How Hot to "open up"

[Steve Blair]

The calculation for barrel expansion is the relatively simple "hoop stress" formula. The formula and explanation may be found here. This is the same formula used for almost all vessels symmetric about the axis. We should use the thick-wall model for rifle barrels.

A hot barrel will expand more than a cool one as material properties degrade with temperature increase. The question we have to ask is: Is it significant? While the movement can be calculated or measured, it may not play a significant role in the problem at hand. Small arms is a heavily explored field and most of this work has already been done at military arsenals. There is a vast amount of information available in books and papers. That does not mean that we should stop asking questions and finding the answers.

The recent application of zone plates to iron sights is a good example. Fresnel described the phenomenon in the early nineteenth century but it took a 21st century shooter working almost two hundred years later to put the two together. This now-obvious application of old technology will enable shooters with old eyes (like me) to see their sights clearly, again.

 

[DBailey]

Hey Jim

Just out of curiosity, is that 7mm riding the bags decently or is it knocking you around a bit?

 

[falconpilot]

David,

That is my next step..It's kicking around a little...as it will for everyone, but I've been careful to watch my setup, and try to keep things lined up and the same for each shot...I seem to be able to keep things running well until toward the end of the string, and that is where things seem to start going downhill...thus me thinking it may be heat affecting my shots...no one else seems to be having this problem(at least they're not talking about it), so it forces me to go back to my setup, as you are suggesting....my next step is going to be strapping the gun into a lead slide or something of the sort, and then running a string while scoring it, and see if the stringing is present with that setup......

For me, it's about having confidence in the gun shooting where I aim...The other day at 1000 yards on string one I shot a 196/7x with the 284, then switching guns and shooting my Dasher, I shot a 200/11X with my Dasher, then switched back to the 284 and shot another 196/7...go figure..on both strings with the 284 things were going very well until toward the end of the strings where I started getting the unexplained flyers...

I hate to say it, but maybe it could be me getting a little "beat up" toward the end of the strings, and subconsciously start letting things get a little loose...My gun only weights 16.9 punds....

Jim

 

[DBailey]

I don't believe it's just you Jim, if you watch closely you will see quite a bit of it.

4.5 lbs. of lead in the stock will help to tame it. You could make it removable if you are worried about having a dual discipline gun.

 

[falconpilot]

Thanks David...I'm going to add that weight as well..it's got to help....We'll see what happens when I latch it down, and take me out of the equation....see you in Houston on the 4th/5th..

Jim

 

[Steve Blair]

I totally agree with spocks logic (to back up pdog), it is the only conclusion that makes sense.
Stainless steel is a poor conductor of heat. It takes a while for the heat to get to the outside so that you have differential heating of the "tube". what this means is the inside has indeed expanded (as metals do when heated) more than the outside of the barrel. It can only go in one direction (in). This is different to heating any other tube with even heat. A barrel (after firing) is always hotter on the inside than the outside.

Many pressure vessel calculations are predicated on a temperature gradient that is greater on the inside, like a rifle barrel. This would be the case pumping hot fluids through a heat exchanger, for instance.

The engineer I consulted to confirm my recollection earned his undergraduate and Masters degree at Cal Poly Pomona and worked for JPL for several years. He then earned his PhD from Stanford. He is a senior stress analyst at my company and a casual shooter.

Just do some research. The answers are available.

 

[camac]

ok comment withdrawn. I will look further into it and do my research - would be good to get a detailed explanation of this on here though. Does your work colleague have access to this site - can he spell it out for us.
It is different to a heat exchanger though - heat exchangers have thin walled vessels or pipes. Something different happens in vessels where walls are greater than 20% of diameter (Barrels are more like 40%). shear stresses start to influence the expansion. I do not pretend to understand the physics involved but would be nice to have someone try and drag us plebs through it

 

[camac]

Steve, In a quick search I found a paper studying the quick heating of a thick tube whilst keeping the outside of tube at room temperature. With my limited knowledge it reads like the forces are initially compressing around the inner radius I can only imagine this forces a
movement of the metal either longitudinally or in. If indeed the arc forces can hold this out, then the metal would be compressed. Can you ask your friend for a laymans explanation...???
http://www.ewp.rpi.edu/hartford/~poworp/Project/4.%20Supporting_Material/References/Jahanian.pdf

 

[camac]

OK - I don't pretend to understand this but thought I would run a simple test. 8 shots fired through my 6.5*284 (old barrel - 1 inch muzzle diameter SS). I took down my reamer bushings and before firing the .2566 bushing would not fit in the muzzle but the .2564 would. I fired my first cold bore shot and then tested the bore diameter. Low and behold the 2564 wouldn't fit anymore (too tight) but the 2652 would (This shot chronoed at 2874fps). I quickly chambered and fired another round (no delay with round in chamber so minimum case heating). Chronoed at 2942 and once again the bore size was still smaller than original but 2562 still fitted. The next shots were 2957, 2948, 2962 and at this point the barrel started opening up again to the point even the 2566 fitted in. I also noted that after about round 4 the outside temp of the barrel was starting to warm up.
I think I will do this test again but the first few shots did indeed appear to "restrict" the barrel before then opening up. Velocity continued to rise though...? Now I am confused.....

 

[TonyR]

Jim.
As Steve suggested, you might want to give IMR7828SSC a try in your Shehane. I had some bolt lift problems toward the end of some match strings with H4831SC that i have not had with 7828SSC. In my guns, it loads to the same MV pretty much grain for grain with 4831SC but you can load it heavier than 4831SC which suggests that it gets the velocity with less peak pressure. What you are describing happens to me when I run the Shehane up to 3000 fps. I can see the harmonic node in the ladder but the vertical gets really bad compared to 2800 fps or 2900 fps. I think that it at least some of it is due to unstable powder burning at high pressure rather than barrel temperature by itself.

 

[camac]

Tony - as a side the 6.5*284 above was also loaded with 4831SC. I found similar things also happening (if not slightly worse) with 4350, yet others see no problems at all......?

I might post these findings back on the main page to stir up some interest and see if we can get a good range of opinions

 

[TonyR]

camac.
i had the same experience with H4350 vs H4831SC in my 6.5X284 as well. I do intend to test H4350 in some larger volume 7mm cases within the next month but I am expecting it to work best at less than the maximum velocities I've been getting with 7828SSC, RL-22 and MRP and/or with lighter bullets. H4831SC and IMR7828SSC are the only ones I have used in matches with the 180VLD so far so I am basing my opinions only on round robin test results with those powders. i think a new thread would be a good idea because we are moving away from Jim's question.

 

[camac]

Tony, I am wondering if it isn't all related to the original question a little. H4350 a faster hotter powder seems worse in my rifle as far as affecting velocity through the string, and hence vertical (When loading 4350 for competition I actually used staggered loads dropping charge slowly through the string). With H 4831 I can better control the gain in velocity by controlled firing rate?? I think a couple of threads may come out of this - one closer tied to the original question - big, hot burning cartridges? and powders? and barrel opening up? The other - still closely related - are the powder choices related to this?? do some do better than others and for what reason. The hodgon powders do very well ES wise if you let the barrel cool back to normal temps (regardless of environmental temperature but seem to react to shot timing and barrel heating. I believe theses may be related to the original thread but propose it isn't the powder or primer reacting to the heat but vice versa, the powder is creating the heat and affecting the barrel expansion/contraction and hence velocity and hence vertical. I think this is also one of the reasons the 6BR and Dasher etc show very little vertical.
Shot timing in the 6.5*284 has become critical for me in controlling velocity spread and vertical. If I punch out a string I know I am going to get climb, and if I break out of the node -vertical scatter.

 

[TonyR]

camac.
I think you are right about the powder-generated heat being one of the prime causes. Barrel length, twist rate, bore diameter, and even bullet diameter can add up to problems at the end of a string. I usually shoot long strings without cooling when I am developing match loads because circumstances beyond our control at matches can result in a lot of shooting without cooling. I want to replicate match conditions as much as possible when I test. NECONOS sells little thermometer strips that tape to the barrel. I put one over the chamber on one barrel to see if barrel temperature had any effect on velocities or vertical. With the Shehane, the temperature would rise from 105 degrees to 150 degrees shooting a 20 shoot string in 10 to 15 minutes. I could see something less than 1 fps per round increase in MV but that can be negated by just sorting cases from heavy to light and bullets from light to heavy in the string. I didn't see any increase in vertical dispersion. I don't bother measuring it anymore. Some of this may be due to my using slower powders in long barrels because I think that combination gives a wider more forgiving accuracy node but they do take some tuning with the VLD bullets. There are enough variables that others could have a completely different experience but this is what has worked for me.

 

[camac]

Thanks Tony, I also like to final test all loads in match conditions as you do. In all my other rifles I see a similar velocity rise to yours (maybe 10-20 fps over a match string - if you ignore the first 2). Shot to shot ES much tighter, just a general rise. The 6.5 * 284 is worse for me though and I ended up with 4831SC over 4350 for the reason of "less rise". I know some guys get very good ES and vertical from 4350 so I (like you) also imagine this is a very complex beast. It also appears to be worse in my old savage barrel than my later Kreiger and Maddco. AS the savage appears much rougher in finish I can only imagine there is another interaction with fouling and or friction coefficient also increasing velocity through the string. A very complex scenario to get my poor old head around (maybe it isn't really) but at least it makes more sense to me than "heat warping" for consistent bullet rise during strings.
I have contemplated more than once converting to a 284 Shehane after going through 3 barrels in 2 years and fighting this velocity rise/heating issue. Very interested to know if the 284 SHehane is more consistent. From the outside it seems to be. I end up shooting my 6BR or 308 at the longer ranges quite often to save barrel life.

 

[Tozguy]

The situation where accuracy seems to go away after a certain number of shots reflects a threshold somewhere. An interesting but unusual explanation was given but I can't remember the source. Best as I can remember it goes like this:
The whole barrel is part of the rifle's combustion chamber. Powders have different temperature stability ranges and may have a stable burn rate within normal weather limits and ammo storage conditions. But if the inside of the bore gets hot beyond a certain point then even stable powders's burn rate will start to increase dramatically. The temperature threshold beyond which the powder burning 'goes nuts' would seem to involve the hottest part of the gun ie the inside of the barrel as opposed to the lower chamber temperature. The temperature of the powder itself has less to do with explaining the fliers than does the temperature of the combustion chamber.

 

[TonyR]

camac.
I do believe that the 284 Shehane has an advantage over the straight 284 Win with respect to the original question of vertical dispersion. I believe that it can get to the same velocities at lower pressures due to the increased case capacity. I shoot with a number of great shooters who use the 284 Win and get velocities close to what I am getting, but I think that may be do to differences in barrels as much as anything else. Most claim to be close to 2900 fps with the 284 Win and that is my basic match velocity for the Shehane. I recently chambered a new barrel that gave me about 50 fps more velocity than I expected and I have another barrel that gives me 50 fps less. I have considered chambering a barrel first with the 284 Win then opening it up to the Shehane just to settle this in my mind, at least, once and for all, but I have too many more important things to do right now. The only negative I have about the Shehane is the loosening of primer pockets that occurs with the rebated case head, but that happened with the 6.5X284 as well and I think that problem is made worse by higher pressures. So if the Shehane does nothing more than let you achieve the same velocities at lower pressures, that is a definite advantage. I also believe that my latest "fast" 7mm barrel would get the Shehane to 3000 fps with the 180s and I doubt that anyone is doing that with a straight 284 Win in any barrel under match conditions. In any event, I think 2000+ rounds with match accuracy and much better wind drift is very reasonable with the Shehane, a big improvement over the 6.5X284.

 

[camac]

Thanks for the feedback Tony, There is a fair following of straight 284's here and I am one of the last remaining 6.5*284 shooters at the local ranges - there are a few left but not many. (No Shehanes yet!). In AUS the price of components and labour (Barrels and gunsmithing) is significantly higher than the states (nearly double) so replacing barrels is not popular. I have gone through 3 since taking up the 6.5 just over 2 years ago. This means I have purchased and chambered 5 barrels in this time - 2 current and 3 older. This is done whilst still shooting my 6BR and 308 fairly regularly too.
Personally I am looking at the Shehane (excitedly) in exactly the way you propose - hopefully acheive the same velocities at lower pressure - maybe enjoy some extended barrel and brass life? I would be very happy if I could get the 180's shooting well at 2900+. My biggest concern is the number of 284 shooters here that cannot get the 180 Bergers shooting well - most shoot the 168SMK which to me doesn't offer as much incentive except 300-600m. If I was 100% certain I could get the Bergers shooting I would switch today.