Firecracking

[Re-Pete]

How would you rate this.............excessive, not much? It's one of our 6.5x47's, a Savage mod 12 F/TR with an 8 twist 26"  Bergara barrel.

Round count is 2680, and it will still do 1/2 moa on a good day.

Pete

[justin credible]

So long as you’ve still got complete rifling I personally wouldn’t worry about it. 

 

[meles meles]

"...it will still do 1/2 moa on a good day."

We think that answers your question, ooman. 

In our opinion, people get far too worked up over barrel wear and, a related issue, cleaning barrels. Allow us to make a few observations. 

Imagine a perfect bar of steel, the initial ingot made to the correct compositional specification, cooled properly and then forged into a concentric round. It now has a fine, uniform, equiaxed grain structure throughout. A hole is then gun drilled right down the middle of it to create the bore. At the face of the bore, all the grain boundaries have now been exposed. Those boundaries are where many of the alloying elements have settled. The sulphur compounds, for instance, used to make the steel free cutting. Let us assume, first, that the bore is now going to be cut rifled. A cutter is passed through the bore, say 5 times, to cut the rifling. More grain boundaries have now been exposed. 

When the barrel is used, the bullet passes down the barrel, accelerating as it does so. Behind the bullet comes a supersonic flamefront, at around 2,500 C and pushed along by a pressure of, let's say, 55,000 psi. That hot, expanding gas is looking for the path of least resistance. There's a bullet acting like a stopper ahead of it, and the brass cartridge case has expanded behind it to obturate the breech. The gas, if the proof test was successful, isn't easily going to break through the perfect steel we've made our barrel from. So, where can it go ? Remember those grain boundaries? The weak paths between the steel grains, full of potentially soft, low melting point compounds? That's where the gas is initially going to find a low resistance path, scouring and melting its way into the grain boundaries. 'Firecracking' we call it.

As the bullet moves forward the pressure is relieved a little and the temperature also drops as heat is absorbed by the barrel's mass. A few inches forward of the chamber the 'firecracking' is far less severe. The bullet is travelling maybe 20" inches or more through this relatively good barrel, picking up spin. Unless the throat is horrendously eroded to the extent that the starting pressure as the bullet sets into the rifling is significantly reduced, it's going to be accurate and not suffer much in the way of velocity loss. Think about it. A Paradox gun only had 2" to 3" of rifling and they were pretty accurate. 

So, don't worry too much about a little throat erosion. If the last few inches of rifling and the crown are good that is far more important than the throat unless the throat wear will almost allow the cartridge case to drop into the bore. 

Cleaning ? Why scrape out the copper that is coating those weak grain boundaries and protecting them ? Why pour solvents in that will eat away at the steel ? 

Cut rifling ? Why expose the grain boundaries more than you need to ? Why not hammer forge or button rifle, or better still, swage or flow-form ? Seal those boundaries and impose a compressive residual stress on them to keep them shut. 

Yes, we know. We're a heretic. 

 

 

 

 

[No i deer]

My 7mm saum barrel throat that's done 1700 shots on mainly H1000.. still shot well at 1000yds back in july

[Popsbengo]

57 minutes ago, meles meles said:

"...it will still do 1/2 moa on a good day."

We think that answers your question, ooman. 

In our opinion, people get far too worked up over barrel wear and, a related issue, cleaning barrels. Allow us to make a few observations. 

Imagine a perfect bar of steel, the initial ingot made to the correct compositional specification, cooled properly and then forged into a concentric round. It now has a fine, uniform, equiaxed grain structure throughout. A hole is then gun drilled right down the middle of it to create the bore. At the face of the bore, all the grain boundaries have now been exposed. Those boundaries are where many of the alloying elements have settled. The sulphur compounds, for instance, used to make the steel free cutting. Let us assume, first, that the bore is now going to be cut rifled. A cutter is passed through the bore, say 5 times, to cut the rifling. More grain boundaries have now been exposed. 

When the barrel is used, the bullet passes down the barrel, accelerating as it does so. Behind the bullet comes a supersonic flamefront, at around 2,500 C and pushed along by a pressure of, let's say, 55,000 psi. That hot, expanding gas is looking for the path of least resistance. There's a bullet acting like a stopper ahead of it, and the brass cartridge case has expanded behind it to obturate the breech. The gas, if the proof test was successful, isn't easily going to break through the perfect steel we've made our barrel from. So, where can it go ? Remember those grain boundaries? The weak paths between the steel grains, full of potentially soft, low melting point compounds? That's where the gas is initially going to find a low resistance path, scouring and melting its way into the grain boundaries. 'Firecracking' we call it.

As the bullet moves forward the pressure is relieved a little and the temperature also drops as heat is absorbed by the barrel's mass. A few inches forward of the chamber the 'firecracking' is far less severe. The bullet is travelling maybe 20" inches or more through this relatively good barrel, picking up spin. Unless the throat is horrendously eroded to the extent that the starting pressure as the bullet sets into the rifling is significantly reduced, it's going to be accurate and not suffer much in the way of velocity loss. Think about it. A Paradox gun only had 2" to 3" of rifling and they were pretty accurate. 

So, don't worry too much about a little throat erosion. If the last few inches of rifling and the crown are good that is far more important than the throat unless the throat wear will almost allow the cartridge case to drop into the bore. 

Cleaning ? Why scrape out the copper that is coating those weak grain boundaries and protecting them ? Why pour solvents in that will eat away at the steel ? 

Cut rifling ? Why expose the grain boundaries more than you need to ? Why not hammer forge or button rifle, or better still, swage or flow-form ? Seal those boundaries and impose a compressive residual stress on them to keep them shut. 

Yes, we know. We're a heretic. 

 

 

 

 

we know you also expert in shiny hard stuff

[Re-Pete]

It's a cheapo Bergara barrel, so button rifled and honed a little. I'd like to know if s/steel barrels vary  much in their reaction to high energy powders like RS60, which is what this one cut it's teeth on.

I know about the metallurgy, and I certainly don't over clean........a little copper and a bit of glassy carbon is of no consequence, as long as the rifle shoots well.

Pete

[Brillo]

18 hours ago, justin credible said:

So long as you’ve still got complete rifling I personally wouldn’t worry about it. 

 

Says he who........😁😁😁

[justin credible]

27 minutes ago, Brillo said:

Says he who........😁😁😁

That wasn’t firecracking mate, it was utter shite chambering work which has now been rectified.

[ejg223]

14 hours ago, meles meles said:

"...it will still do 1/2 moa on a good day."

We think that answers your question, ooman. 

In our opinion, people get far too worked up over barrel wear and, a related issue, cleaning barrels. Allow us to make a few observations. 

Imagine a perfect bar of steel, the initial ingot made to the correct compositional specification, cooled properly and then forged into a concentric round. It now has a fine, uniform, equiaxed grain structure throughout. A hole is then gun drilled right down the middle of it to create the bore. At the face of the bore, all the grain boundaries have now been exposed. Those boundaries are where many of the alloying elements have settled. The sulphur compounds, for instance, used to make the steel free cutting. Let us assume, first, that the bore is now going to be cut rifled. A cutter is passed through the bore, say 5 times, to cut the rifling. More grain boundaries have now been exposed. 

When the barrel is used, the bullet passes down the barrel, accelerating as it does so. Behind the bullet comes a supersonic flamefront, at around 2,500 C and pushed along by a pressure of, let's say, 55,000 psi. That hot, expanding gas is looking for the path of least resistance. There's a bullet acting like a stopper ahead of it, and the brass cartridge case has expanded behind it to obturate the breech. The gas, if the proof test was successful, isn't easily going to break through the perfect steel we've made our barrel from. So, where can it go ? Remember those grain boundaries? The weak paths between the steel grains, full of potentially soft, low melting point compounds? That's where the gas is initially going to find a low resistance path, scouring and melting its way into the grain boundaries. 'Firecracking' we call it.

As the bullet moves forward the pressure is relieved a little and the temperature also drops as heat is absorbed by the barrel's mass. A few inches forward of the chamber the 'firecracking' is far less severe. The bullet is travelling maybe 20" inches or more through this relatively good barrel, picking up spin. Unless the throat is horrendously eroded to the extent that the starting pressure as the bullet sets into the rifling is significantly reduced, it's going to be accurate and not suffer much in the way of velocity loss. Think about it. A Paradox gun only had 2" to 3" of rifling and they were pretty accurate. 

So, don't worry too much about a little throat erosion. If the last few inches of rifling and the crown are good that is far more important than the throat unless the throat wear will almost allow the cartridge case to drop into the bore. 

Cleaning ? Why scrape out the copper that is coating those weak grain boundaries and protecting them ? Why pour solvents in that will eat away at the steel ? 

Cut rifling ? Why expose the grain boundaries more than you need to ? Why not hammer forge or button rifle, or better still, swage or flow-form ? Seal those boundaries and impose a compressive residual stress on them to keep them shut. 

Yes, we know. We're a heretic. 

 

 

 

 

Na, I don't buy it.

Grain boundaries are always there no matter what you do with steel. Fire cracking can be seen in many other areas and materials such as in furnaces without any pressure. Fire cracking is a function of thermal shock.  Extreme hot surface expands more than the materials allows,  only option left is to crack on the surface.  To decrease amount of fire cracking a steel with higher thermal conductivity would help as well as a material with lower Youngs modulus as well as a material with higher yield strength. (More thermal stretch before break/crack)  Those with a lot of time on their hands could calculate the surface stress due to thermal shock in a barrel.

edi

 

 

[meles meles]

Interesting non sequitur there, ooman, but we aren't selling anything and you can believe the earth is flat if you wish.  For the others, yes, extreme temperature alone can cause thermal cracking, but does anyone seriously believe that a supersonic flow of erosive and corrosive gas at over 55,000 psi and 2,500 C isn't going to melt the intergranular compounds preferentially and make matters worse? Oh, and these days, the thermodynamic calculations don't take long, a relatively proficient student can churn them out in minutes on a decent cluster.

[ejg223]

It does not make any sense what you are saying.

think it through again and start over. Maybe a bit more research?

edi

[Re-Pete]

One more point......the exposure to the flow of hot high pressure gas is very brief, probably of the order of millisecs, so comparison with firecracking in applications where the heat is continuous rather than momentary, isn't really valid.

Pete

[meles meles]

1 hour ago, ejg223 said:

It does not make any sense what you are saying.

Attached is an electron micrograph of the throat of a carbon steel gun barrel. To our  small, simple and clearly inferior mind, the cracking does indeed seem to follow the grain boundaries rather than being transgranular and forming along those atomic planes which would represent the low resistance paths to simple thermally induced stresses. Equally, we interpreted the back scattered electrons fluorescing at the grain boundaries as being indicative of the presence of sulphides, particularly as they generated the same emissions spectra as suphur. Maybe we got that wrong too. As for the brief duration of the pressure, we foolishly assumed that the Arrhenius reaction rate applied. Aren't we silly ? For those of you who know better than us, feel free to correct us. We're open minded and eager to benefit from your wisdom. Perhaps you could explain to us why, given that the temperature duration is so small, the surface layer of the steel undergoes a transformation from ferrite to austentite  and then cools to form martensite ?  Maybe collar us at the 32nd International Ballistics Symposium on May 8 - 9th, next year ?

Something to bear in mind is that though we hobbyist shooters refer to the crazing in the throat as 'firecracking', maybe we shouldn't consider it to be the same 'firecracking' as seen in boilers, furnaces and bread ovens. The service conditions are different, thus the causal mechanics may be different too. Just a thought. Feel free to ignore it.

 

[ejg223]

Exactly Pete,

we do know that the heat is transformed into the barrel, we do know that there are only millisecs. From the amount of heat and the short time and the heat transfer rate of the material one can assume how hot the surface of the barrel can be. Also how much it expands vs a tiny bit into the surface. This difference in temperature leads to stress and cracking. In material with or without sulphur, with or without chrome etc.  Of course there is the chemical as well as abrasive element also but I think the main killer is firecracking/thermal shock.

Heckler and Koch did tests and came to the conclusion that 20 round bursts then cool had longer barrel life than rifles that were shot single shot on the range.  

edi

[meles meles]

2 hours ago, ejg223 said:

think it through again and start over. Maybe a bit more research?

edi

 

*chucks qualifications in bin, pops on the Dunce's cap, sits attentively at the front of the classroom*

"Educate us, edi."

[Re-Pete]

So shooting strings is better for the bore then?................that's a good thought when considering how much ammo I'm currently getting through on the electronics...............😊

Pete

[No i deer]

4 hours ago, meles meles said:

 

*chucks qualifications in bin, pops on the Dunce's cap, sits attentively at the front of the classroom*

"Educate us, edi."

😂🤣

[Shuggy]

I have read a few research papers on this subject and I kind of think that you are 'both right'. The papers stated that small arms barrel damage is caused by thermal stress cracking, which forms along the exposed grain boundaries. There is a reason why a lot of military small arms use hammer forged barrels with chrome lining.

Mind you, having seen the accuracy tests on the LMT Sharpshooter rifle after 10,000 rounds, I sometimes think that we overthink the effects of fire cracking on stainless barrels, as long of course that the cartridge is not an 'over bore' design. 

[Brillo]

This thread is proving to be very interesting, especially meles meles light hearted technical explanations.

Correct me if I’ve misunderstood the bones of this, but reading between the lines it seems the consensus is that fire cracking in the throat has no real bearing on accuracy or barrel longevity. If that’s the case, what does?

A few of years ago I had a 1:12 twist barrel on my FTR rifle that was like a laser. I went the whole one season with a lot of competition successes. Then during a 300 yd comp at the Imperials the groups opened up wildly. What caused that?

[Leeman]

My take on that and it's only an opinion is that the roughness of the bore either starts to strip copper off the jacket leading to inconsistencies of flight, ie flyers, or that build up and roughness combined affects the dwell time in the barrel causing a previous tuned load to shoot erratically.  I do notice a build up of copper as the round count gets beyond a certain level, I don't do enough testing at that stage to check whether the ES's have started to increase as well, meaning the groups are likely to open up.    Once I notice the V count starting to drop in what was a previously good barrel and borescope inspection shows roughness, I just generally shrug my shoulders and get a new barrel fitted.

[KABOOM]

On 10/23/2020 at 4:54 AM, meles meles said:

Attached is an electron micrograph of the throat of a carbon steel gun barrel. To our  small, simple and clearly inferior mind, the cracking does indeed seem to follow the grain boundaries rather than being transgranular and forming along those atomic planes which would represent the low resistance paths to simple thermally induced stresses. Equally, we interpreted the back scattered electrons fluorescing at the grain boundaries as being indicative of the presence of sulphides, particularly as they generated the same emissions spectra as suphur. Maybe we got that wrong too. As for the brief duration of the pressure, we foolishly assumed that the Arrhenius reaction rate applied. Aren't we silly ? For those of you who know better than us, feel free to correct us. We're open minded and eager to benefit from your wisdom. Perhaps you could explain to us why, given that the temperature duration is so small, the surface layer of the steel undergoes a transformation from ferrite to austentite  and then cools to form martensite ?  Maybe collar us at the 32nd International Ballistics Symposium on May 8 - 9th, next year ?

Something to bear in mind is that though we hobbyist shooters refer to the crazing in the throat as 'firecracking', maybe we shouldn't consider it to be the same 'firecracking' as seen in boilers, furnaces and bread ovens. The service conditions are different, thus the causal mechanics may be different too. Just a thought. Feel free to ignore it.

 

Not being expert the lines of the cracks look to be nothing more to my uneducated eye than a path of least resistance.  

[Bangbangman]

Interesting thread- thanks for the expertise, Badger.