markm
03-25-14, 15:18
I mentioned this topic in a few threads here recently, and I think it's worth posting as its own thread.
From this thread on thefiringline.com
http://thefiringline.com/forums/showthread.php?t=532039
When you realize that all powders are not equal, and some projectiles create more friction heating in certain types of barrels combined with the twist, rifling and bore coating, there are a few things you can do to prolong barrel life.
There are three parts of the barrel that are of concern, the throat, the gas port and the muzzle. When any of these three are damaged, accuracy will suffer. However, only the gas port is significantly affected by heat. The hot gases make a 90 degree turn (initially) which eventually turns the port into a teardrop kind of oval shape. With some bullets in shorter systems, this can lead to jacket rupture in the bore. Muzzle device strikes and "vaporizing" bullets are typically due to a worn gas port combined with other factors.
The faster the twist, the higher the burn rate temperature and the faster the bullet is pushed are the factors that really "cook" barrels. Chrome-lined, 1/7 twist, hot and slower powders (such as Varget) with light bullets has been a problematic recipe for heat related barrel damage.
When I ran some tests for a manufacturer in such a barrel with 50 grain bullets pushed at 3300 fps, the surface temp of the barrel after 100 round beta mag dump was about 105 degrees hotter than a sister barrel shooting 77 grain slugs at 2800 fps with a cooler powder. The peak temp on the hotter load occurred 75 seconds after the last round was fired and just 12 seconds after the last round on the cooler load. Those 100 round mag dumps were just over 14 seconds, or just under 7 rounds per second. It was slower than the FA rate the barrels were intended to be used at, but the hassle of a FA lower was not worth it. When the barrels were examined with digital x-ray, I was able to see gas port erosion on the barrel that was run hotter around the 1400 round mark. The one that was run cooler, not until just past 11,000 rounds was I able to detect any gas port erosion. In the 1/9 barrels, pretty much all else equal, gas port erosion was first detected at about 7,000 rounds.
If you have fast twist barrels, light bullets, especially with hot slow powders, should be avoided. I avoid anything under 55g in 1/7 and 1/8 barrels and anything lighter than 50g in 1/9 barrels. The new VDC coatings (what I was actually testing) do perform much better than chrome lined when exposed to high heat cycles. Lengthening gas systems also helps.
Barrels can have the extension removed, cut off 0.05" and be re-chambered, so too can barrels be re-crowned ever easier. Granted the cost to re-chamber and re-crown is not insignificant. You can't fix gas port erosion except by using loads that don't cause increased wear in the first place.
__________________
Good Shooting, MarkCO
www.CarbonArms.us
From this thread on thefiringline.com
http://thefiringline.com/forums/showthread.php?t=532039
When you realize that all powders are not equal, and some projectiles create more friction heating in certain types of barrels combined with the twist, rifling and bore coating, there are a few things you can do to prolong barrel life.
There are three parts of the barrel that are of concern, the throat, the gas port and the muzzle. When any of these three are damaged, accuracy will suffer. However, only the gas port is significantly affected by heat. The hot gases make a 90 degree turn (initially) which eventually turns the port into a teardrop kind of oval shape. With some bullets in shorter systems, this can lead to jacket rupture in the bore. Muzzle device strikes and "vaporizing" bullets are typically due to a worn gas port combined with other factors.
The faster the twist, the higher the burn rate temperature and the faster the bullet is pushed are the factors that really "cook" barrels. Chrome-lined, 1/7 twist, hot and slower powders (such as Varget) with light bullets has been a problematic recipe for heat related barrel damage.
When I ran some tests for a manufacturer in such a barrel with 50 grain bullets pushed at 3300 fps, the surface temp of the barrel after 100 round beta mag dump was about 105 degrees hotter than a sister barrel shooting 77 grain slugs at 2800 fps with a cooler powder. The peak temp on the hotter load occurred 75 seconds after the last round was fired and just 12 seconds after the last round on the cooler load. Those 100 round mag dumps were just over 14 seconds, or just under 7 rounds per second. It was slower than the FA rate the barrels were intended to be used at, but the hassle of a FA lower was not worth it. When the barrels were examined with digital x-ray, I was able to see gas port erosion on the barrel that was run hotter around the 1400 round mark. The one that was run cooler, not until just past 11,000 rounds was I able to detect any gas port erosion. In the 1/9 barrels, pretty much all else equal, gas port erosion was first detected at about 7,000 rounds.
If you have fast twist barrels, light bullets, especially with hot slow powders, should be avoided. I avoid anything under 55g in 1/7 and 1/8 barrels and anything lighter than 50g in 1/9 barrels. The new VDC coatings (what I was actually testing) do perform much better than chrome lined when exposed to high heat cycles. Lengthening gas systems also helps.
Barrels can have the extension removed, cut off 0.05" and be re-chambered, so too can barrels be re-crowned ever easier. Granted the cost to re-chamber and re-crown is not insignificant. You can't fix gas port erosion except by using loads that don't cause increased wear in the first place.
__________________
Good Shooting, MarkCO
www.CarbonArms.us