MBTcustom
06-01-2015, 10:40 PM
It has been a question that has been tossed around quite a bit: What happens when you shoot a LOT of HV cast bullets at 2500+ FPS?
After all, this is the recipe to quickly turn your barrel into a crispy critter:
1. Slow powders
2. Overbore cartridge
3. High operating pressures
4. Heavy for caliber bullets
5. Fast twist rate
6. long strings of fire through a hot barrel
When shooting cast bullets in the XCB project, it's very hard to predict because several of these criteria are met, and several are not. We have slow powders, medium to high operating pressures, and long strings of fire, but the XCB cartridges are designed to be inherently balanced, with cartridges that are neither over nor underbore, bullets of the ideal weight for caliber, and very balanced twist rates on the barrels.
Well, it's been a year and a half since I started sending out XCB rifle systems to the participants of this project, and Bjornb has been shooting his "Bertha" rifle and posting groups here with his current barrel since November of last year. He has fired over 2000 HV rounds since that time. Today, the rifle was returned for upgrades and for me to inspect the progress of throat erosion. It has been postulated that shooting bullets made of Linotype and heat treated alloy will erode the throat at an accelerated rate, and this is the first time I have had a barrel come through the shop that has been shot almost exclusively with such alloys and always at elevated velocities.
I brushed the barrel about 15 strokes with a bronze brush sprayed with Hoppe's bore cleaner, then followed it with several patches.
I looked in the throat with my borescope and saw waht looked to be a tar like black substance extending about 1.5" down the bore. It was glossy black, and reminded me of looking in the necks of pulled LC LR brass where they have that black sealant on the inside of the necks.
This black substance extended about 5" up the barrel but only on the crests of the lands.
I scrubbed it many more times with a bronze brush, but this black varnish ignored it.
Finally, I wrapped the brush with 000 steel wool and took several passes down the barrel.
This succeeded in removing the black coating and I was finally able to see what lay beneath.
I saw that the fine reamer scratches had been swept away completely and had been replaced with minute linear striations that are typical.
I saw the faintest bit of gator checking on the ramps of the lands in the throat, and the edges of the lands had been worn smooth, but the angle of the throat was still there and in good form.
The linear striations (obviously caused by shooting, and different from the lapped finish that came from the factory) extended down the barrel about 4".
It's very hard to quantify the erosion of this barrel because it's as different from normal erosion as cast bullets are from jacketed bullets. That said, if this were a 30-06 and I were to guess at how many shots had been fired through it, I would have tentatively said less than a thousand.
Very interesting results. If I were to guess at the supposed barrel life of this rifle using the same loads that got it this far, I would tentatively say it was less than 1/4 of the way to burn out. In other words, this barrel with these HV loads should give 8000-10,000 rounds before wear starts to effect groups.
The black substance is puzzling me more than anything at this point. I spoke with Bjorn about this a while ago and he wondered if it might be the coating on the kernels of the WC860 powder he enjoys shooting in this rifle. I honestly don't know. I've never seen anything like it before. I know that stuff was thick and hard though.
After all, this is the recipe to quickly turn your barrel into a crispy critter:
1. Slow powders
2. Overbore cartridge
3. High operating pressures
4. Heavy for caliber bullets
5. Fast twist rate
6. long strings of fire through a hot barrel
When shooting cast bullets in the XCB project, it's very hard to predict because several of these criteria are met, and several are not. We have slow powders, medium to high operating pressures, and long strings of fire, but the XCB cartridges are designed to be inherently balanced, with cartridges that are neither over nor underbore, bullets of the ideal weight for caliber, and very balanced twist rates on the barrels.
Well, it's been a year and a half since I started sending out XCB rifle systems to the participants of this project, and Bjornb has been shooting his "Bertha" rifle and posting groups here with his current barrel since November of last year. He has fired over 2000 HV rounds since that time. Today, the rifle was returned for upgrades and for me to inspect the progress of throat erosion. It has been postulated that shooting bullets made of Linotype and heat treated alloy will erode the throat at an accelerated rate, and this is the first time I have had a barrel come through the shop that has been shot almost exclusively with such alloys and always at elevated velocities.
I brushed the barrel about 15 strokes with a bronze brush sprayed with Hoppe's bore cleaner, then followed it with several patches.
I looked in the throat with my borescope and saw waht looked to be a tar like black substance extending about 1.5" down the bore. It was glossy black, and reminded me of looking in the necks of pulled LC LR brass where they have that black sealant on the inside of the necks.
This black substance extended about 5" up the barrel but only on the crests of the lands.
I scrubbed it many more times with a bronze brush, but this black varnish ignored it.
Finally, I wrapped the brush with 000 steel wool and took several passes down the barrel.
This succeeded in removing the black coating and I was finally able to see what lay beneath.
I saw that the fine reamer scratches had been swept away completely and had been replaced with minute linear striations that are typical.
I saw the faintest bit of gator checking on the ramps of the lands in the throat, and the edges of the lands had been worn smooth, but the angle of the throat was still there and in good form.
The linear striations (obviously caused by shooting, and different from the lapped finish that came from the factory) extended down the barrel about 4".
It's very hard to quantify the erosion of this barrel because it's as different from normal erosion as cast bullets are from jacketed bullets. That said, if this were a 30-06 and I were to guess at how many shots had been fired through it, I would have tentatively said less than a thousand.
Very interesting results. If I were to guess at the supposed barrel life of this rifle using the same loads that got it this far, I would tentatively say it was less than 1/4 of the way to burn out. In other words, this barrel with these HV loads should give 8000-10,000 rounds before wear starts to effect groups.
The black substance is puzzling me more than anything at this point. I spoke with Bjorn about this a while ago and he wondered if it might be the coating on the kernels of the WC860 powder he enjoys shooting in this rifle. I honestly don't know. I've never seen anything like it before. I know that stuff was thick and hard though.