Benelli MP90S .22LR feed "contact"

[-TT-]

I now understand the problem. It's in the design of the mags, maybe generic to all rimfire mags, but due to the entry into the Benelli chamber, more of an issue for mine.

The top round in these magazines tips up less (i.e. rides "flatter") when the mag is full. The tip-up angle rises quite a bit by the time the last round is ready to feed. This means the first round is especially likely to contact the bottom edge of the chamber. This contact, in turn, can cause it to bump at the top (inside the chamber itself) as it pushes further into battery. Later rounds go straight in, which is what I've been seeing.

It seems to have less to do with the mag lips (i.e. where the round rests as it tips), and more to do with the width of the space at the very back, where the rim of the brass stops. The full stack seems to "pinch" the rim of the round tighter, kind of jamming it into the top of the mag and preventing it from rotating upward as much. As the stack empties, the spring pressure decreases, and the rounds slide up more readily.

Here are photos as the mag empties (SK Pistol Match ammo). See how the tip of the round rises noticeably as it progresses?

Loaded with 5

Loaded with 4

Loaded with 3

Loaded with 2

Loaded with 1

Now - what to do to correct it? Polish the inside of the mags? Modify the gap at the very rear to force the rims into the upper corner and make them tip? Or is it basically a within-tolerance thing, and simply a result of the mag's shape and design?

[Gwhite]

Two questions: 1) Does it function reliably? 2) does it shoot accurately?

Because of the rimmed case design, feeding .22's is always a bit tricky. I have seen a fair amount of variation in how sharp the chamber edge is on Benellis, and the sharper edge can shave lead off the bullets if things are really bad. Doesn't seem to hurt the accuracy to a visible degree.

Basically, I think it's the nature of the beast and if it isn't actually causing problems, I wouldn't worry about it.

[-TT-]

Two questions: 1) Does it function reliably? 2) does it shoot accurately?

1) Mostly 2) Absolutely

The operational issue is the main reason I'm still looking at this. The lower edge of the chamber mouth can get quite built up with lead shavings and wax, and it's just not always cycling properly, IMO.

[Gwhite]

The change in angle is not so much a magazine issue as it is the results of stacking rimmed cases on one another. The Benelli compensates for this a bit by staggering the rims, but not by much. If you look at the follower, the last round has the back end forced off to the right. Subsequent rounds will alternate. Pardini magazines stack rounds more straight one on top of the other, and deal with the changing angle by having the follower pivot a bit.

The team I help coach has one Benelli that shaves lead & lube a bit. The one thing I noticed is the the rear of the chamber edge is mush sharper than most. I've been meaning to stone the lower edge a tiny bit, but I haven't figured out a good way to do it very efficiently. I used to be concerned about leaving part of the case unsupported until I took a look at a Pardini barrel. They have a fairly significant feed ramp built into the barrel.

[-TT-]

The change in angle is not so much a magazine issue as it is the results of stacking rimmed cases on one another.

Indeed. In fact, I can gently lift the nose of that top round and get it into a similar high angle. Alas, it immediately tips itself down due to the spring action on the lower rim. If I peek "under" the round from the nose, the rim (far back to the underside rear) is really the only point that actually touches anything.

Opening up that lower edge of the chamber mouth isn't something I'd take lightly. Irreversible, and all that. I'll keep meditating in my search for chambering enlightenment for now. And get a plastic scraper in the meantime to de-crud more often. :-)

[-TT-]

Gwhite, if you have a chance I'd love to see a snapshot of the chamber opening of one of those MP90S's that did not have this issue.

[Gwhite]

I'd actually hoped to do that today, but the team is getting ready to go to the Collegiate National Championships next week, and we've had a rash of pistol problems. Maybe I can squeeze it in tomorrow.

[-TT-]

Good priority! I had a chance to study my friend's Pardini today, interesting differences in mag rear surface, integrated ramp and chamber shape. No hurry.

[-TT-]

Just a followup with a new photo, showing the chamfering at the edge of the chamber entry. It's pretty much uniform all the way around.

The contact is at 6 o'clock, and the feed issue is minimal if the chamber area is kept clean, however that spot shaves lead and wax over time. Once it builds up to a certain point, around 500-1000 rounds, things definitely start to act up. Yes, I know that 500-1000 rounds in a .22 match pistol is about when I should be cleaning it anyway, but the issue really sneaks up and can hit at any time. So slowing it down to where I'm in charge is important to me.

Note, there's no kind of feed ramp in this gun, either on the barrel or the magazine. So I'm again considering relieving the area at 6 o'clock. Seeing how the gun has become a bit irreplaceable in the US, I'm really torn. Talk me out of it!

[Gwhite]

Yours doesn't look too awful, and if you have to clean it every 400 rounds, that's actually not too bad.

Here are photos of three Benelli chambers for comparison. Yours is similar to the middle one, with a single bevel at the chamber mouth, The pistol that shaves the least lead has a roughly 45 degree bevel like yours, and then a small, shallower bevel going inside a bit.

I'd leave it alone, or go at it VERY gently.

[-TT-]

Fascinating, thanks! Mine is definitely just a single-bevel, and it seems somewhat sharp to me. But comparing with your examples I agree it's right in the ballpark.

I guess I'll go back to focusing on the mags, and see if I can get those top rounds to sit more consistently. I've tried tweaking the way I start a string, by manually moving the slide forward into battery instead of slingshotting it, and while most rounds seat smoothly, every once in a while they'll still hang up at that edge. This tells me it's not about the force of the recoil spring, or the mags getting sticky. Subtle...

[-TT-]

That double-beveled one at the right - what do you think are the angles? The outer one seems extremely shallow, is it possible that the inner bevel is a regular 90 degree countersink, then a (say) 120 degree "on top"?

It seems odd to me that the factory would change this. Is there a big gap in the date stamps (Italian standard two letters in a rectangle on side of barrel frame) between this one and the others? Mine is "CP" - 2016.

[Gwhite]

I think it's more a manufacturing variation than a design change. All of the pistols I showed are at least 15 years old.

If I had to guess, the main bevel at the rear is 90 degrees (45 w.r.t. the breech face), and the inner one is ~ 60 degrees. They may have skipped the second bevel operation as a cost cutting step, or just forgot to do it. My understanding is that Benelli makes shotguns about 51 weeks out of the year. Every once in a while, they switch over production to making pistols. I wouldn't be surprised if details like the 2nd bevel get forgotten occasionally.

[-TT-]

I got an opinion from a gunsmith I highly trust, who recommends "throating" the chamber by polishing (and ONLY polishing) the entry bevel and inner surface, to a depth of no more than half the brass. I demonstrated the ding(s) and he immediately called them the classic symptom. He suggests that polishing will also help with extraction and improve ejection since the round will exit the chamber more easily/rapidly.

The job could be done from the rear of the frame, with some sort of holder extension in order to avoid removing the barrel itself. Apart from possibly fabricating that, it seems fairly straightforward.

I'm giving it some thought. But he did a similar job on a 9mm barrel for me and it was night and day...

[Gwhite]

I've been working on making up a polishing setup for Benelli chambers for a while. For polishing, I am planning on using VFG felt cleaning pellets. I have a 3D printed rod guide that latches into the back of a Benelli to keep things straight.

Rev 2 Rod Guide Model.gif (23.44 KiB) Viewed 2241 times

The latch (with the button) snaps over the top of the grip bolt. I'm still iterating to get the latch stiff enough to hold well without snapping off...

[-TT-]

Ooh, I like it! If you need additional units to do a 3d print fabrication run, I'm in.

I am using a similar approach now for brushing the barrel and chamber from the rear. I have a brass .22 "muzzle guard" (*) which is circular and sits at just the right height when resting on the bolt rail area of the frame. I use one hand to hold it down, along with the frame itself, while rodding the brush with the other hand. The guard makes a perfect rod guide. But it's a little bit awkward and your snap-in approach would be great.

I am also considering the VFG felts, I use them for cleaning and really like them. The main thing for the polishing job though is that they're cylindrical. The "felt bobs" typically used for chamber throating are kind of bullet-shaped. This lets them polish the entry bevel, as well as the outer region of the chamber. With a little finesse, I guess the VFGs could do both jobs though.

(*) http://www.proshotproducts.com/Muzzle-G ... p_485.html

[Gwhite]

You could get some higher caliber VFG's and spin them against a sharp file or knife edge to shape them. The felt "bobs" work too, but you'd need a different guide to center the shaft they would go on. The felt bobs the size we need will probably have a 1/8" diameter shaft, and they won't be nearly long enough to reach through the Benelli frame to the chamber, so you'd need something like a drill bit extension which will probably have a 1/4" shaft. Like this one:

https://www.mcmaster.com/3016a817

I looked at the felt polishing bobs that McMaster Carr sells, and it's very hard to tell exactly what shape, point diameter & angle they are.

[-TT-]

Hmm, interesting idea on reshaping the larger VFG. But the McMaster Carr reference is exactly the "some sort of holder extension" that my gunsmith proposed to make. Options, thanks!

[-TT-]

Success! The McMaster-Carr holder extension works perfectly to sneak a tool in from the rear of the frame. It's on the short side so I removed the elevation screw and flipped the sight up to get an extra 1/2".

We:

• Used a cone-shaped stone to gently add roughness (rotating by hand) to the inner edge of the bevel
• Polished the bevel (and only the bevel) using a felt bob with 400 compound
• Cleaned the felt and switched to 800
• Cleaned again and finished with 2000

Here's the result:

There is *no* trace of a ding on the underside of any chambered round after this. Fairly amazing what a big difference this small change makes.

[Gwhite]

Thanks for the update! I was wondering if the 4 1/2" extension would be long enough. They also sell a 9" one (for a few cents less, for some reason...)

https://www.mcmaster.com/3016a844

I'd be curious to know what size & shape bobs you used, and also the stone. The trick is getting the angle right at the diameter of the chamber.

I think rather than using a stone to start, I'd try one of the abrasive filled rubber points like these:

https://www.mcmaster.com/rubber-bonded-abrasive-points

I've got a couple pistols that could probably use the same treatment.