Benelli MP90S .22LR feed "contact"

[-TT-]

My Benelli MP90S .22LR has been running very reliably lately, but the other day I had occasion to eject an unfired round and noticed the side of the nose was dented. Wondering if other MP90S owners might be willing to try an experiment.

Load a single round and mark the "top" like this:

Now, load the mag and manually chamber the round by releasing the slide catch as normal.

Then, manually eject the unfired round. Does it have a dent 180 degrees opposite the mark like this arrow points to?

This is SK Pistol Match ammo, and it appears to be striking the lower edge of the chamber mouth as it enters. If I cycle the slide by hand, i.e. slowing its movement to watch each transition, it slides up into the extractor position with a positive click, and enters the chamber perfectly horizontally.

I suspect that in action, the round is not "coming up" as quickly as it needs to, but bullets exhibit the same behavior regardless of mag or position in the stack, so I don't suspect the spring, or a sticky follower. The recoil spring is fairly new, and the slide is clean. I also played with raising the mag, and even removing the bumpers from the trigger assembly and holding it, and the mag, up tightly - same thing.

I'm a little stumped and beginning to wonder if this is normal (albeit undesirable). Input appreciated!

[deadeyedick]

A good friend of mine is experiencing something similar with his mp90s world cup. In his case the gun will feed and fire two or three rounds and then drive the next round into the upper front edge of the breech bringing everything to a grinding halt.

Prior to this the pistol has digested over 5000 rounds without a hitch. Mags checked, cleaned, new unused mags tried recoil buffer replaced......nada.

I will follow this post with interest but unfortunately have nothing positive to offer.

[-TT-]

drive the next round into the upper front edge of the breech

Just to note, yes this happens, but that problem is kind of the opposite of mine. Hitting the top of the chamber mouth is because the round gets picked up "too early" and its rim fails to click into the recess on the slide, and the slide overruns the brass. In my experience it's generally a dirty buildup right around and inside the firing pin. It can also be a broken or distorted firing pin itself, which interferes with the round's rotation upward. A sticky mag can also affect this. There are some threads here about it.

Mine is not that. Mine is coming into battery at both ends of the round just fine. But it's touching at the lower edge on the way in.

Can you ask your friend to try my experiment?

[Gregbenner]

TT, I'll try it with mine, although might be a couple days, getting ready for a match.

[-TT-]

Thanks a bunch. I have a couple more tidbits.

1) I tried with CCI SV ammo, same ding appears.

2) I took a 240fps video (fastest my phone could manage). It's all over in 4 frames, but you can see in frame 2 that the front of round has reached the chamber mouth, yet the rear has not yet reached the "notch" in the mag lips that allow it to rotate. In frame 3 that has happened however. When cycling slowly, these two critical moments do not overlap.

Frame 1

VID_20180905_143151.mp4_snapshot_00.04_[2018.09.05_14.40.15].jpg (23.04 KiB) Viewed 5111 times

Frame 2

VID_20180905_143151.mp4_snapshot_00.04_[2018.09.05_14.40.33].jpg (23.05 KiB) Viewed 5111 times

Frame 3

VID_20180905_143151.mp4_snapshot_00.04_[2018.09.05_14.40.41].jpg (22.84 KiB) Viewed 5111 times

Frame 4

VID_20180905_143151.mp4_snapshot_00.04_[2018.09.05_14.40.48].jpg (22.44 KiB) Viewed 5111 times

I am soooo tempted to modify the "front of the back" magazine lips...

Or, is it a headspace issue maybe?

[Gwhite]

It doesn't look like the ding in the lead is that severe, and I suspect it is pretty common. I doubt it has any impact on accuracy.

A couple things to watch out for:

1) Any sort of burr on the face of the bolt where it pushes the round forward (and up). We had a pistol that started jamming badly after we replaced a broken firing pin. It turns out that when the tip broke off, it must have gotten jammed in the action and gouged the bolt face. There was a tiny burr that impeded the back of the round from sliding up the bolt face.

2) Similarly, any burr on the tip of the firing pin or impediment to its travel can mess up feeding. The Benellis don't have a spring to retract the firing pin. They rely on inertia and the bevel on the bottom front of the tip (which you can see in the above photo) to allow the rim of the next round to push the firing pin back. If the tip is rough, or there is fouling built up so the firing pin doesn't retract smoothly, that can also slow down the rise of the rim of the next round.

If the slide isn't coming back far enough to pick up the back of the next round, that means you've either got weak ammo, too much drag, or too stiff a recoil spring (rare). The usual culprit is a dirty chamber that isn't getting cleaned properly by a worn bore brush. See: http://www.targettalk.org/viewtopic.php ... 5&#p276366

One weird problem I ran into recently was a pistol that would trap brass not very long after a cleaning. I also noticed the pistol was REALLY dirty for the number of rounds fired. It turns out somebody had cut the recoil spring down. The slide was opening too fast for the extractor to hang onto the case, and it was also causing incomplete combustion of the powder. A new spring fixed both issues.

[Gregbenner]

I just tried your test on mine. No ding/mark whatsoever.

[-TT-]

Gregbenner - thanks, so it's not universal. I guess that's good to know, anyway.

Gwhite - yes I'm well aware of the firing pin bevel, and it's the first thing I checked. But a burr on the bolt face is a new one. Honestly, the lower edge, where that little "step" is, has always been something I've been suspicious of. I'm going to take it down and look more closely.

I'm with you on the minimal nature of the ding, but it bugs me, and it gets worse as the round count and residue increases. I was getting some pretty severe ones the other day, which is what motivated the cleaning and re-testing. So my goal is to either eliminate or root-cause them at this time.

[Gwhite]

For the OP's issue: you could try tweaking the magazine feed lips. I've got almost 25 Benellis to maintain, and I've never found it necessary, but it's critical for some pistols.

High Standards are a prime example. On those, the drag from the magazine lips controls how much the nose of the round pops up on its way to the chamber. Under static conditions, a round will fall between the lips, but if they are too wide the round will pop up too high and catch on the top of the chamber, and if they are too tight, it will hit at the bottom. There's a fair amount of wiggle room in the middle. I've got about 8 magazines for my two Victors, and I just measured the width between the lips of a couple magazines that worked well, and adjusted all the others to match within +/- several thousandths on an inch. I've never had a problem with feeding since.

Because yours is hitting at the bottom of the chamber, theory says the lips may need to be spread ever so slightly. Before getting out the pliers, I'd measure the ones you've got and see if there is any correlation between the spacing of the feed lips and the size of the ding on the bullet. If not, you need to look elsewhere. If you do decide to try adjusting them, I'd go REAL slow, and use a pair of smooth parallel jaw pliers like these: https://www.amazon.com/Parallel-Pliers- ... B009436GDO

[-TT-]

If the slide isn't coming back far enough to pick up the back of the next round, that means you've either got weak ammo, too much drag, or too stiff a recoil spring (rare).

I ruled out drag and stiff spring, the gun is clean and the spring is replaced with a factory part a few months ago.

Weak ammo is out of the picture on the first-round chambering, but it is something I've wondered about at other times. I used to shoot SK Standard Plus through it, with very few issues. But a friend suggested I try SK Pistol Match and I like it because it's a bit "softer". Still, it's not *that* different. And in this gun it shoots the tightest groups of any ammo I tried. So I'm pretty reluctant to blame it, or change.

[-TT-]

Because yours is hitting at the bottom of the chamber, theory says the lips may need to be spread ever so slightly. Before getting out the pliers, I'd measure the ones you've got and see if there is any correlation between the spacing of the feed lips and the size of the ding on the bullet.

Hours of fun ahead! Another good strategy, thanks.

[Gwhite]

If the slide isn't coming back far enough to pick up the back of the next round, that means you've either got weak ammo, too much drag, or too stiff a recoil spring (rare).

I ruled out drag and stiff spring, the gun is clean and the spring is replaced with a factory part a few months ago.

Weak ammo is out of the picture on the first-round chambering, but it is something I've wondered about at other times. I used to shoot SK Standard Plus through it, with very few issues. But a friend suggested I try SK Pistol Match and I like it because it's a bit "softer". Still, it's not *that* different. And in this gun it shoots the tightest groups of any ammo I tried. So I'm pretty reluctant to blame it, or change.

I was referring to the post about "the slide over-running the brass".

Benellis can be relatively ammo agnostic, but there are lots of minor issues that can add up to make them fussy. I have two. One bought new eons ago that has never had a problem, and one I bought used a few yeas ago. That one had all sorts of issues, but now runs just fine. It will go over 500 rounds between cleanings.

[-TT-]

More testing. I have a total of 8 mags, 7 of them the regular 6-round, and one 9-round. I'll name them "A" through "H". Typically, I use mags A-F.

I loaded one round in each, chambered/ejected and observed the mark. All were visible, and I graded them "1" (barely touched) to "5" (dinged similar to photo earlier in thread). None of the rounds were what I'd call "damaged".

[edit] Realized I should have measured one more gap, added

I think the critical mag dimensions are these (see photo):

• Mag height - light blue
  - top of catch slot to top of mag lip
  - relative height of round in feed area
• Front lip width - green
  - rearmost opening of forward area, where brass touches mag lips
• Rear lip width - purple
  - the wider the gap, the greater the tip-up angle
• Right rear lip length - orange
  - right side
  - the longer the lip, the farther the brass travels forward before rising up
• Left rear lip length - yellow
  - left/ejector side
  - ditto, with side-to-side considerations

Here are the results, sorted by depth of ding (dimensions in inches):
It's an image, I can't figure out how to format a table in the forum...

I see barely any correlation between results and dimensions. The tallest mag was the lightest hit, but the shortest mag was second lightest. The lip width is all over the map. The rear lengths, which were my favorite theory, are too.

The silver lining here is that there is at least some variation, so it's perhaps not the gun, though I guess I could run more rounds through each mag. And, mag "G", which I rarely use because it's a bit sticky for the middle rounds in the stack, is obviously ripe for tweaking. But, what to try, if anything? Again, just sharing here. Ideas welcome. Thanks.

[Gwhite]

One thing to watch out for when doing tests like this: loading one round only tests how the rounds feed off the follower. The other 4 rounds in a magazine will be feeding off the brass case of the next round, which may be very different.

I don't see an "smoking gun" in your table that might affect the issue. One thing I have noticed is that different Benellis have varying amounts of radius on the bottom edge of the chamber. I don't know how many of them came from the factory that way, or if a previous owner/coach may have tweaked them. My recollection is that there are similar variations in the radius at the bottom of Pardini chambers, although they tend to have a MUCH larger radius than the Benellis.

[-TT-]

Noted, and agreed on the one-round test. I'll do more.

My gun (about 3 years old) has a rather tight chamber and no discernable additional radiusing at the mouth. There is a hint of a chamfer, that's it.

I'm much more strongly thinking the issue is the forward edge of the rear magazine lips, and how far the round has to travel to clear them. Some of the mags are slightly longer, and others uneven side-to-side, at that spot, and I suspect this leads the round to wobble. Since the nose of the lead is already entering the chamber at that point, this causes it to contact the sides of the opening. Still trying to prove that hypothesis.

[Gwhite]

I'd ignore it & go back to shooting...

[-TT-]

I'd ignore it & go back to shooting...

Almost there. :-)

I did adjust some of my mags, both tightening the rear lip openings on the ones that tipped rounds up more than others, and very gently shaping/smoothing the forward edges where the rims roll "up" when chambering. The rounds are positioned quite uniformly among all mags now, and they operate much more crisply. That's the good part.

I also manually cycled a full stack of 5 rounds through each and discovered that the top round is getting much more dinged than the 4 below it. In fact for those, two dings appear, both on the lower side, one directly on the nose, and the other, already observed, at the midpoint. (I've stopped worrying about the latter.) Here's a picture of the dinged nose.

Things in favor of not worrying too much:

• It only happens on the top round of 5
• It doesn't always happen
• It's not very deep

That said, it's not something I like to see, and seems like it should be fixable. Maybe the friction of the tight stack is just a tad too much. Only, please don't suggest I oil the top round. ;-)

Oh, one other thing, I did check the bolt face and it was quite smooth, although I did give it a little touch with 800-grit paper at the lower edge where the rim is pushed. Firing pin is smooth and bevel retracts fully.

[Gwhite]

Another thing that can affect feeding is any burrs on the edges of the magazine lips where the round is supposed to slide. Either inside the lips, or the rear edges of the forward lips where the rim rides up.

If you want something else to obsess about, fire a round, and check the top of the round in the chamber, and the one in the magazine The case will often show a single dent (I believe) from getting hit by the rim of the previous round, or a double dent from the corners of the notch around the firing pin on the bottom of the slide.

Both bullets also show a pair of scratches from the edges of the firing pin notch. As long as the dent isn't too big, it has no apparent effect on cycling or accuracy. I've never had time to really dig into it, but I suspect an overly strong magazine spring may contribute to this. Because Benelli has made 3 different capacity magazines over the years, it's easy to accidentally get an overly long spring from one of the higher capacity magazines, especially given the mix of vintages of spare parts we have for the college pistols I maintain.

[-TT-]

I frequently see the double-dented brass. It seems to me they're pretty much expected due to the design - as the top round moves up to engage the bolt and extractor, the rising round moves with it. But, the bolt is only halfway into battery, so that little "step" inevitably hits the brass. The concern for denting is a tight fit in the chamber, which could slow the extraction. But it doesn't seem to be a big problem, so in the absence of a solution, I ignore it.

The only reason I'm obsessing on the rounds being dinged at the nose is that it does seem to impede the cycle. I had my first alibi in a while the other day, a failure to feed, and it's not something I want to see!

Thanks again for all the ideas and observations. Very helpful.

[deadeyedick]

Keep at it TT as I am sure there should not be a ding on the lead and it will undoubtedly have an effect on the flight of the bullet. That’s not obsessive....it is aerodynamics.

This in depth examination often helps others with similar problems.