MAC11 (open-cartridge) firing problems - only 1-3 at a time

Anyone have a good guide to loading the open cartridges and making sure the gun can fire sustained full-auto. Mine jams all the time, and can't even get off more than 1-3 shots although much of the time it won't fire at all.

well could be a lot of things, what det pin do you have is it the right one for open rounds? are the rounds the right ones not as stupid as it sounds as there are a lot of open rounds for models like m76 M39 etc and there are long and short open rounds you may have the wrong size. is the bolt Ok does it hold the rounds frimly for ejextion and is the extractor working correctly. have a look at these things first then we can go from there.

Several types of faults are described below, narrowing down symptoms should help with further troubleshooting :-)

If bolt goes forward, but cap does not ignite: dud cap (check another batch of caps), too weak blowback force from previous cap (check another batch of caps), too much friction slows bolt and it does not have force to detonate cap (check for friction, clean parts, put some silicone oil but not too much - excess can "gum up" parts).

If cartridge stovepipes: too weak blowback does not eject cleanly (check another batch of caps), too weak blowback force (bolt does not travel back far enough to give time to eject cartridge, check another batch of caps), extractor does not work correctly (check spring, try cycling manually to see if cartridges are correctly ejected and not left in chamber, change speed of cycling from slow to very fast), ejector is damaged (manual cycling should show if it is the problem).

If fired cartridge is left in the chamber: extractor does not grip cartridge correctly (visually inspect cartridge rims to see if they are not deformed or damaged, verify proper extractor operation by cycling manually).

If cartridge is not correctly stripped from magazine or blocks before entering chamber: misaligned mag lips (manual cycling should show if that happens), damaged bolt face (inspect for cracks or swelling).

If there is no cartridge in chamber: weird feeding problem that should not happened if previous cartridge was correctly ejected.


When cycling manually check for any friction - you should feel it by hand. Doing that do not use live caps, but empty (already fired) ones. This will help to align cartridge correctly for depth on detonator pin in the chamber, and prevent damage of cartridge due to ramming the pin.

claymore wrote:

well could be a lot of things, what det pin do you have is it the right one for open rounds?

Yup, that's another point to check. Mark had similar problem (jams after 2-4 rounds) and he discovered he used wrong type of cartridges: 9x24 instead of 9x22:
http://www.mp40modelguns.com/Mac11.htm
https://mp40modelguns.forumotion.net/modelgun-general-discussion-f1/mgc-m11-full-set-t452.htm

Don't know how to tell the det pins apart...

This the right cartridge size for the mag? Or is it supposed to fit completely flush (meaning I am using too small carts)?

There is a gap (see below picture from a Japanese blog), but from your picture it is difficult to say if these are correct.
If you could measure them, it should be easier to tell if they are correct.



Detonator pin for open type cartridges is very wide at the end, it fills completely cartridge inner diameter.
Depending on design it could be either just wider metal ring, or a thinner metal ring with an o-ring.
CP pin is much thinner and is usually more uniform in diameter.

Your cartridges are correct , the other ones would reach the end of the follower in the mag

Correct det pin should look like this




Is yours pretty clean looking ? if its been allowed to rust and has pitting it will be difficult to obtain peak performance.

Your cartridges look pretty clean and new , on older more used carts a little test is always to have the det pin out and place inside each cartidge before the cap is fitted...if it slides all the way down without snagging its good , if it snags it may missfire ( due to drag etc) if it drops down way too easily it may not blowback enough due to loss of gas pressure.

Snagging cartridges can be cleaned again, much too loose ones are best binned or at the least marked so they are used as the last round etc

DOC

DOC wrote:

Your cartridges are correct , the other ones would reach the end of the follower in the mag

Correct det pin should look like this




Is yours pretty clean looking ? if its been allowed to rust and has pitting it will be difficult to obtain peak performance.

Your cartridges look pretty clean and new , on older more used carts a little test is always to have the det pin out and place inside each cartidge before the cap is fitted...if it slides all the way down without snagging its good , if it snags it may missfire ( due to drag etc) if it drops down way too easily it may not blowback enough due to loss of gas pressure.

Snagging cartridges can be cleaned again, much too loose ones are best binned or at the least marked so they are used as the last round etc

DOC

Hmm...thanks for the tips.

They are all pretty loose...guess thats the problem.

Having struggled with c.p. cartridges using 5mm caps and those temperemental 5mm o-rings for so long I converted my MAC to the open cartridge system.
All it needed was a change of Detonator Pin to the type shown above and a supply of cartridges which I got from Francky.

Firing performance was far, far better. The first mag' full ripped through just like I'd hoped so a second mag was shoved in as quickly as I could manage.
This time, a couple of stoppages through failed extraction from the chamber or ejected rounds not clearing the port spoiled the fun but that's modelguns for you I guess

The third mag was a series of constant jams. Rounds would fail to extract, to the point one was jammed so tightly in the chamber I couldn't get it out. Stripping the gun to get at the barrel and chamber it was necessary to clench the cartridge with pliers and work it free.
Once out, the cause of all the problems was obvious. So much dirt and residue had built up around the Det.Pin's head it was jamming inside the cartridge.
A good scrape and clean later and the MAC was ripping through a mag'full again.

I've learned now that firing 30 rds creates enough dirt to warrant cleaning the Det.Pin. 60 rds will cause stoppages, trying 90 just ain't worth the hassle of constant jamming.

This is true with almost every model sub machine gun I have. 90 c.p. type rds is about the most they'll manage before the Det Pin and chamber has to be cleaned. The MAC fires so fast, 30 open type rds. have fired in about a second and a half, but it only takes that long to get the wider head of the Det. Pin filthy too.

The best way to prolong the fun is to have two MACs