MP90s last round issue

[Rudi]

Good day, hope everyone is well.
This last few months, I'm experience an issue with the last round in my 5 round magazine. The round gets chambered, but the hammer isn't cocked, off-putting to say the least. It's happening about 75% of the time and only the last round. I can of course sucessfully fire, when I load a single round in the magazine and cock manually, but this issue is with anything over one round in the magazine 2 or 5 doesn't matter.
The pistol and magazines are 26 years old. I clean the pistol every 200-300 Shots, but only today have I fully disassembled the magazines in the 2 years I owned it. The springs were a little gritty and the sides of the followers had some hardened build up so I wonder if that's causing issues, I don't know how though. I've shot Aguila successfully for two years, but replicated the issue with Eley Club and Eley OSP.
I'll get to test tomorrow with the newly cleaned magazines, but is there anything else I should keep an eye on? Main spring is clean and lightly greased, top of slide a drop of oil, and the side runners a drop. Buffer is great. Do the magazines need any additional care? Thanks as always.

[Gwhite]

Cleaning that magazines will probably help. One sign they've gotten really dirty is when the follower sticks momentarily, and then pops up, throwing the rounds up & out of the pistol... (I've seen this with a couple Benellis)

Make sure your chamber is REALLY clean. Running a used .22 caliber brush through it won't do the job. Even using an oversized brush (like .25 caliber) won't always remove very tough carbon that builds up over time. At some point "chemical warfare" is needed.

See: https://www.targettalk.org/viewtopic.ph ... 05#p300805

[-TT-]

Sticky chamber. Does the recoil feel different on that fourth round? You'll come to recognize the chamber that way, over time. :)

[Rudi]

Thanks both, so by sticky chamber, do you mean it's losing energy on the extraction?
Why then not on the 1st or 3rd shot of any magazine, always the last, I get the round chambered but pulling the trigger doesn't even give a click, I mean its NEVER the 2nd round, or 4th, always the last round in the magazine (even I load 2, the first fires, the second is chambered, but no click) . I have to pull the slide back which ejects the bullet from the chamber, then load it into the magazine, and bring the slide forward as usual. This might happen 5 or 6 or 7 magazines in a row.
I use an angled chamber bronze brush every 500-750 shots, but a nylon and pull through every session or two, and the barrel looks spotless. I could use some heavier cleaning product than balistol, to take any metal build up away, but I see no signs of any not even a single flake when bronze brushing.

yep, if this was random fair enough, but I feel there's too much consistency to be a dirty gun.
I'm having trouble to take a pic of the chamber, but its very smooth, very clean, no flakes after bronze brush.

[Gwhite]

I still suspect the magazine cleaning is the secret, but all sorts of problems can happen or be amplified if the chamber has baked on crud. If the magazine follower is tipping due to lube build up, or there's so much gunk that the pistol can't smoothly feed the 5th round, I think that would produce the symptoms you are seeing. All the earlier rounds are sliding brass on brass. Having fouling in the chamber would increase the resistance to feeding fully.

When you say it doesn't click, how far open is the slide? The lack of click can be from two things: 1) slide didn't come back far enough to cock the hammer, or 2) the slide isn't closed enough to clear the disconnector. That will depend on the vintage of your pistol. The really old ones could fire out of battery, no matter where the slide was. They fixed that by adding a lever to the top of the trigger to keep the disconnector depressed until the slide was closed. If the slide is open by more than ~ 1 mm, it won't fire. A combination of drag stripping the 5th round out of the magazine and fouling in the chamber would allow that to happen.

The only thing in the pistol that varies with round count is the magazine. 1st round jams can be aggravated by the upward force of a full magazine. The 5th round has the least upward force on it, but it's feeding off the follower, which is very different from earlier rounds.

If you don't have a borescope, you can't really see how much fouling may be in the chamber. My regular cleaning protocol sounds similar to yours, and I still had enough carbon built up that it affected things. I've checked several problematic Benellis the team I help coach has, and they all have baked on carbon in the chambers.

[Rudi]

Thanks! My version has no disconnector, straight or duck.
I have the old Benelli 1cm thick red dry fire block that it would have come with, so even a chamber stuck open till 10mm would still click.
Cleaning regime was from absorbing your wonderful videos even well before I bought the pistol but I'll give it a hit with something to dissolve out any carbon regardless.

I was met with this when I took apart the magazine today, as well as a hard to see buildup on the inner walls of the magazines which I scraped off with a blunt wooden saté stick then got the rest with a thin toothbrush. I'll get to test tomorrow at the club, I'm thinking this might just be the issue. Magazines only ever got a minimal clean, so I'm glad I dismantled it. I can order a couple of new followers from Sweden, even when clean these aren't as smooth as I think they could be.

[Gwhite]

Yeah, that looks like it could be an issue...

If you don't have a disconnector lever, and the pistol doesn't go click, then the bolt is not coming back far enough to cock the hammer. That means there's too much drag someplace. The only thing I can figure is that the magazine is so crudded up that it's adding drag, but I'm not sure how/where.

The Benelli magazines aren't the easiest to work on. I will occasionally push the follower down & scrape out accumulated lube & fouling. They will usually behave for over 10K rounds before they need a full disassembling. If you take one apart, you only need to remove the rear roll pin. The only tricky part is getting the roll pin properly lined up with the hole on the far side when you reinstall it. I much prefer the Pardini design, which you can open up in seconds with almost any small pin/punch/toothpick, etc.

It certainly won't hurt to have nice clean magazines. Hopefully that will fix it.

[Rudi]

thanks as always, its been a gem of a pistol since I bought it, and with the exception of a bad batch of Aguila (pre red logo boxes) where the heads were a little "rattly" as if there was no proper crimping, and we has many stovepipes and inaccuracies - it's performed excellently. (of course not the pistols fault)
Good tip about taking out the single pin, although it was a pretty straight forward rebuild, with no issues.

I'll be sure to update if this fixes things.

[-TT-]

I have the old Benelli 1cm thick red dry fire block that it would have come with, so even a chamber stuck open till 10mm would still click.

There's a special, second depression to allow the sear to release in the exact position that dryfire plug holds the slide. It's safe, because the hammer strikes the underside of the bolt and cannot hit the firing pin.

I bet cleaning your magazines will help. As mentioned, that last round feeds off the follower. If there's crud that keeps it from riding at the correct height to feed the round, or if the round doesn't tip up properly, then the slide will have extra friction and may be stopping early. Did you check that the mag spring was in the correct orientation front to back? The "high" end goes forward under the follower.

These guns are subtle. Little things can add up to cause problems. But when they're in shape, they'll run like clocks.

If it still happens, you might consider trying different ammo, or at least a different lot. The recoil energy it produces is one element of those little things...

[Gwhite]

There's a special, second depression to allow the sear to release in the exact position that dryfire plug holds the slide. It's safe, because the hammer strikes the underside of the bolt and cannot hit the firing pin.

That's only on the newest ones that have a disconnector lever with a bump on the top at the front. The really old ones without any lever will fire with the slide in pretty much any location. There was an intermediate version with a lever that is flat across the top. Those will only fire with the bolt 99% closed. They will NOT work with a dry fireplug/block. My two personal Benellis are of that vintage, and I've modified one by fitting it with the newer lever (with the bump) so I can dry fire it.

[-TT-]

Understood, but I thought Rudi was talking about another Benelli, not this misbehaving one.

The bolts all have the second disconnector depression, right? So If you install a trigger module with the duck-shaped lever, it will dryfire? I always thought it was odd, that the pistol was designed for it but the dryfiring disconnector wasn't installed in the module.

[Gwhite]

There is nothing special about the bolts. The disconnector lever is operated by the hardened cross pin that holds the black takedown plate for the recoil spring & firing pin in place. You can install a new style trigger in an old pistol with an old bolt, and (for the most part), it should work fine. The disconnector levers are fitted at the factory so the pistol will only fire when the bolt is less than ~ 1 mm from fully closed. If you put the trigger in a different pistol, that may be off a bit.

[Rudi]

An update for those that would like to know.
So in a nut shell my pistol would fail on the last shot of the magazine, it would chamber but the hammer wasn’t engaged.
Two days ago it was way worse sometimes two or three or four rounds from each magazine would chamber, without the hammer cocking. They fired if manually i brought the slide forward. Thought it was a magazine issue, but no.

Tonight I fired 60 shots with no problems.

I turned the cam adjustment in the trigger unit about an 1/8 turn anti clockwise, and locked it with the grub screw, which seems so far to have solved the issue.

Possibly the hammer was falling on the slide coming back but only just on the start of the return, I have an old system with no disconnector so I fear it could have been much worse.

Hope this helps someone that has this weird issue in the future.

[Gwhite]

Thanks! I will add that to my collection of weird things that can happen. The team I help coach has had some 5th round issues, but they've mostly been magazine problems. There is one that I know from personal experience. When the magazines get really dirty, the last round will hang up briefly, and then it can pop loose, which spits the last round up & out of the pistol landing on the bench in front of you. One of my students had that happen a few years back as well.

There are supposed to be two scratches on the left side of the housing for the trigger, and a single mark on that end of the cam. The mark on the cam is supposed to be in between the marks on the housing. Had the cam slipped so that it was outside of that window? I remember my Benelli messing up at one point because the cam had slipped, but I don't recall exactly how it behaved. I think it stopped cocking altogether.

We have a match tomorrow, and I think I will double check a few pistols before the shooting starts.

[Rudi]

Indeed there are two lines on the bottom at 7 and 5 o’clock which the manual say to stay between.
I had put mine about 1mm to the outside of the 7 o’clock two years ago as it brought the trigger further back for me and felt great . And it ran fine for that time until a couple months ago.
I don’t believe it slipped as I had made a photo of it but it’s possible it was a minuscule amount. Bringing it back , right onto or a tiny amount to the right of the 7 o’clock line and within the lines fixed it but brought the trigger forward several mm which I’m getting used to.
It’s been a horrible issue, and tiring to focus to overcome the anticipation a failure , a good lesson maybe but a huge relief to get it sorted.
Thanks for all the suggestions, it was also a great reason to get my magazines apart and spotless.

[-TT-]

Now you mention it, I adjusted that cam a few years ago and had some difficulties. Of course I don't remember the details now. Currently, the dot is aligned just slightly clockwise past the mark, say 7:10. I haven't touched the settings in years.

Your forward cam setting has a locking grubscrew? Mine does not, it has them only on the length and weight settings, behind the trigger.

[Gwhite]

On the older ones, the locking grub screw and the marks are on the left side, not the right.

MP90S 1st Stage Length Adjustment Cam & Locking Screw.jpg (21.16 KiB) Viewed 2441 times

"H" is the grub screw.

[-TT-]

D'oh! I see it now. Yep, on the right for mine. I did say I haven't touched it in years, right? :)

Because this cam adjusts the sear engagement, it is definitely possible to go too far - and doing so would cause the hammer to miss the sear and simply follow the bolt forward, unsafely.

[Gwhite]

On mine, the grub screw had loosened up to the point where the sear engagement varied with every shot. Occasionally that meant it wouldn't fire at all. It drove me nuts until I took it home & dry fired the trigger a bit. It didn't long to figure out what was going on.