

Inertia! Just no way to get around Newton's Laws regarding motion.
Remember the old trick of jerking a tablecloth out from under a table setting? This is the principle at work here.

We have a round in the chamber and one round in the magazine.

Bang! The slide starts to move as the pistol torques up and back. The slide holds the last round slightly below the feeding position until it moves far enough to uncover it. Just as the magazine spring is struggling to move the round into position, the round is in a sort of "Limbo" while the pistol continues to move backward in recoil. The round obeys Newton, and stands still while the gun is moving away from it.

The magazine spring catches up, and gets the round up and into the underside of the feed lips, but because the pistol pulled backward away from it (Even though the pressure from the slide drags the round backward in the magazine)...it settles down forward of the feeding position. At this point, if the magazine spring is strong enough to keep it there, the slide pushes it ahead of the extractor. The pistol either fails to go to battery with the round fully chambered, and the front of the extractor rammed against the back of the rim. Extractor breakage is an eventuality.

If the spring isn't strong enough, the round is forward of optimum feeding position just as the slide smacks the impact surface in the frame, and triggers a second recoil impulse. The gun makes a short, hard jerk upward and backward...and the round is in limbo once more because the mass of the round has caused the magazine spring to compress slightly. The round...already too far forward in the magazine...jumps the follower, and is free of the magazine. The follower pushes the slidestop up as the slide moves forward, and the slide locks. If the magazine spring is weak enough, the next to last round will be ejected from the magazine, and the last round feeds. Ever found live ammo among your brass? Heeere's yer sign!

The odd "Double Feed" is another symptom. Live round in the chamber with another live round trying to feed.

The problem is two-fold. One is the spring that has fewer coils to make room for the extra round. There is ample tension to feed until it gets to the last round...and tension is at a mimimum...but sometimes it can happen before the last round. Upping the spring rate helps, but doesn't address the other issue.

The other part of the problem is the smooth follower. Browning knew how
inertia would affect things, and he put a small dimple on the top of the magazine follower. The dimple's function is two-fold. It adds a small amount of height to the follower in order to give it a "Leg Up"...and it stops the forward movement of the round. More accurately, it keeps the pistol from moving out from under the round in recoil. In this function, it's basically a back-up for the spring as it fatigues, and provides a better opportunity for the round to stay in position to be stripped from the magazine by the slide instead of being pushed ahead of it...or... in the extreme cases, escaping from the magazine completely.

John Moses designed a 7-round magazine and he put a dimple on the follower for very good reasons. Whenever we try to change things in order to "improve" the gun...we very often cause problems. There just ain't no such thing as a free lunch, I'm 'fraid.

 

[JulChris]

Tuner,
I have read (almost) all your threads and responses in many forums, included the one you have attached.
I have also “dimpled” many magazines I use with another Kimber (the true Kimber, no firing pin stop and internal extractor) which showed the “last round” syndrome: now it functions flawlessly.
I have not dimpled the magazines for my Springfield (over 7K rounds with no problems), neither those of this new Kimber II, I bought circa 2 months ago.
I am very intrigued with this new pistol, and following your hints I have get rid of initial problems, as stem bind, FTE, FTF and etc.
I have also rebuild a new extractor that now functions perfectly (at least for the last 3 K rounds).
My new external extractor don’t act as loaded chamber indicator, but only as extractor, pure and simple. Its travel is limited and not interferes with the path of the round, so no more stem bind (but I have also elongated the slide stop hole in the barrel link).
My Kimber II now function flawlessly and is very accurate but shows rarely the double feed prob.
It’s possible that even the last round escapes the magazine lips and, given the structure of my new extractor, the slide can close without effort on a chambered round, but I wonder why I have never had a slide stop with the last round in the chamber.
I repeat, just curiosity… but I would know how it happens.
Chris

 

[1911Tuner]

Dimpled?

Chris said:


>I have also “dimpled” many magazines<<<

Hi Chris,

The location of that dimple is pretty critical. It has two functions. One is controlling the last round when the sporing tension is at a minimum. The other is in timing the release of the last round, which...due to not having the convex surface of another round under it...feeds at a slightly different angle than the rest. Minimum spring tension also comes into play here. There is also the matter of the length of the rear follower leg itself to consider. If the followers that you added the dimple to were 8-round followers, it may worsen the situation. The shortened, stamped steel 8-round follower rocks forward more readily, and adding a dimple for the rim to catch makes it more pronounced. The standard 7-round followers should already have the dimple,
and are long enough to maintain stability.

If your dimples are correctly located and dimensioned, adding a Wolff 11-pound mag spring will probably cure the problem, even with an 8-round follower, assuming that the follower design is compatible with the spring.
Wilson-Rogers followers aren't. McCormick/Devel followers are.

 

[JulChris]

Tuner,
I have distinct magazines for my 1911s, and the problem of Double feed is only on the new Kimber and only (may be) with one magazine, a Mec-Gar 7 Rounds, with NO dimple (BTW mecgar makes many types of follower, with and without the dimple, cast and stamped steel and some are exactly as USGI follower)
I have planned to get stiffer springs for all my magazines, but I wonder if also a stronger or weaker return spring could have some role in dampening the slide impact on the frame. Until now I have had 2 Double feed with the standard 16# and 2 with a 18.5# (I use always full house rounds, factory and reloaded).
Chris

 

[mpthole]

Hey Tuner ol' buddy! Timely thread... I just picked up some Chip McCormick 10-round mags for USPSA competitions and I'm getting this

Live round in the chamber with another live round trying to feed.

Seems to happen on the 2nd or 3rd round in the mag. The followers are flat, without a dimple. Sound like this is magazine related as well? Only 1 of the 4, 10-round mags I bought seems to do this. I've been keeping them loaded... maybe I should keep them unloaded when I'm not expecting to shoot anytime soon...?

BTW, did you catch my thread on the loaded vs mil-spec?

 

[1911Tuner]

10-Round Mag Burp

Hey hey Marcus mah fren! Didn't check your thread yet...Time is the main issue. I'll get on it in a few.

Your problem is with the magazine. As it happens, I just tweaked 3 of those critters for 19112XS last week when he came to visit. He had trouble aplenty with'em. I noticed two things when I tore'em down...One is that the OEM springs are a bit soft for best follower stability and feed angle. The other is that they will accept the standard Wolff 11-pound, 13 turn spring. The problem is that they won't take the full 10 rounds with that spring. (By the way...the standard Officer's Model 6-round mags will also accept that spring and the standard follower native to the GM-length 7-round mags. Neatly corrects mag-related problems with the little shorties, and they'll accept 6 rounds as designed. They just won't accept 7, but life's fulla little trade-offs I guess.)

Since the mags wouldn't take 10 rounds anyway, I just swapped out the Devel/McCormick folded followers for some standard 7-round followers...and the magazines that wouldn't, suddenly would...just like magic. Whowouldathunkit? Now his magazines not only run fine in the half-dozen pistols that I've tried'em in...including two Commanders and an early LNIB 1991A1 Compact/OM...the followers don't eat the feed ramp in alloy-framed pistols any more and they lock the slides on empty with utter authority. I ask ya HOOOO....wouldathunkit???

Sooo...Get a pack'o'Wolffs and stick'em in the mags. If that still doesn't do the trick, order a few of Metalform's stainless 7-round followers and be prepared to see those mags work like they oughter.

Oh yeah...PS The dimple essentially affects the timing and release of the last round only, so that's not your problem.

 

[mpthole]

Cool. One more question though...

One is that the OEM springs are a bit soft for best follower stability and feed angle. The other is that they will accept the standard Wolff 11-pound, 13 turn spring. The problem is that they won't take the full 10 rounds with that spring.

Do you think I can trim the Wolff spring until it takes 10 rounds? That extra round can really make the difference in winning a stage in a match.

 

[1911Tuner]

Trim Spring

Try the spring as is first...It just MAY accept 10. The ones that I tried didn't miss by more than a 64th of an inch, and tolerance stacking on a different mag may allow it. If not, I'd try the Wolff +5% springs that are amde for 8-round magazines. They would probably offer a bit more tension than the stock springs at the top of their run. At any rate, you've still got the problem of that smooth topped, folded Devel-type follower to contend with if you have to have 10 rounds...which may put you right back to square one.
If you do manage to get 10 rounds in with the 11-pound spring, be aware
that you'll compress the springs beyond their design limit in which case they'll probably require more frequent replacement.

Might be best to just practice 'til ya get so good that ya don't need the tenth round. Front sight...Press!