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Unobtanium
10-19-2017, 02:59 PM
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.

http://i68.tinypic.com/2ex0y0j.jpg

Odin Bravo One
10-19-2017, 03:00 PM
None of my RDS equipped shotguns do this.

Unobtanium
10-19-2017, 03:09 PM
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
10-19-2017, 03:14 PM
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.
10-19-2017, 03:23 PM
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
10-19-2017, 03:27 PM
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
10-19-2017, 03:33 PM
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.

20987

Unobtanium
10-19-2017, 03:37 PM
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.

20987

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
10-19-2017, 03:49 PM
If such shifting is happening to some shotguns, wouldn’t an S1 on the rib solve the problem?

RevolverRob
10-19-2017, 04:08 PM
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.

GJM
10-19-2017, 05:20 PM
This is starting to sound like one of those Glock threads, where the answer is to stick with OEM parts.

txdpd
10-19-2017, 05:48 PM
Wandering zeros on shotguns aren't unheard of.

A four inch shift in zero after ripping off 25 rounds isn't anything that I would worry about. That's a whole lot of shooting in a short time frame and it's still close enough for a shotgun. The only place your going to realistically do that kind of shooting is a three gun match and you have a pretty good idea about your shift in zero.

Thy.Will.Be.Done
10-19-2017, 05:55 PM
If such shifting is happening to some shotguns, wouldn’t an S1 on the rib solve the problem?

I would imagine it COULD, assuming it is mounted far enough forward... assuming we really KNOW why the OP has noticed stringing after heated up.

Odin Bravo One
10-19-2017, 07:29 PM
None of my RDS equipped shotguns do this.

870 with a 12” barrel + T-1 Micro
Super X-2 with a 22” cut barrel + RMR
Super X-3 with a 28” barrel and 7 round magazine + Burris Speed Bead

Unobtanium
10-19-2017, 08:02 PM
The mag tube wouldn't heat at the same fate as the barrel no matter what it was made of.
I have verified that it is square and true.
When the zero shifts, remkcong and replacing the cap instantly restores zero, and it is noted that the cap is tighter when removed, indicating impingement due to thermal expansion, as it does not feel tighter to remove if I wait until the weapon has cooled, which also returns zero.

Unobtanium
10-19-2017, 08:04 PM
870 with a 12” barrel + T-1 Micro
Super X-2 with a 22” cut barrel + RMR
Super X-3 with a 28” barrel and 7 round magazine + Burris Speed Bead

Those shotguns have hangers that attach much closer to the receiver. Less barrel length to heat by far and thus less expansion and tension change at the juncture.

RevolverRob
10-19-2017, 09:40 PM
The mag tube wouldn't heat at the same fate as the barrel no matter what it was made of.
I have verified that it is square and true.
When the zero shifts, remkcong and replacing the cap instantly restores zero, and it is noted that the cap is tighter when removed, indicating impingement due to thermal expansion, as it does not feel tighter to remove if I wait until the weapon has cooled, which also returns zero.

No, neither the barrel nor mag tube would heat at the same rate. But in this case, titanium and steel heat at different scales and thus have different linear change per temperature increase.

Linear Length of Change is:

Length of Change (dL) = Original Length in meters (L0) x alpha (thermal linear expansion coefficient) x change in temperature in Cº (dT).

In this case, titanium has an alpha (Ti-A) = 8.4x10^-6 (0.00000084) and 4140 steel (most specific steel I could find) has an alpha (4140-A) = 12.3x10^-6.

So let's look at those dTs -

L0 = ~19" = 0.4826m
Let's go with a temperature change of 75ºF to 150ºF (that's a dT of 41.6667ºC)
Ti-A = 8.4x10^-6
4140-A = 12.3x10^-6

dL-Titanium = 0.000016891m or 0.00665"

dL-Steel = 0.00024735m or 0.00973"

So based solely on thermal expansion coeff - titanium should not change length more than steel, given the same temperature change. But we're not dealing same temperature range...across the two tubes.

Instead, you have to calculate both thermal conductivity and the heat transfer coefficient. In this case, steel has higher conductivity than titanium and as a result steel has a lower heat transfer coefficient. For a given thickness, length, and area, steel will transfer heat less than titanium.

This means that when you're firing and your barrel heats up the same temperature range, your Ti tube and steel tube do not. And in this specific instance, it takes more barrel radiant heat to heat the steel tube up to make it move. How much more? Well, the thermal conductivity of steel is twice that of titanium and as a result the heat transfer coefficient is ~1/2 that of titanium. Which means the temperature needed to establish the same temperature change in steel as titanium is twice the amount of temperature.

So, we calculate half the temperature change for the steel tube to the titanium tube.

So what is dL of steel with half the temperature change? 0.0032" - OR half the change of titanium.

____

Seriously, swap the magazine tubes and find out if it cures the problem. I'd bet it does.

Odin Bravo One
10-19-2017, 10:20 PM
Those shotguns have hangers that attach much closer to the receiver. Less barrel length to heat by far and thus less expansion and tension change at the juncture.

Ah.

So it sounds like it’s your shotgun that has a hidden liability with an RDS rather than shotguns in general.....

txdpd
10-19-2017, 11:14 PM
Ah.

So it sounds like it’s your shotgun that has a hidden liability with an RDS rather than shotguns in general.....

It sounds like people are putting a whole lot of though into a non-issue. This thread reads like "why does the engine and brakes on my Honda Accord overheat when I race it at the track?" or "My gun does weird things when I do things with it that it's not designed to do."

Whether it's drilled or forged, you barrel is going to have to get straightened and stressed relieved as part of the manufacturing process. There isn't a lot of incentive for barrel makers to sink a ton of money into stress relieving shotgun barrels so that people can heat them up stupid hot and then try to test for zero. If it doesn't doesn't permanently warp when it gets hot, it's probably stress relieved enough. If you're lucky your barrel will hold zero when you do retarded things with it. If you're not so lucky, you just have a normal shotgun barrel.

RevolverRob
10-19-2017, 11:27 PM
It sounds like people are putting a whole lot of though into a non-issue. This thread reads like "why does the engine and brakes on my Honda Accord overheat when I race it at the track?" or "My gun does weird things when I do things with it that it's not designed to do."

First, I've tracked a couple of stock Honda Accords before. It's not the brakes or engine that overheats, it's the clutch, always a weak link when you try to roll hot out of a corner or launch hard.



Whether it's drilled or forged, you barrel is going to have to get straightened and stressed relieved as part of the manufacturing process. There isn't a lot of incentive for barrel makers to sink a ton of money into stress relieving shotgun barrels so that people can heat them up stupid hot and then try to test for zero. If it doesn't doesn't permanently warp when it gets hot, it's probably stress relieved enough. If you're lucky your barrel will hold zero when you do retarded things with it. If you're not so lucky, you just have a normal shotgun barrel.

In this case, I think it's not the barrel. I think it's the mag tube. I think the use of Ti in the mag tube results in more movement of the mag tube when the gun gets to the same barrel temperature than it would with the stock steel mag tube. In turn the tube is clamped to the barrel and causes a wandering zero at a temperature/operating range that is probably close to "normal".

Unobtanium
10-20-2017, 12:18 AM
No, neither the barrel nor mag tube would heat at the same rate. But in this case, titanium and steel heat at different scales and thus have different linear change per temperature increase.

Linear Length of Change is:

Length of Change (dL) = Original Length in meters (L0) x alpha (thermal linear expansion coefficient) x change in temperature in Cº (dT).

In this case, titanium has an alpha (Ti-A) = 8.4x10^-6 (0.00000084) and 4140 steel (most specific steel I could find) has an alpha (4140-A) = 12.3x10^-6.

So let's look at those dTs -

L0 = ~19" = 0.4826m
Let's go with a temperature change of 75ºF to 150ºF (that's a dT of 41.6667ºC)
Ti-A = 8.4x10^-6
4140-A = 12.3x10^-6

dL-Titanium = 0.000016891m or 0.00665"

dL-Steel = 0.00024735m or 0.00973"

So based solely on thermal expansion coeff - titanium should not change length more than steel, given the same temperature change. But we're not dealing same temperature range...across the two tubes.

Instead, you have to calculate both thermal conductivity and the heat transfer coefficient. In this case, steel has higher conductivity than titanium and as a result steel has a lower heat transfer coefficient. For a given thickness, length, and area, steel will transfer heat less than titanium.

This means that when you're firing and your barrel heats up the same temperature range, your Ti tube and steel tube do not. And in this specific instance, it takes more barrel radiant heat to heat the steel tube up to make it move. How much more? Well, the thermal conductivity of steel is twice that of titanium and as a result the heat transfer coefficient is ~1/2 that of titanium. Which means the temperature needed to establish the same temperature change in steel as titanium is twice the amount of temperature.

So, we calculate half the temperature change for the steel tube to the titanium tube.

So what is dL of steel with half the temperature change? 0.0032" - OR half the change of titanium.

____

Seriously, swap the magazine tubes and find out if it cures the problem. I'd bet it does.

It really doesn't matter their differences in thermal properties, as the mag tube is heated by the barrel 1/4" away through radiant conduction, and the barrel is heated directly by friction and combustion. The thermal expansion coefficients are the smallest part of that puzzle.

DocGKR
10-20-2017, 12:19 AM
I have not noticed this problem with any of my RDS equipped 870's...

Unobtanium
10-20-2017, 12:19 AM
It sounds like people are putting a whole lot of though into a non-issue. This thread reads like "why does the engine and brakes on my Honda Accord overheat when I race it at the track?" or "My gun does weird things when I do things with it that it's not designed to do."

Whether it's drilled or forged, you barrel is going to have to get straightened and stressed relieved as part of the manufacturing process. There isn't a lot of incentive for barrel makers to sink a ton of money into stress relieving shotgun barrels so that people can heat them up stupid hot and then try to test for zero. If it doesn't doesn't permanently warp when it gets hot, it's probably stress relieved enough. If you're lucky your barrel will hold zero when you do retarded things with it. If you're not so lucky, you just have a normal shotgun barrel.

It isn't a stress issue of the barrel. It's physics. No-matter how flawless the barrel is, when you double the pressure on one side of it...it's gonna bend. Ever so slight. Just like hanging a can on a rifle barrel. You often get a POI shift. It's not the barrel's inherent stressors that cause it, either.

Unobtanium
10-20-2017, 12:20 AM
I have not noticed this problem with any of my RDS equipped 870's...

Do your 870's have the near full length of barrel before the hanger, or are they more conventional with the barrel hanger mid-barrel or closer to the receiver? It seems the barrel hanger location is the issue, as you have t he whole length of the barrel vs. just half or less of the barrel expanding.

JBhunter45
10-23-2017, 12:38 PM
I have a similar issue with my M4.
Sighted irons in at 50 yards with Rio slugs to shoot the 12x12 stage at last years Pin Shoot.
On the 90yd targets, after a tube full of slugs, point of impact shifted 18" to 2' low.

I haven't tried replicating on paper at the range to confirm or tried different ammo.
Using a Numrich Gun Parts full length steel tube, only other mod is a Mesa side saddle.

RevolverRob
10-23-2017, 04:47 PM
It really doesn't matter their differences in thermal properties, as the mag tube is heated by the barrel 1/4" away through radiant conduction, and the barrel is heated directly by friction and combustion. The thermal expansion coefficients are the smallest part of that puzzle.


Do your 870's have the near full length of barrel before the hanger, or are they more conventional with the barrel hanger mid-barrel or closer to the receiver? It seems the barrel hanger location is the issue, as you have t he whole length of the barrel vs. just half or less of the barrel expanding.


It isn't a stress issue of the barrel. It's physics. No-matter how flawless the barrel is, when you double the pressure on one side of it...it's gonna bend. Ever so slight. Just like hanging a can on a rifle barrel. You often get a POI shift. It's not the barrel's inherent stressors that cause it, either.

So, if you're convinced it is the barrel attachment ring positioning, have you contacted Benelli about this, to find out of it is normal?

Unobtanium
10-23-2017, 07:42 PM
So, if you're convinced it is the barrel attachment ring positioning, have you contacted Benelli about this, to find out of it is normal?

No, I doubt it's worth my time. They are just an importer.

Thy.Will.Be.Done
10-23-2017, 07:44 PM
No, I doubt it's worth my time. They are just an importer.

Who is the maker then?

Unobtanium
10-23-2017, 07:49 PM
Who is the maker then?

Benelli Italy, subsidiary of Beretta Holdings. The phone number you call is just to their American importer side. They can handle warranty issues and tell you part numbers and check availability, but the only "building" they do is sawing off the barrels on SBS guns that come in with 19.75" barrels and setback sights. They don't do any actual technical stuff beyond that , much less have the kind of data I'd need of want, etc.

Shotgun
10-23-2017, 10:01 PM
Some shotgun barrels are worse than others with walking shots when they heat up and warp. I have actually seen a barrel where the warping was visible to the naked eye after the barrel got hot. Cryogenically treating the barrel is thought to lessen point of impact changes with a cold to hot barrel. Many shotgun target shooters Cryo their barrels. Visit 300below.com in this regard. This is just an example of the first vendor I pulled up from a Google search regarding cryogenically treating a barrel. Cryo might not be on the radar of most defensive/combat shotgunners, and it might not be worth the cost for a home defense shotgun. But, if one is trying to eliminate changes in point of impact from a cold to hot barrel, it is worth investigating.

Unobtanium
10-23-2017, 11:15 PM
Some shotgun barrels are worse than others with walking shots when they heat up and warp. I have actually seen a barrel where the warping was visible to the naked eye after the barrel got hot. Cryogenically treating the barrel is thought to lessen point of impact changes with a cold to hot barrel. Many shotgun target shooters Cryo their barrels. Visit 300below.com in this regard. This is just an example of the first vendor I pulled up from a Google search regarding cryogenically treating a barrel. Cryo might not be on the radar of most defensive/combat shotgunners, and it might not be worth the cost for a home defense shotgun. But, if one is trying to eliminate changes in point of impact from a cold to hot barrel, it is worth investigating.
Cryo won't alter this. Design change is necessary to address it.