Terry, I’ve never seen any hard numbers/data but it’s pretty much agreed on by different ammo/bullet makers I’ve talked to that in a given caliber a longer bullet / longer bearing surface will also effect/shorten barrel life.
So like in a 30cal…. A 150gr match bullet vs a 190gr match bullet the 190 is going to be harder on the barrel. A bullet maker/lab guys will be at the shop on Tuesday. I’ll bring the question up and see if they can shed any light on it.
Thanks for sharing your input Frank. I appreciate your expertise and opinion.
Here is where I was coming from on the friction thing and somewhat dismissing the pressure thing when it comes to bullet weight...
Disclaimer: I will try to explain where my opinions originated from and I will do an extra sucky job of writing this...
I regularly find out a lot of things I took as fact in the past were wrong or have changed considerably. I'm all ears and definitely don't want to pass on bad info to other, newer rifle peeps.
Friction?
I remember talking to the guys at Sierra years ago that did the accuracy testing for different lots of bullets, I think all the match stuff specifically. I'm sure you know all of them by name and likely do barrels for them.
Anyhow, I recall them saying that they produced and sold a shit ton of jacketed 9mm and 45 ACP bullets. Like WAY more than I could have imagined. Lots going to US ammo companies but a surprising number to Intl. companies. Some SKUs of the 9 and 45 line had to meet certain accuracy specs so they test fired them just like the rifle bullets. I think 10rds per target pretty much as fast as they could load and fire the fixtured actions.
They said the rifle barrels used in the test fixtures were all being replaced at round counts that we would expect.
What the big deal to us was that they were pretty much using the same test barrels in the pistol fixtures that they started with. Some of the pistol fixtures had like >100K rounds fired through the original barrels but the chambers and bores looked excellent.
With this in mind, the conclusion was that the friction from jacketed projos had a very minimal impact on barrel bore wear. Of course the pistol ammo had miniscule pressures and heat compared to modern rifle ammo so that took more of the heat and pressure variables out of the wear equation leaving only jacket to bore friction as a potential major player. Again, seemingly little or no wear from friction.
Pressure differences due to bullet weight/bearing surface.
This is where I came to believe that pressure combined with the short pulse of plasma hot gasses are the main contributors to bore erosion.
But I have a hard time aligning my logic with comments above regarding pressures of light versus heavy bullets.
Assume that we use the OP's 6GT cartridge as an example.
SAAMI MAP is around 62K psi. Loading and shooting 70gr 6mm bullets and 107gr 6mm bullets, the MV would be higher for the lighter bullets but wouldn't the chamber pressures be the same or almost the same at publicly accepted SAAMI CUP or PSI ?
I would have thought 62K psi with a 70gr versus 62K psi with a 107gr would be a constant.
Maybe slight dwell times with getting the heavier bullet going might affect the pressure curve between the two but then usually bullets of such different weights usually call for powders with different burn rates. Even then, the two different powders are being loaded to the same MAP.
We had come the the opinion that almost all of the cracking and throat erosion was attributable to the material compression and relaxation coinciding with the hot plasma gases looking for an exit.
We figured that the interior of the chamber and throat is expanding during the ride up the pressure curve. Guessing that the interior surfaces are trying to expand faster and more than the exterior surface, there is a good bit of barrel material compression taking place then relaxing again as the pressure subsides.
The material at the interior surface is the most abused and the most stretched so it would become less flexible, start to crack, allow deeper fire cutting and exposing underlying material to the direct expansion loads. So the first surface flaws to appear will allow accelerated damage which continues to speed up the damage even more. Finally chunks start coming loose as the interior surface sloughs off.
Kinda reminds me of the ablative heat shields on the earlier Mercury, Gemini and Apollo.
If I am thinking correctly, this momentary exterior stretching or expansion is why wire strain gauges mounted to the exterior of the barrel can feed information to the instruments?
Obermeyer years ago said that was a true thing as well. I remember him saying when breaking in a barrel to use the longest / heaviest bullet you can shoot at a moderate load/pressure/velocity to break in the barrel.
Yes sir. I talked with Boots several times in the 80s and 90s. Maybe put on about 100 of his blanks. He was a really awesome resource. I forgot the name of it but he would write several articles about muzzle wear, bore wear, high power, bullets, etc. At one point I had them all printed out. They were called "Boots on (something I can't remember).
I remember him mentioning the heavier, longer bullets for barrel break-in. I always took it to mean that we were just wanting to burnish the new throat and get a good initial uniform foul down the bore. Thus the longer bearing surface at moderate velocity.
If my context was correct with his intent, using the heavier bullets for that chore would be a totally different discussion/cause/effect than the actual bore erosion conundrum?
It might have to do with a combination of things…. Type of bullet, type of powder etc… more than just the bullet. Friction, heat?
Later, Frank
Please do bring this up with your brainiac friends and let us know what floats to the top.
Again, I am not arguing that the above is the way things are. I'm just explaining how I came to my opinion. I welcome the critique of what I got wrong.
