High Velocity Throat Erosion

[Mike Matteson]

Interesting and watching. Order out a bore scope yesterday. It will be in Montana before me. There only two rifles that I have put a lot of rounds thru. 1 I have a petty close count what I have put down range with. The other I would be only guessing. The one thing is I just don't go out and shoot and shoot. Once I develop a load for that rifle, that's what I stay with from then on. That can take several hundred rounds to to that including brake in.

 

[jessej]

You would need a new reamer made from something even harder to chamber it. Then hope it cuts smooth enough not to chatter it up. Makes me think a new process similar to nitriding that makes the surface of the metal harder and smoother might be a more cost effective route.
Just thinking out loud, and enjoying the brainstorming.

Possibly ruff cut the chamber by EDM process then finishing with some other method.

 

[Pdvdh]

What's possible versus affordable are two different matters. Not gonna by a $300K NASA toilet seat for my outhouse.

 

[mcdil]

A very interesting thread.
As a note: we have a .416 barrel that had not been cleaned on a regular basis. In fact it was so "dirty" that for 12+" the lands were fully filled and the only way you could detect them was the corrosion event. Upon cleaning the barrel- a few days and 80patches- the bore showed fire cracking for 18" plus. This barrel had 500 rounds in it. IF the grooves are packed/dirty/filled - just what are we engraving? Or are we sending a "button" down the barrel? Something has to give and the "friction heat" probably approaches a welding event.
Our 260Rem has light fire cracking for about 1" with 1500 rounds through it - a gas gun.
Inversely we have a .460barrel with the same round count- immaculately taken care of- and the fire cracking is only a few inches in length.
Our 260Rem has light fire cracking for about 1" with 1500 rounds through it - a gas gun.
We have some barrels in which the "fire cracking" appears to have lifted off leaving a "oatmeal" like structure behind. I have seen this phenomena in another field of work and could be replicated quite easily - MEMS, super smooth finishes (Single digit Angstrom level). We could reproduce this "finish" up into the single digit micron values. Testing to see if the fire cracking lifted or it is bad material is another problem, but it sure is not helping things concerning the "accuracy" formula.
We are running a series test of materials attempting to directly replicate the fire cracking and isolate the individual variables. To the point of material- 416 is not a heat resistant material. It should also be noted, that outside of powdered metals, you can have large differences in a single bar of material resulting in a barrel being different than the immediate barrel following on the same stick. "Large" being how big of a "microscope" do you view the parameters with. We are testing materials with 3x the heat resistance- however like all engineering attempts "compromise" is the gorilla in the room.
The large gain twist- in our opinion works.... and works very well.

The only thing that comes quickly to mind would be a PM'd D2 Tool Steel with the third tempering cycle conducted in concert with a plasma nitride. Now, how on earth to do anything to it for engraving and chamber reaming would be quite interesting. At least the chamber could be r eamed with a diamond grinder and honed as such. Have fun with the cost of that tooling. Broaching, ha, ha.

D2 is designed to perform when red hot, so I don't think it would be susceptible to fire cracking, unless it chemically doesn't play well with nitrocellulose and nitroglycerine at superheated levels, but the nitriding helps here as well. Not too sure in that regard. Either way, complete through harden ability is off the chart, and the PM process would ensure charpies are doubled over standard D2 bar.

I think the only manufacturing method would be to understand the density and associated growth rate from testing samples, then premachining accordingly, then heat treating. After heat treating, you can't do anything to D2 steel, practically anyway.

 

[jessej]

Steve, Sorry for contributing to your thread sliding off the road like a pickup truck on a dirt road in a rain storm west of Gillette.


Ya know. Old fashions made at home always taste better.

 

[Fiftydriver]

Coming late to the party. Have a bit of experience with extreme rounds and have learned a little bit about throat life. Many factors effect throat life. In my testing, velocity has very little to do with it. I have done extensive ballistic testing with my wildcats using a very wide range of velocity loads. For example, My 26 Stalker will do 4100 with a 100 gr TTSX bullet. Top loads with the 156 gr berger will top 3450 fps out of the same rifle. And most of my wildcats have similar velocity potential spreads.

keys to long barrel life no matter the velocity i would rank in this order:

1. simple bore temp control

2. throat dimensions

3. positioning of bullet in throat

4. powder used

5. expansion ratio of chambering

lets review each.
1. Bore temp eats more throats then anything. Keep a barrel cool and they will last a very long time, get them hot and keep shooting and it really does not matter how fast your velocity is, you will damage your throat. A very non exact example i tell my customers. shots 1-3 out of a cold bore will do X amount of bore damage, a fourth shot will do as much damage as the first three and a fifth shot will do twice the damage as the first three……. Point being, bore wear increases exponentially the more rounds you shoot in a string which is why those that demand a minimum of 5 shot groups to mean anything is a joke. Does not matter if your running 4100 fps or 3200 fps, if you keep your bore temps low, your bore will last a very long time.

2. throat dimensions
there is a reason factory barrels often erode faster then a properly machined custom throat. It is largely because throat diameter is much looser then most custom throats. When a throat is over sized, the hot gases from the powder will blow by the bullet body and have a blow torch effect on the throat because it will be hyper accelerated as it passes through this small space between the bullet body and throat. Its not uncommon to see factory throats be 0.001 to 0.002" larger in diameter then nominal bore diameter. In contrast, every reamer i have made has a throat diameter that are 0.0003-0.0005" over nominal bullet diameter. Not only does this very tight fit limit the flame jetting action but also greatly improves a rifles accuracy, consistency and also makes the rifle much less dependent on bullet seating depth for accuracy and consistency. Loose throats burn throats faster, tight throats slower.

3. related to this, if you seat a bullet within 10 thou of the lands it will also limit the blow torch effect of hot gas blowing by the bullet before the bullet seals in the bore. If you have a 0.300" long freebore, alot of hot gas will blow by your bullet before it seals in the bore. A properly dimensioned throat diameter as mentioned above helps limit this no matter the seating depth but longest throat life is generally had with the bullet seater close to the lands.

4. some powders simply produce more heat then other powder. Some feel ball powders are less abrasive to the throat the. Stick powders and i am sure there is some degree of truth to this. However flame temp is much more critical to bore life. most recent manufactured powders have chemicals that help keep temps down as much as possible. One comparision i often share is how VV570 and RL33 will produce nearly identical performance and results but RL33 will have a measurably lower bore temp with same number of rounds down the bore because of its lower flame temp.

5.expansion ratio is a huge factor in bore life. Much more then velocity has. For example, a 22-250 loaded with a 40 gr bullet to 4100 fps will have a dramatically longer bore life then my 26 Stalker with 100 gr loaded to the same 4100 fps. Why is that. Well, the expansion ratio of the 22-250 is quite high. Meaning the powder charge in the 22-250 will convert to gas and expand several times over as it fills the volume of the bore. The 26 Stalker has a much lower expansion ratio. Case powder capacity volume will expand very little. this means fast burning powder can be used in the 22-250 compared to the 26 Stalker. Fast burning powder generally produces lower bore temps the slow burning powder simply because the slow burning powder releases more energy over a longer period of time, subjecting the bore to more heat.

tips i give my customers customers have nothing to do with limiting velocity. I tell them to limit shot strings to only 3 shots and let barrel cool. Seat your bullets close to lands if bullet design permits it. keep the bore clean.

i also do tell customers not to red line their pressures but this has more to do with the increased blow torch effect as pressures increase, not because of velocity being hard on barrels. Plus, they just eat up brass life for an extra 50-75 fps.

if you really want to increase bore life, lets talk about forward ignition systems. That will blow your mind as far as how performance will dramatically increase as will bore life!! If you want to discuss those tests we can on a different post.

 

[Pdvdh]

You figured a way to make forward ignition cost effective for the masses?

 

[Fiftydriver]

You figured a way to make forward ignition cost effective for the masses?

Nope, not in function but in theory. thats why none of my wildcats have this feather. The practicality on smaller chamberings just is not there. For 408 cheytac and larger, certainly more practical. Lapua class borderline. If there is enough interest i will post a refresh of my testing of this idea which is very interesting as far as results go. Probably learned more about internal ballistics with this testing then from anything else.

 

[mcdil]

Coming late to the party. Have a bit of experience with extreme rounds and have learned a little bit about throat life. Many factors effect throat life. In my testing, velocity has very little to do with it. I have done extensive ballistic testing with my wildcats using a very wide range of velocity loads. For example, My 26 Stalker will do 4100 with a 100 gr TTSX bullet. Top loads with the 156 gr berger will top 3450 fps out of the same rifle. And most of my wildcats have similar velocity potential spreads.

keys to long barrel life no matter the velocity i would rank in this order:

1. simple bore temp control

2. throat dimensions

3. positioning of bullet in throat

4. powder used

5. expansion ratio of chambering

lets review each.
1. Bore temp eats more throats then anything. Keep a barrel cool and they will last a very long time, get them hot and keep shooting and it really does not matter how fast your velocity is, you will damage your throat. A very non exact example i tell my customers. shots 1-3 out of a cold bore will do X amount of bore damage, a fourth shot will do as much damage as the first three and a fifth shot will do twice the damage as the first three……. Point being, bore wear increases exponentially the more rounds you shoot in a string which is why those that demand a minimum of 5 shot groups to mean anything is a joke. Does not matter if your running 4100 fps or 3200 fps, if you keep your bore temps low, your bore will last a very long time.

2. throat dimensions
there is a reason factory barrels often erode faster then a properly machined custom throat. It is largely because throat diameter is much looser then most custom throats. When a throat is over sized, the hot gases from the powder will blow by the bullet body and have a blow torch effect on the throat because it will be hyper accelerated as it passes through this small space between the bullet body and throat. Its not uncommon to see factory throats be 0.001 to 0.002" larger in diameter then nominal bore diameter. In contrast, every reamer i have made has a throat diameter that are 0.0003-0.0005" over nominal bullet diameter. Not only does this very tight fit limit the flame jetting action but also greatly improves a rifles accuracy, consistency and also makes the rifle much less dependent on bullet seating depth for accuracy and consistency. Loose throats burn throats faster, tight throats slower.

3. related to this, if you seat a bullet within 10 thou of the lands it will also limit the blow torch effect of hot gas blowing by the bullet before the bullet seals in the bore. If you have a 0.300" long freebore, alot of hot gas will blow by your bullet before it seals in the bore. A properly dimensioned throat diameter as mentioned above helps limit this no matter the seating depth but longest throat life is generally had with the bullet seater close to the lands.

4. some powders simply produce more heat then other powder. Some feel ball powders are less abrasive to the throat the. Stick powders and i am sure there is some degree of truth to this. However flame temp is much more critical to bore life. most recent manufactured powders have chemicals that help keep temps down as much as possible. One comparision i often share is how VV570 and RL33 will produce nearly identical performance and results but RL33 will have a measurably lower bore temp with same number of rounds down the bore because of its lower flame temp.

5.expansion ratio is a huge factor in bore life. Much more then velocity has. For example, a 22-250 loaded with a 40 gr bullet to 4100 fps will have a dramatically longer bore life then my 26 Stalker with 100 gr loaded to the same 4100 fps. Why is that. Well, the expansion ratio of the 22-250 is quite high. Meaning the powder charge in the 22-250 will convert to gas and expand several times over as it fills the volume of the bore. The 26 Stalker has a much lower expansion ratio. Case powder capacity volume will expand very little. this means fast burning powder can be used in the 22-250 compared to the 26 Stalker. Fast burning powder generally produces lower bore temps the slow burning powder simply because the slow burning powder releases more energy over a longer period of time, subjecting the bore to more heat.

tips i give my customers customers have nothing to do with limiting velocity. I tell them to limit shot strings to only 3 shots and let barrel cool. Seat your bullets close to lands if bullet design permits it. keep the bore clean.

i also do tell customers not to red line their pressures but this has more to do with the increased blow torch effect as pressures increase, not because of velocity being hard on barrels. Plus, they just eat up brass life for an extra 50-75 fps.

if you really want to increase bore life, lets talk about forward ignition systems. That will blow your mind as far as how performance will dramatically increase as will bore life!! If you want to discuss those tests we can on a different post.

Great write up. The only area I would debate with you is in number 3 above. Here's why. If a bullet of normal cup and core design is touching the lands at the time of ignition, it has no momentum prior to any velocity achieved. Therefore, to overcome the static friction it has against the lands to engrave the bullet from a standing start is substantial. At this point, the bore is still not sealed as the bullet is sitting in the throat held out of the bore area by the lands like cleats. Since the effort to engrave in this scenario is very high, the only thing that will allow that effort to be achieved is a high differential pressure at the back of the bullet. Remember, the bore is still not sealed. So, this higher differential pressure has to be attained while jetting is taking place. In other words, the jet is stronger prior to sealing, and it's concentrated right at the critical throat to lands juncture. Now, because it is close to sealing, it will stop fairly quickly, but the jet is very high during the event.

Counter that with an appropriate solid copper design that, with the right geometry, reduces friction yet is more snug in the throat area while also starts back further having substantially more momentum into the lands with less starting resistance and extrusion gap around the perimeter. Sure, the distance is further, but I'm not sure the time to get there is any different due to the ease of acceleration, and the differential pressure at the point of sealing is significantly less, meaning the jet is also significantly less. This jet is also displaced over a larger area of the throat, less concentrated at the initiation of the lands.

Okay, back to analyzing cup and core only. I agree with your logic on number 3 in general for a bullet of that design, but I do think there is a hill crest for optimum distance off the lands to reduce the effect as opposed to an ever increasing slope of improvement to touching the lands, if that makes sense. Of course, every individual chamber will be different, because of how it is reamed. A very loose chamber at SAAMI maximums will suffer immensely no matter what. This is also another reason I slightly dread when we get to the .358" and .375" calibers as SAAMI specs and barrel manufacturers play differently in this regard. This is in relation to custom barrel manufacturers mainly. Trying to get any detailed information from the factory folks is somewhat futile, and I'm sure the tolerances are at least double the custom guys anyway, probably more.

 

[jessej]

You figured a way to make forward ignition cost effective for the masses?

Undoubtedly someone has opened up and threaded a flash hole, screwed a tiny tube into it that ends just below the base of a seated projectile.

 

[Fiftydriver]

Great write up. The only area I would debate with you is in number 3 above. Here's why. If a bullet of normal cup and core design is touching the lands at the time of ignition, it has no momentum prior to any velocity achieved. Therefore, to overcome the static friction it has against the lands to engrave the bullet from a standing start is substantial. At this point, the bore is still not sealed as the bullet is sitting in the throat held out of the bore area by the lands like cleats. Since the effort to engrave in this scenario is very high, the only thing that will allow that effort to be achieved is a high differential pressure at the back of the bullet. Remember, the bore is still not sealed. So, this higher differential pressure has to be attained while jetting is taking place. In other words, the jet is stronger prior to sealing, and it's concentrated right at the critical throat to lands juncture. Now, because it is close to sealing, it will stop fairly quickly, but the jet is very high during the event.

Counter that with an appropriate solid copper design that, with the right geometry, reduces friction yet is more snug in the throat area while also starts back further having substantially more momentum into the lands with less starting resistance and extrusion gap around the perimeter. Sure, the distance is further, but I'm not sure the time to get there is any different due to the ease of acceleration, and the differential pressure at the point of sealing is significantly less, meaning the jet is also significantly less. This jet is also displaced over a larger area of the throat, less concentrated at the initiation of the lands.

Okay, back to analyzing cup and core only. I agree with your logic on number 3 in general for a bullet of that design, but I do think there is a hill crest for optimum distance off the lands to reduce the effect as opposed to an ever increasing slope of improvement to touching the lands, if that makes sense. Of course, every individual chamber will be different, because of how it is reamed. A very loose chamber at SAAMI maximums will suffer immensely no matter what. This is also another reason I slightly dread when we get to the .358" and .375" calibers as SAAMI specs and barrel manufacturers play differently in this regard. This is in relation to custom barrel manufacturers mainly. Trying to get any detailed information from the factory folks is somewhat futile, and I'm sure the tolerances are at least double the custom guys anyway, probably more.

Have had this debate many times and there is value and truth on each side. I would reply that with a bullet seated within 10 thou of the lands, when the primer ignites the base of the powder column, the entire powder column is pushed forward long before even 1/2 the total powder has been ignited. This means the powder granuals are actually, physically pushing the bullet out of the neck and into the bore to seal it very quickly. Yes SOME amount of gas does blow through the powder column and around the bullet, that is a given and that is unavoidable, however you can limit the flame torch effect dramatically.

i would also add that having a neck fit properly that is only 1/2 thou larger in diameter then the loaded cartridge neck diameter will also limit the amount of blow by you see on each firing.

it is i possible to stop all blowby but its very easy to dramatically reduce it on each shot.

as far as how much force is needed to start a bullet into the leade angle of the rifling, not much at all. I know this because i can take a properly dry lubed case and seat s bullet with very little effort needed to seat that bullet into the case. I can take that same cartridge and test fit it in a rifle and the rifling will engage and engrave the ogive with very little noticed effort closing the bolt.

however when that same round is extracted and measured the bullet will not always have been pushed back into the case mouth at all, even though it is very easy to do so as there is very light neck tension on the bullet.

conversely, a solid copper or alloy bullet will contact the lands and easily be forced back into the case with very little actual engraving of the ogive by the lands. Which also makes it very easy to find the exact OAL seater to just kiss the lands.

as far as pressure increasing with a bullet seated to just touch the lands compared to the same bullet getting a run at the lands, that is certainly true reguarding internal ballistics and internal pressure curves but that has very little to do with the topic the OP asked.

so while i can see good points in some of your comments i dont agree with some of them which is perfectly ok both way!!

 

[Fiftydriver]

Undoubtedly someone has opened up and threaded a flash hole, screwed a tiny tube into it that ends just below the base of a seated projectile.

Thousands of times personally, it works great but a topic for another post.

 

[Philward]

So what is the temperature of combustion in a chamber at 60k psi?

 

[cabelasken]

So what is the temperature of combustion in a chamber at 60k psi?

I believe it is north of 3000 degrees - only instantaneous.