High Velocity Throat Erosion

[VenatusDominus]

The primary cause of throat erosion is the initial super heated gases created as the powder burns in a confined space. As the gases expand and cool further down the barrel, there is less erosion. This is the same process that happens when these super headed gases escape past the primer and etch the bolt face. The degree to which this happens also depends on the composition and temperature of the barrel material. Friction from the bullet is negligible when compared to the effects of the super heated gases.

 

[RidgeTop]

Another factor is the initial barrel temp. A barrel after 10rds rapid fire will be more susceptible to erosion than a cold one.
Also, I have heard some powder kernels wear a barrel more than others - shape, hardness. A third is the temp and duration of the flame.
BTW, I took my digital IR temp to the range to watch temp heating. To my surprise, the hottest part of the barrel is about 10 to12" down from the chamber. I was using 7mmRemMag and .300Wby Mark V. I did not believe it either - try it.

 

[Pdvdh]

Given same pressures does it come down to the temp and time of the powder burn?

I think the answer is yes. The throat is the portion of the bore that receives the highest duration of high temps and pressures. The throat gets toasted first because of those high temps/pressures.

The heavier the bullet, the longer the duration of the high heat and temperatures at the throat. The heavier bullet remains near the chamber/throat slightly longer than a lighter bullet, because it takes a longer period of time to get that heavier bullet accelerated away from the chamber/throat. And I think that's what the OP was asking about.

For what that's worth... I should note my comments are primarily based on theory, much more so than personal experiences and bore scoping after the fact. And this theory doesn't include any consideration of different powder burning temperatures. A powder that generates a higher throat temperature is certain to heat crack the throat faster than a powder generating a lower throat temperature.

 

[CA48]

I'm no expert but I believe that powder kernel size and shape have an affect on throat erosion. The same reason they use different grades of media when sandblasting different materials. As coarse media can put holes through thinner or softer metals. Then add extreme heat and pressure into the mix.

 

[Litehiker]

Asi I understand the latest research it is HEAT that causes throat erosion.
That said some things can slow down this process such as a stainless steel barrel, cold hammer forged barrels, chrome plating the throats and cryogenic treatment.

Me? I prefer cold hammer forged stainless steel but I have a 6.5 CM "normal" chrome/moly cold hammer forged RUGER Precision Rifle barrel and a 6.5 PRC stainless steel Browning X-Bolt Pro barrel.

Soon I'll need a new RPR barrel. RUGER does sell a "Custom Shop" 6 mm PRC RPR with a cold hammer forged stainless barrel and I'm hoping I can buy a barrel like that for my 6.5 CM RPR. Fingers crossed.

 

[bigngreen]

I think the answer is yes. The throat is the portion of the bore that receives the highest duration of high temps and pressures. The throat gets toasted first because of those high temps/pressures.

The heavier the bullet, the longer the duration of the high heat and temperatures at the throat. The heavier bullet remains near the chamber/throat slightly longer than a lighter bullet, because it takes a longer period of time to get that heavier bullet accelerated away from the chamber/throat. And I think that's what the OP was asking about.

For what that's worth... I should note my comments are primarily based on theory, much more so than personal experiences and bore scoping after the fact. And this theory doesn't include any consideration of different powder burning temperatures. A powder that generates a higher throat temperature is certain to heat crack the throat faster than a powder generating a lower throat temperature.

You can see during pressure testing that you can maintain the same peak pressure but duration or pressure under the curve varies. H1000, Retumbo and R33 at the same pressure in a 6.5 Sherman but had a nice spread of time under pressure, I can't remember the microseconds number now but it was very interesting. Some will definitely say H1000 is easier on barrels than R33.

 

[VenatusDominus]

Kernal size and shape does affect the throat erosion, but not due to mechanical abrasion. Modern gun powder is made of nitrocellulose. Other common forms of cellulose are cotton fibers and sawdust. Cellulose (relatively soft) would cause negligible erosion of steel unless moving at much higher speeds. Unburned gunpowder is moving at the speed of the bullet and is at its slowest at the throat. If it were responsible for erosion, it would cause this where is is moving at its fastest, much further down the barrel or even at the crown.

The size and shape of the kernel affects its surface area, which (along with the chemical make up of the powder) affects how quickly the powder burns and how hot the gases become. The hotter the gases and the longer they are exposed to the barrel, the more erosion will take place.

 

[cabelasken]

Another factor is the initial barrel temp. A barrel after 10rds rapid fire will be more susceptible to erosion than a cold one.
Also, I have heard some powder kernels wear a barrel more than others - shape, hardness. A third is the temp and duration of the flame.
BTW, I took my digital IR temp to the range to watch temp heating. To my surprise, the hottest part of the barrel is about 10 to12" down from the chamber. I was using 7mmRemMag and .300Wby Mark V. I did not believe it either - try it.

the barrel is typically thinner there and you'll get a higher reading. The thicker part near the case is thick and the radiant heat is still moving outward. And it has more surface area which aids cooling. The inside temp is much different.

 

[VenatusDominus]

Asi I understand the latest research it is HEAT that causes throat erosion.
That said some things can slow down this process such as a stainless steel barrel, cold hammer forged barrels, chrome plating the throats and cryogenic treatment.

Me? I prefer cold hammer forged stainless steel but I have a 6.5 CM "normal" chrome/moly cold hammer forged RUGER Precision Rifle barrel and a 6.5 PRC stainless steel Browning X-Bolt Pro barrel.

Soon I'll need a new RPR barrel. RUGER does sell a "Custom Shop" 6 mm PRC RPR with a cold hammer forged stainless barrel and I'm hoping I can buy a barrel like that for my 6.5 CM RPR. Fingers crossed.

Stainless steel is softer than chrome moly steel. Chrome moly steel will last longer than stainless steel when used in barrels. Stainless steel is softer and easier to tool, making it easier to produce an accurate barrel than the harder chrome moly. Chrome lined barrels are even less prone to wear but it is difficult to get a perfectly consistent lining and so can have a negative effect on accuracy.

 

[cabelasken]

Stainless steel is softer than chrome moly steel. Chrome moly steel will last longer than stainless steel when used in barrels. Stainless steel is softer and easier to tool, making it easier to produce an accurate barrel than the harder chrome moly. Chrome lined barrels are even less prone to wear but it is difficult to get a perfectly consistent lining and so can have a negative effect on accuracy.

I've always been told that stainless barrels hold accuracy slightly longer than CM barrels. I guess they lied!!!!

 

[VenatusDominus]

This may

I've always been told that stainless barrels hold accuracy slightly longer than CM barrels. I guess they lied!!!!

This may be true because in general a SS barrel will start out more accurate than the CM barrel. But if you take a SS barrel and a CM barrel that both shoot 0.5 MOA and shoot the same ammo through them, the CM barrel will outlast the SS barrel. If you could produce a chrome lined barrel with that accuracy, it would outlast both.

 

[BillLarson]

I have no explanation, only picture evidence for the discussion. aprox 2000 rounds and still shot pretty good

edit

55gr nosler ballistic, 43.4 SuperF, 26 inch barrel, 3800FPS.
Thats fast for that bullet

If that were my barrel,I would clean it good with J-B let it soak overnite with 7.62.... and keep shooting it til it shot
2 moa.
OMHO...
mileage may vary....
.

 

[Naymola]

Wow, you guys are making this topic way too complicated for sure. It's simply BTU's generated at the case mouth, where the heat generated is the greatest. Otherwise like the man said, the muzzle would have more wear than the throat. Keep firing successive bullets one right after another for 20 rounds or so and you'll have a throat so hot the lands have softened to where the bullets are eroding them. I ruined a very accurate .233 Rem at a competitve varmint hunting club shoot. Hart Varmint contour, stainless steel, 1:14 twist, 50 grain V Max, at 3500 fps. Who could hit a 6"gong at 200 yards, sitting on a stool with only extending shooting sticks, shoot reload shoot, the most in one minute, took the prize. I hit it 18 times with a bolt gun. That gun would easily shoot 1/2" groups at 200 yards. Afterwards, groups opened up to over 1-1/2". I'd bet the farm, if a guy shot a 30-06 the same way it would be toast as well. I'd never be that stupid again.

 

[Northkill]

I think the big difference here is that machine guns are not given time to cool down between shots like most guys on here are doing with their bolt action rifles. So you kind of have an apples to oranges scenario. We were swapping out hot barrels not because of loss of accuracy per se, but because getting the barrel way too hot is not only going to warp the barrel, but at a certain point it will start cooking off rounds and thus firing on its own which is a bad situation. We also weren't very concerned about accuracy because we relied on quantity over quality.

As for the overall title of this thread I don't have much to add, but I recently got some hammer bullets to try in my cheapo Savage 110 with a 22" sporter barrel chambered in 30-06. I shot several loads this weekend. Some factory Winchester Power Point 150gr @ 2900fps , some 190gr ABLR running 55.3gr of H4350 at 2800fps, some 165gr Sierra GameKings running 61.7gr of 6.5 StaBALL, and then some 12 shot ladders of 178gr Absolute Hammers with 55.5gr to 61.8gr of Hybrid 100V at 2860fps to 3068fps; and some 101gr Hammers running 51.8gr to 57.7gr of Varget at 3164fps to 3467 fps.

I cannot speak for barrel wear, but the big thing I noticed is on every shot of the 101gr Hammers the case was uncomfortably hot to the touch, while none of the other ones were. Now there are certainly some variables here because I changed powders and who knows what my pressures are, though I think the 190gr, 165gr and 178gr are at their limits, while I appear to still have some room to run the 101's faster. I meant to check barrel temp on the 101's but I got distracted so I'm not sure if the barrel was significantly hotter as well or not.

You're running a faster powder with the 101 AH. Makes sense to me that your case would be hotter with that. Maybe I'm green.