The hidden liability of a RDS on a shotgun

[Unobtanium]

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]

I have not noticed this problem with any of my RDS equipped 870's...

[Unobtanium]

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]

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]

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]

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]

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]

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

Who is the maker then?

[Unobtanium]

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]

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.