Warm barrels live longer?

[ejg223]

A few years ago I read about army rifles used for occasional target shooting on single fire had lower round count until accuracy fell off compared to equivalent rifles that shot full auto. It seemed that a few studies were conducted but I could not find them on the net again. On a German forum the same subject of fire cracking etc. was discussed and this link was attached. http://sv-muenchwilen.ch/wp-content/uploads/2019/04/laufverschleiss.pdf  Only issue, it is written in German. Fire cracking is something I know from developing high performance ceramics and always thought we are missing a big point in barrel wear which is thermal shock resistance / behaviour. One part of it is that stiffness reduces at higher temperature which in turn reduces stress... less stress = less cracks.

Not sure what Bartlein did to their new barrel material which claims much higher (double) round count.

edi

 

 

 

[Ronin]

Looking forwards to trying one of their new recipe barrels in high heat use rifle newt year 

[baldie]

A barrel with twice the round count and knackers reamers in short order ?

They are going to be very expensive.

[No i deer]

How many barrels can you chamber on one reamer before its knackered approx..??

[lapua]

Emperors new clothes

[ejg223]

On ‎7‎/‎30‎/‎2020 at 5:49 AM, baldie said:

A barrel with twice the round count and knackers reamers in short order ?

They are going to be very expensive.

Have you made up some of the new Bartleins? Are they that much harder on tooling? How expensive are reamers? A barrel change at almost £1000 is also not cheap.  The other way round... material that is easier to machine than say 416, with less tool wear and less round count was also not really a goal... at least I never heard of developments going in that direction.

If the function of fire cracking is mainly due to thermal shock behaviour of the material then the cure to less fire cracking is not necessarily making the material harder. My understanding an improvement is achieved by one or more of the following: lower Youngs modulus, higher thermal conductivity, longer elongation before break (bet there is a fancy term for that). Hardness might go hand in hand with some but is not the feature that helps against fire cracking.

I would love to try the new Bartlein materials in one of their carbon barrels.

edi

[brown dog]

15 minutes ago, ejg223 said:

Have you made up some of the new Bartleins? Are they that much harder on tooling? How expensive are reamers? A barrel change at almost £1000 is also not cheap.  The other way round... material that is easier to machine than say 416, with less tool wear and less round count was also not really a goal... at least I never heard of developments going in that direction.

If the function of fire cracking is mainly due to thermal shock behaviour of the material then the cure to less fire cracking is not necessarily making the material harder. My understanding an improvement is achieved by one or more of the following: lower Youngs modulus, higher thermal conductivity, longer elongation before break (bet there is a fancy term for that). Hardness might go hand in hand with some but is not the feature that helps against fire cracking.

I would love to try the new Bartlein materials in one of their carbon barrels.

edi

Sort of how what I got from what you wrote - stretch rather than crack to which the answer may or may not be hardness

I've not read more widely though, are they claiming that 'firecracking' is the primary contributor to erosion In cold barrels and addressing that is how they've doubled barrel life? 

[ejg223]

The initial report is a few years old and has nothing to to with the Bartlein development. I don't know in which way Bartlein achieved there improvement. My guess would be they reduced fire cracking as that is i think the biggest evil. 

Yes right, stretch not crack. Or thermal expansion without / less cracking. Low thermal conductivity leads to higher surface temp which induces more thermal expansion in shallower depth = more stress= more cracking. Meaning higher thermal conductivity = longer barrel life.

Edi

 

[brown dog]

Just idle and thinking:

At target bolt rifle (as opposed to machine gun) rates of fire and heating; I'm struggling to see how thermal expansion rates, or thermal coefficients are even in the game - for target, it's all over by the time the barrel warms through; wouldn't this have to be all about the properties of the surface itself?  

Other than lining the barrel, the only thing that pops into mind is pre-stressing the surface - a la autofrettage

 

[baldie]

On 7/30/2020 at 8:46 AM, lapua said:

Emperors new clothes

This.

Along with gain twists and carbon wrappings.

Non of them work. Just bull to sell something different.

I would rather have a barrel in a much shorter time frame, with better internal finish, any day of the week.

Harder steel such as Walther , will last longer, that's a fact. Softer steel such as Krieger, doesn't last, but machines better and is usually more accurate.

Reamer costs are irrelevant to me, I have over 150 of them...never been shy of buying tools. The problem comes when you want another, EXACTLY the same.

 

[Shuggy]

On 8/2/2020 at 8:30 AM, brown dog said:

Just idle and thinking:

At target bolt rifle (as opposed to machine gun) rates of fire and heating; I'm struggling to see how thermal expansion rates, or thermal coefficients are even in the game - for target, it's all over by the time the barrel warms through; wouldn't this have to be all about the properties of the surface itself?  

Other than lining the barrel, the only thing that pops into mind is pre-stressing the surface - a la autofrettage

 

My assumption is that ‘fire-cracking’ is caused by thermal stress cracking. This failure mechanism is directly related to the thermal expansion coefficient of the material. That might also give a clue to the difference in damage from single shots as opposed to full auto fire. Single shots will cause a heating/cooling cycle, which is probably more stressing to the material than putting it through a constant thermal load.

I think that Edi is also right about the importance of thermal conductivity on barrel life. This is part of the thinking behind the development of the ‘StraightJacket barrel’ by Teludyne Tech. As I understand it, this is basically a sporter weight barrel enclosed in a sleeve of thermally conductive ceramic potting compound. This was originally developed with military applications in mind.

[brown dog]

4 hours ago, Shuggy said:

My assumption is that ‘fire-cracking’ is caused by thermal stress cracking. This failure mechanism is directly related to the thermal expansion coefficient of the material. That might also give a clue to the difference in damage from single shots as opposed to full auto fire. Single shots will cause a heating/cooling cycle, which is probably more stressing to the material than putting it through a constant thermal load.

I think that Edi is also right about the importance of thermal conductivity on barrel life. This is part of the thinking behind the development of the ‘StraightJacket barrel’ by Teludyne Tech. As I understand it, this is basically a sporter weight barrel enclosed in a sleeve of thermally conductive ceramic potting compound. This was originally developed with military applications in mind.

Understood, but at single shot, surely it's all over too quickly for conductive materials to have time to 'do their thing' - aren't we talking about, effectively, dealing with a sort of surface-level hoop-stress shock load?    

[KABOOM]

We use the term fire cracking and I often wonder is it a combined action of thermal action plus erosion. Full auto is in my mind one of the harshest things you may do to a barrel. When I served in uniform 5-10 shot bursts were the norm . That had to induce a huge thermal change between bursts or if fire was more continuing the need to change barrels at 100-200 rounds to cool them off. Having set the chamber end of a M-60 MG barrel on wood right after it came off the gun at 200 rounds instant fire was the result so a large rock to keep the chamber out of the dirt.

[brown dog]

1 hour ago, brown dog said:

Understood, but at single shot, surely it's all over too quickly for conductive materials to have time to 'do their thing' - aren't we talking about, effectively, dealing with a sort of surface-level hoop-stress shock load?    

And, from that, the solution, for single shot is going to lie in either going for greater material elasticity or pre-stressing (?)

- but since autofrettaged barrels would have to be supplied ready chambered, the commercial single shot solution would most likely lie with elasticity (youngs modulus, as per edi) rather than thermal coefficient (?)

[brown dog]

27 minutes ago, KABOOM said:

We use the term fire cracking and I often wonder is it a combined action of thermal action plus erosion. Full auto is in my mind one of the harshest things you may do to a barrel. When I served in uniform 5-10 shot bursts were the norm . That had to induce a huge thermal change between bursts or if fire was more continuing the need to change barrels at 100-200 rounds to cool them off. Having set the chamber end of a M-60 MG barrel on wood right after it came off the gun at 200 rounds instant fire was the result so a large rock to keep the chamber out of the dirt.

I would stand correction, but most mg barrels are hammer forged, which does all sorts of advantageous things (for mgs) to the metal - it isn't autofrettage, but some of the effects are similar , but I agree, I would think that once they're really hot, it's about erosion: mechanical and gas wash

My most marked hot barrel memory was seeing a minimi barrel thrown on snow at about -5degC melt straight down through a foot of snow and start the vegetation beneath smoking!

[TSG]

I seem to remember GPMG barrels had stelite ( probably spelt differently) liners in the first few inches to stop any damage. All of our G36s (not a popular carbine) have chrome lined barrels which get very hot but accuracy is not that much of an issue at shorter ranges. Only my opinion but I suspect suppressors do not help much for accurate target shooting as I think they cause much overheating. 

[Shuggy]

3 hours ago, brown dog said:

Understood, but at single shot, surely it's all over too quickly for conductive materials to have time to 'do their thing' - aren't we talking about, effectively, dealing with a sort of surface-level hoop-stress shock load?    

Propellant flame temperatures can get to about 3000 degrees C over a couple of milliseconds. That’s plenty long enough for the response to be affected by the barrel material properties.

[KABOOM]

10 hours ago, brown dog said:

I would stand correction, but most mg barrels are hammer forged, which does all sorts of advantageous things (for mgs) to the metal - it isn't autofrettage, but some of the effects are similar , but I agree, I would think that once they're really hot, it's about erosion: mechanical and gas wash

My most marked hot barrel memory was seeing a minimi barrel thrown on snow at about -5degC melt straight down through a foot of snow and start the vegetation beneath smoking!

That's a bit warm sir!