The hidden liability of a RDS on a shotgun

[Unobtanium]

Many people are moving to the RDS on the shotgun, myself included. The shotgun has always shot where the sights indicated, and this has been enough. You take it to the range, and it continues to do so, except that you constantly seem to be chasing your zero during sight-in. Finally it is zeroed, but when you test the cold-bore shot, it shoots high. The process begins again.

Or, you zero the gun, are happy, but note when it heats up, it begins shooting to 6 o-clock.

I fell into both camps above.

See, the shotgun barrel is typically (on combat type shotguns) retained with a magazine cap in the 6 o-clock location, which is of course affixed to the magazine tube, and the "ring" under the barrel is impinged against the cap, holding the assembly in place.

This works great as originally designed. However, during a course of fire (and shotguns heat up FAST!), the relatively thin barrel will expand. This places tension from the 6 o-clock direction on the barrel. With iron sights, the front sight will track with the barrel during this movement. With a receiver-only mounted optic, however, there is no front reference point in physical conjunction with the barrel, and shift can be noted.

Variables exist, such as how long is the mag tube, how thick is the barrel, and where is the anchor point in relation to muzzle/receiver respectively.

This is what I noted on my Benelli M1014. The first shot fired was from a cold, pristine barrel, and using the RDS. It was fired prone at 25 yards, and is seen top right. The second shot was fired directly thereafter, using the irons through the optic. Then, 25 field loads were run through the weapon as fast as humanly possible, and the 3rd shot was fired in the bottom right corner using the optic. Immediately thereafter, the irons were used to fire the 4th shot to the bottom left corner bull. Immediately after that, a 5th shot was taken usuing the RDS to confirm on the bottom right. You can see the results in this photo.

[Odin Bravo One]

None of my RDS equipped shotguns do this.

[Unobtanium]

None of my RDS equipped shotguns do this.

What models are they/where is the retention system? Using an M1014, you have nearly 16" of steel in play. Using something with a retention system much closer the receiver, you have just that much less in play.

[GJM]

Unobtanium, is it just this shotgun, or something you have noticed generally? I recall a recent post where you did a lot of stuff to your M4.

I have an M4 with a Scalarworks mount and T2, M2 with the same mount and optic, and an M2 with an Aimpoint S1 (all 14 inch barrels), and my zero seems steady from session to session with Brenneke slugs.

[David S.]

IF this is true, that would explain why I had such a hard time sighting in my PA Micro equipped 870. After 50 rounds, I'm still not satisfied that it's acceptable for shots beyond about 10 yards.

[Unobtanium]

Unobtanium, is it just this shotgun, or something you have noticed generally? I recall a recent post where you did a lot of stuff to your M4.

I have an M4 with a Scalarworks mount and T2, M2 with the same mount and optic, and an M2 with an Aimpoint S1 (all 14 inch barrels), and my zero seems steady from session to session with Brenneke slugs.

My zero is steady session to session. This is the first time i have done an optic on a shotgun, but all the math supports my observation. Try the same test I did, on yours. Let us know. The 14" Barrel may well give a less notable shift, though.

[GJM]

My zero is steady session to session. This is the first time i have done an optic on a shotgun, but all the math supports my observation. Try the same test I did, on yours. Let us know. The 14" Barrel may well give a less notable shift, though.

Is the test to shot several slugs at 25, shoot a box of something to heat the barrel up, then repeat the slug zero test at 25 yards?

How much of this do you think is related to the mag extension and barrel clamp? I don’t have barrel clamps on my 14 inch shotguns.

[Unobtanium]

Is the test to shot several slugs at 25, shoot a box of something to heat the barrel up, then repeat the slug zero test at 25 yards?

How much of this do you think is related to the mag extension and barrel clamp? I don’t have barrel clamps on my 14 inch shotguns.

I've never run mag extensions or barrel clamps. Full length mag tubes only. Milspec attachment.
You outlined the test accurately. Your barrel hanger in that photo is much closer to the receiver. I'd expect much less shift out of that than with a milspec m1014.

[GJM]

If such shifting is happening to some shotguns, wouldn’t an S1 on the rib solve the problem?

[RevolverRob]

IF this is true, that would explain why I had such a hard time sighting in my PA Micro equipped 870. After 50 rounds, I'm still not satisfied that it's acceptable for shots beyond about 10 yards.

If this is true, it becomes an excellent argument for guns that have tubes and barrels integral to the chassis of the gun (i.e., Kel-Tec KSG) or magazine fed guns with fixed barrels.

My concern here is that stringing of shots should also effect fixed sights. For instance, shooting lever guns with tube clamps not properly tightened or fitted will result in shot stringing by shot 6 or so. But if you remove the clamp or properly the clamp and fit the barrel, you won't get stringing. Regardless, stringing occurs with fixed sights and optics both. If folks are not seeing stringing/zero changes with receiver mounted ghostrings or rifle sights, then I suspect the barrel heating and moving hypothesis isn't correct nor the (sole) source of this problem. I would check tightness of the optic mount and any mounted accessories to the barrel/mag tube/receive (side saddle?), along with the stock.

If I still couldn't find a solution, I'd swap the optic to something that is known to hold a strong zero and check it. If the optic is good. I'd leave the optic off the gun and shoot several dozen rounds, rapidly to get the gun nice and hot and check zero of the iron sights. Then check the gun over for anything wiggling or loose in the barrel fit/tube nut/clamps/mounts. I'd let the gun cool down, check the tightness of everything again, and re-shoot the test a couple of more times (cooling in between). If it's a metal expansion problem, it will eventually show up in the iron sights. If barrel mounted iron sights hold zero the suggest it isn't the barrel, so use a receiver mounted rear sight and repeat the test (this will tell you if it is the receiver).

Somewhere in there, you're going to figure out what combination of sighting and shooting is causing the stringing. That'll narrow down the list of potential culprits.

PS: If you do really think it's the barrel. I'd check the square of the shoulder of the barrel as it fits the receiver during cold and hot conditions. In addition, check the tension and fit of the barrel mag-tube ring hot and cold. If loose when hot you might try shimming it with something and running a couple of dozen rounds to see if impact changes as the gun warms up. If super tight, you might consider taking a fine toothed file or stone and ever so slightly stoning the surface for a wee bit looser fit (or maybe just oiling it heavily, anything to sort of allow it to move easier when the gun is hot).

ETA: I just read your build thread. I'd be willing to bet cash money, the problem is associated with the Ti mag tube. First thing's first, check the square of the mag tube to the receiver. Just because it threaded in properly, doesn't mean it is actually square to the receiver. Next, get the gun hot, REALLY HOT, like too hot to handle without gloves hot. Strip off the handguard and check the square of of the mag tube and barrel to the receiver and to each other. Then let the gun cool down and check it again. If everything appears square, swap the Ti mag tube for the OEM tube and repeat the test.

My guess is? The titanium magazine tube is the source of your problem, because the tube has very different thermal conductivity properties than steel. They may be similar in strength, but the thinner Ti heats up AND dissipates heat much faster than steel. So the tube will heat up and move at a different rate relative to the the steel tube the gun was engineered for. It would not surprise me if the thinner Ti tube moves around more than a steel tube does as well when hot vs. cold. I bet the original steel tube acts more like an anchor for the barrel when the barrel gets hot, than the Ti tube does.