The answer to Benelli slugs and pattern off from point of aim

[kampr]

This is pretty cool stuff, and it makes perfect sense. I am curious though, how much is this an issue on brands other then Benelli's ? Is it a problem likely to happen on other brands? Logically, I guess so as the mechanics are the same, but in practice is this an issue for say a Versamax or Mossberg, or are their assembly procedures less likely to lead to this issue?

I know of two VM receivers that were replaced by Remington after the guns were sent in to correct slug POI.

Their explaination, "the receiver was out of spec."

[Ragnar]

This is interesting. This issue is what pushed me to adjustable sights on my M2

[kurtm]

Booker's is my favorite! No one needs to drink my share as the doctor said its all good!

A quick up date Mr. 343 met me at the range and after ascertaining his M2 indeed did shoot 9-13" high, after a bit of interpretive tuning it now hits poa/ poi. THE GAUGE WORKS!!!

[No.343]

Just yesterday I saw Kurt work with his new gauge and fix my M2. This M2 shot slugs about eight inches high at 25 yards. After he finished I was holding at the Bravo/Charlie line on an USPSA target and hitting on the lower part of the B zone from a good cross legged sitting position at about thirty yards. I'm certainly convinced. Thanks Kurt.

[bigbrowndog]

RNT baby!!!.............................. we don't just get things done,................... we get things done RIGHT!!!

Trapr

Edited September 26, 2013 by bigbrowndog

[TRUBL]

I still like the sand bag

[J.Schmitt]

Nice work Kurt, this knowledge evolution is the stuff of legends.

Jay

[assaulter]

Kurt, there is an old adage that goes something like, "wherever the sights are when the shot breaks is where the bullet will print", or some version of that. So, assuming the "sights" are lined up perfectly when you break the shot on the shotgun, and the tube is off allignment, is the barrel shifting under recoil to align itself with the tube while the slug is still in the barrel? Then settling back into a different position after recoil?

[bigbrowndog]

When dealing with iron sights there are waay too many reasons for a bullet not to go where you intended, aimed, or pointed. Even if you thought they were perfectly lined up.

[assaulter]

That's not what I mean at all. If the gun is grouping well, you have to assume that you are aligning the sights correctly each time. If the barrel is straight, that means it has to be moving while the shot is exiting the barrel....right?

[ericm]

Kurt, there is an old adage that goes something like, "wherever the sights are when the shot breaks is where the bullet will print", or some version of that. So, assuming the "sights" are lined up perfectly when you break the shot on the shotgun, and the tube is off allignment, is the barrel shifting under recoil to align itself with the tube while the slug is still in the barrel? Then settling back into a different position after recoil?

Different pressure and/or different pressure points will change the harmonics of a barrel and change the POI (point of impact).

If YOUR sights are aligned for YOUR gun (i.e. POI is same as POA) you will never know what harmonics are acting on the barrel, and, for your purposes (hitting targets), you aren't concerned with them.

Example: Let's say I have a Glock w/ iron sights that shoots to POA @ say 25 yards. If I change out the barrel for a new barrel with all the attendant differences (dimensions, lug height, points of contact, etc.) I will probably need to adjust the sights so that the POA coincides with the NEW POI. The new POI has been determined by how the new barrel "acts" with the existing pistol frame and slide.

As this applies to our shotgun dilema, many are finding that the SGN is not "shooting where it looks" i.e. when you line up the bead with the rib and receiver (your iron sights in this case), your slugs group high and right, for example. This is due to harmonics or barrel side pressure or barrel nut tightness or whatever.

I believe the "whatever" has been found to be the alignment of the magazine tube with the receiver, which is what the gauge can measure.

If you have a correctly sighted barrel (POI) (and now I'm talking about a front AND rear iron sight set-up) that hits to POA on a "tweaked" receiver, I would suspect that the POI would shift if the barrel was put on another receiver, just like our Glock example above, and your POA would no longer coincide with your new POI.

ericm

[kurtm]

The barrel loosely slides into the receiver. The only place it is firmly attached to the receiver is around 13" forward of the receiver and about 1.33" below the centerline of the bore where the barrel ring sits around the MAG TUBE and is screwed together with the end cap. In this configuration the barrel will always point and impact where the magazine tube is pointing. Asaulter , you are trying to think of this like a rifle barrel which is screwed firmly to the.receiver and is centered on the center of the receiver, which is no where near where we are with shotgun barrel on a Benelli M1/M2/

Edited September 27, 2013 by kurtm

[assaulter]

Actally, I'm more or less looking at it like a revolver, where the sights are fixed to the barrel. I'm just trying to understand what's happening to change the point of impact. I'm guessing the barrel is actually shifting to align with the mag tube while the shot is being fired, then coming to rest in a slightly different position. I wouldnt think it would matter if the tube is pulling it one way or the other, you are looking at the sights either way, UNLESS, the mag tube is causing the barrel to move while the shot is fired. Thats all I'm trying to understand. Don't get me wrong, I'm not disagreeing at all, I'm just trying to break it down and see whats happening. If the tube is pushing/pulling the barrel off a couple moa, isn't that where you are sighting anyway due to the sights being fixed to the barrel?

Edit:After rereading what eric said, maybe that is pretty much what I'm getting at but trying to be more specific.

Edited September 28, 2013 by assaulter

[assaulter]

Also, since it fits loosely in the receiver, could you achieve better accuracy if you could make a tighter fit in the receiver?

[Ontarget]

How is the mag tube attached to the receiver?

And how is the mag tube installed during manufacture that allows so much variation between guns?

Thanks

[midengine3sgte]

this topic if very interesting. It has me thinking about trying to have the mag tube straighten or squared up to the reciever before sending my M2 to taran to have it fully worked up

[kurtm]

Assaulter it isn't a matter of the guns being "inaccurate" they will almost all throw slugs as well as a non spinning slug can be thrown. No one is saying they shoot 13" groups at 50m. What they are saying is they are tired of a good group being 13" high and left!

Yes making the barrel fit the receiver tightly would help, but only if it was screwed straight in and was torque to say 50 ft/lb, but then we would have a Super Blackeagle, or a nice bolt action Mossberg, which may be the heat for 3-gun.

[kurtm]

Ontarget the magazine tube is screwed into the receiver with about a 1 in 13 pitch Ajax thread, and it is screwed down on a molded plastic collar.......now how could that cause any variation???

[TonyH]

This is good stuff Kurt,,,, Nice job figuring this out.

Tony

[rlscriv]

Thanks again, Kurt, for checking the mag tube to barrel square on my M2, and for the awesome carrier release bar. Will wear the Redneck Tactical decal with much pride!!!!!

[kurtm]

Tarantactical, and Triangle will have gauges by early next week. Accurate Iron already has access to one.

[Colodrew]

Kurt- would an aftermarket precision mag tube solve the problem? Or only half the problem because of the female acme thread in the receiver has a loose tolerance also?

[kurtm]

The Benelli factory magazine tubes are a thing of beauty! They are very uniform dimensionally inside and out. The finish is outstanding and they seem to be very straight. I really don't think you could make a better tube as an after market part.

[jcarpenter82]

This is interesting stuff Kurt! I (finally) have an M2 on the way, I definitely have a lot to learn about this gun. May have Mike take a gander at it sometime soon.

[Michael Flatley]

Kurt/Trapr,

My M2 shoots slugs just fine, my SuperNova is a different story!

My Supernova will not hold any kind of point of aim with the mag clamp on. Now this is not a case of shooting high left or high right, but more a case of high left THEN high right and THEN somewhere else! I now shoot it with no mag clamp, though even then it does not seem as accurate as the M2. I don’t know if this is because the gun does not shoot as straight or that I don’t shoot the gun as well. Is this something you more experienced guys have come across?

I just wondered if this was a related issue with the Benelli pump!

Kurt this is excellent work you have been doing and thanks for sharing it with the rest of us.

My best from across the pond.

Mick