Interesting malfunctions

[strobro32]

My RPB M11A1 380 small grip, small metal mags has been malfunctioning all this week. I called the big Kahunas and they are stumped. The word "impossible" was mentioned by 3 different people.

With both semi and full auto, the gun stops shooting with the bolt back, rounds in the mag, and a live round in the chamber. I drop the mag, shoot the round in the chamber and reinsert the mag. 1 X would be a fluke but it happened about 40 times in 500 rounds with 9 different MAC/RPB mags. I've tried switching uppers, 2 OEM cast bolts and 1 CNC bolt. Same result.

Richard recommended stretching the mag springs, checking feed lip contact and using high speed video. I tested about 250 rounds and it happened at lease once to 8 out of 9 mags.

Chili wanted me to find out what Jake was wearing.

Sam recommended checking selector holes and replacing the safety which I did. I still need need to test.

Has anyone else experienced this?

 

[Offmarksman]

Just spitballing, but any chance the mag catch or the holes holding it in are out of round and allowing thr mag to move around?

 

[strobro32]

Negative, no more than usual.

 

[Slowmo]

That is intriguing. I won’t call it “impossible,” but I can’t envision how it is possible. Has the primer been struck on the live round in the chamber?

 

[strobro32]

I had 2 out of 40 rounds with light strikes.

 

[skoda]

Sounds like the disconnector is disconnecting when it's not supposed to though I'm not sure about the live round in the chamber when the bolt is half way across the receiver.

 

[sniperdoc]

I'm not a MAC expert, but I've seen Semi Auto pistols that would, because of a worn/out of spec Extractor or Ejector;
1) Fire a round,
2) Extract the fired case from the chamber,
3) Not eject the fired case,
4) Feed a new round, but still retain the previously fired case

Usually, this causes a "stovepipe jam", but, sometimes, the new round will feed and the old case will catch in the edge of the slide, and cause the slide to lock back. In an Open Bolt, if the last example occurs, perhaps the empty case would simply fall out, because the sear would catch the bolt, and the spent case wouldn't have any pressure holding it in the gun

 

[K2]

Hi Strobo32,

Just curious.

- When it stops firing you have the trigger pulled?
- Assuming the trigger is pulled is the sear down or is the sear holding the bolt open?

I've seen the Macs "spew" live ammo from the magazines after the shooter gets off the trigger. In the little Mac it might be taking a bank-shot into the chamber after the round spews from the magazine.

I guess I'd look into what is stopping the bolt. Is it the sear popping up or something else.

Great puzzle,

K2

 

[BlackBelt]

Ok, this one has me stumped. Only thing I could think of that hasn't already been mentioned would be is there a crushed spent casing getting down in the fcg?
And are you using factory ammo?

 

[strobro32]

Sam thinks it's a combination of a worn safety and enlarged selector pin hole. The safety was a little worn and there was a small burr on the selector pin hole. I have fixed both. A worn safety can allow the sear to overtravel and result in stoppages mid mag, like a worn trip.

I will test soon and if the problem continues, I will try to get video for Richard but I don't have a high speed camera. Even if I can't see what's happening, I want video just to prove it's happening.

While replacing the safety, It was interesting to see some of the older RPB parts. The safety was very loose and would not go into safe mode. It uses a plunger instead of the ball bearing and spring. The trigger spring only had one loop on the left side for the trigger pin to pass through.

 

[skoda]

While replacing the safety, It was interesting to see some of the older RPB parts. The safety was very loose and would not go into safe mode. It uses a plunger instead of the ball bearing and spring. The trigger spring only had one loop on the left side for the trigger pin to pass through.

You've been around long enough to know that as soon as you get a MAC you replace as many of the fire control parts with Lage or PS parts that you can get. Those factory parts are made out of slag, old soup cans, and dog turds.

 

[strobro32]

Ok, this one has me stumped. Only thing I could think of that hasn't already been mentioned would be is there a crushed spent casing getting down in the fcg?
And are you using factory ammo?

I was using my reloads and the range ammo, which is reloads.

 

[brenbuilds]

Is the sear geometry okay? A worn or out of spec sear could allow the bolt to bounce forward slightly when it engages the corresponding surface on the bolt. This could be worsened by a worn selector, or selector hole allowing the bolt to slightly slap the top cartridge in the magazine into the chamber.

Many of the old MAC cast parts are not always in spec, especially after wearing in. My JAWS Avenger had stock RPB parts that caused all sorts of issues and would not even allow the MAX 10/45 Mk. 2 from being installed without the gun running away or inducing odd FTF's. It seems to have been related to the sear and selector. I replaced the suspected parts with CNC machined parts from FTF industries and it ran flawlessly.

 

[strobro32]

I seem to have many parts in my MAC box but not too many of the newer CNC made ones,

I tested this morning after tweeking the mags and replacing the safety. The replacement safety was all kinds of wackadoo and was loose in the upper. I did have one of the weird malfunctions which I caught on slow mo video. It was too dark in the shooting bay to see how it happened. Then I got a dead trigger and went home.

I replaced the safety with another safety and it functioned well. I just got back from the range again and could not reproduce the weird malfunction with 4- 30 round mags, although a had 1 case crushed in the ejection port and 5 failures to fire with a bolt dropped on a live round in the chamber.

That is an improvement.

 

[skoda]

Have you cleaned it in the last 20,000 rounds?

The OEM cast safety on my M11/9 was worn and bent when I first got it. It didn't even keep the gun from firing on 'Safe' but it would allow the sear to catch the bolt occasionally. Mr. Lage's magic got rid of that problem.

 

[strobro32]

I put another 150 rounds through it today. No sign of the impossible malfunction. Something about adjusting the mags and replacing the safety did the trick. I will look into getting some of the new CNC. I like the idea of a steel safety that won't dent like the aluminum one.

The whole gun is clean and lubed for the N TX SMG match tomorrow.

 

[chili17]

I put another 150 rounds through it today. No sign of the impossible malfunction. Something about adjusting the mags and replacing the safety did the trick. I will look into getting some of the new CNC. I like the idea of a steel safety that won't dent like the aluminum one.

The whole gun is clean and lubed for the N TX SMG match tomorrow.

Match is rained out!

 

[MitchWerbellsGhost87]

Have you tried running the gun with the trip removed?

Nevermind I see you solved it.

Replacing the safety can help because if the safety is missing or worn it can allow the sear to drop too far, which will cause the disconnector to disengage the sear when it isn’t supposed to…. Or something like that?

 

[Jmacken37]

Dang Matt, you get to hang out with Chili at those awesome TX matches. Totally jealous!

I'm wearing my brown protected by Chris&Wesson T-shirt...

 

[strobro32]

I need one of those.