What causes over pressure loads?
- Thread starter Dusty
- Start date
Re: What causes over pressure loads?
We just ran into that. My friend and I rebarreled our guns but with different barrel manufactures. Both of our guns were rechambered right after one another with same reamer by SSK. My friends original factory 260 barrel shot a certain load fine. He also has a Savage in 260 that also shoots that particular load fine. My barrel is a Lilja 3 groove and I run .5 grain more powder to get same velocity as his but it also shoots fine. His rebarrel had to be dropped nearly 3 grains less powder and still was a bit hot.
We've not run into that much of a difference with 3 guns being fine and another being that much more sensitive. Only thing we can think of is possibly the barrel is a tiny bit undersized. He's taking that new barrel off and going back to the orginal again since it shot great but was only looking to get a bit better improvement in accuracy and it also had a brake. Mine also has a brake so we don't think that has anything to do with the pressure problems. The throat and chambering of his and mine are identical. Maybe when the smith gets the barrel off he can check it out and check for undersize.
Topstrap
We just ran into that. My friend and I rebarreled our guns but with different barrel manufactures. Both of our guns were rechambered right after one another with same reamer by SSK. My friends original factory 260 barrel shot a certain load fine. He also has a Savage in 260 that also shoots that particular load fine. My barrel is a Lilja 3 groove and I run .5 grain more powder to get same velocity as his but it also shoots fine. His rebarrel had to be dropped nearly 3 grains less powder and still was a bit hot.
We've not run into that much of a difference with 3 guns being fine and another being that much more sensitive. Only thing we can think of is possibly the barrel is a tiny bit undersized. He's taking that new barrel off and going back to the orginal again since it shot great but was only looking to get a bit better improvement in accuracy and it also had a brake. Mine also has a brake so we don't think that has anything to do with the pressure problems. The throat and chambering of his and mine are identical. Maybe when the smith gets the barrel off he can check it out and check for undersize.
Topstrap
Re: What causes over pressure loads?
I have chambered two barrels from same manufacturer to min headspace and run throat erosion gage in to record unfired throat depth and had a full ring of difference due to the barrel internal dimensions.
You can really see the differences in barrels with a borescope.
Some barrels the bullets just touch the bottom of the grooves here and there and others the bullets show full contact with lands to varying degrees all the way to the muzzle.
Some barrels look like they are rifled with a mill bastard file and lapped with a cement block. Others are nice and shiney with good surface finish all the way down.
Some barrels don't have straight internal drilling and this can cause problems at long range unless the bend directs the bullet towards 12 o'clock when it leaves the muzzle.
There is a saying in the gun industry:
"If it won't shoot bent straighten it, if it won't shoot straight bend it"
Used to be these were the highest paying jobs in factory were the folks that tweaked the barrels straight. One can see old pics of guys straightening barrels in a jig that has a multi handled wheel on it like the wheel on a sailing ship.
Then there are those that don't stress relieve their barrels at all or not enough all the way to those custom guys that stress relieve them before and after rifling is cut. That is where you really get into trouble is getting a barrel that has not been stress relieved as it will start to walk on the second or third shot and the hotter it gets the more the group walks.
Finally you have internal loose spots (or tight spots) that cause problems and I forgot variable pitch rifling which means the twist rate varies before it gets out the muzzle. Ideally if the rifling increases spin rate you are OK and there have been barrels made to do just this. They are known as gain twist rifling. There were/are some bores that have gain twist and squeeze bore built in to achieve higher velocity with heavy bullets which are very expensive to manufacture.
If you want to track the barrel performance load up say 200 rounds with the same lot of brass and same componant lots. When the barrel is new or low rounds fire a continious 30 round string on the chrono and record all velocities and run SD, ES, hi/lo on each 10 rounds in the 30 round string. At 2500 rounds do another 30 rounds, then again at 5000, 7500 and 10,000 and track the differences.
Ideally you fire on the same range at the same temperature every time at the same rate of fire. On a long range rifle one shot every 30 seconds.
If you have a IR thermometer record the barrel temp 6" in front of chamber and 6" from the muzzle at the end of firing.
You will most likely discover some very unique things about that barrel and it will fascinate you how much the barrels change during the history. Some get better, some get worse.
A barrel that loses 200 fps average velocity in the gov't is considered to have failed but then again if it is shooting fine for you keep on laying them out there. I have heard of barrels going 20,000 rounds (7.62) that were well cared for.
Rounds alone do not destroy barrels. Lack of cleaning and improper cleaning have ruined more barrels than high round numbers.
I have chambered two barrels from same manufacturer to min headspace and run throat erosion gage in to record unfired throat depth and had a full ring of difference due to the barrel internal dimensions.
You can really see the differences in barrels with a borescope.
Some barrels the bullets just touch the bottom of the grooves here and there and others the bullets show full contact with lands to varying degrees all the way to the muzzle.
Some barrels look like they are rifled with a mill bastard file and lapped with a cement block. Others are nice and shiney with good surface finish all the way down.
Some barrels don't have straight internal drilling and this can cause problems at long range unless the bend directs the bullet towards 12 o'clock when it leaves the muzzle.
There is a saying in the gun industry:
"If it won't shoot bent straighten it, if it won't shoot straight bend it"
Used to be these were the highest paying jobs in factory were the folks that tweaked the barrels straight. One can see old pics of guys straightening barrels in a jig that has a multi handled wheel on it like the wheel on a sailing ship.
Then there are those that don't stress relieve their barrels at all or not enough all the way to those custom guys that stress relieve them before and after rifling is cut. That is where you really get into trouble is getting a barrel that has not been stress relieved as it will start to walk on the second or third shot and the hotter it gets the more the group walks.
Finally you have internal loose spots (or tight spots) that cause problems and I forgot variable pitch rifling which means the twist rate varies before it gets out the muzzle. Ideally if the rifling increases spin rate you are OK and there have been barrels made to do just this. They are known as gain twist rifling. There were/are some bores that have gain twist and squeeze bore built in to achieve higher velocity with heavy bullets which are very expensive to manufacture.
If you want to track the barrel performance load up say 200 rounds with the same lot of brass and same componant lots. When the barrel is new or low rounds fire a continious 30 round string on the chrono and record all velocities and run SD, ES, hi/lo on each 10 rounds in the 30 round string. At 2500 rounds do another 30 rounds, then again at 5000, 7500 and 10,000 and track the differences.
Ideally you fire on the same range at the same temperature every time at the same rate of fire. On a long range rifle one shot every 30 seconds.
If you have a IR thermometer record the barrel temp 6" in front of chamber and 6" from the muzzle at the end of firing.
You will most likely discover some very unique things about that barrel and it will fascinate you how much the barrels change during the history. Some get better, some get worse.
A barrel that loses 200 fps average velocity in the gov't is considered to have failed but then again if it is shooting fine for you keep on laying them out there. I have heard of barrels going 20,000 rounds (7.62) that were well cared for.
Rounds alone do not destroy barrels. Lack of cleaning and improper cleaning have ruined more barrels than high round numbers.
Re: What causes over pressure loads?
One of the barrel guys will probably chime in on this. But barrels even from the SAME company can have very slightly different inside bore diameters. And when you are dealing with pressures in the 10s of thousands of PSI it doesn't take much to make those kind of differences.
I know barrel makers that advertise that they "air guage" their barrels to .00005" (fifty millionths of an inch, or half a tenth). I have seen it done. And even very small increments can make a difference. That's why everyone always says to start your load a little low and work up on it.
One of the barrel guys will probably chime in on this. But barrels even from the SAME company can have very slightly different inside bore diameters. And when you are dealing with pressures in the 10s of thousands of PSI it doesn't take much to make those kind of differences.
I know barrel makers that advertise that they "air guage" their barrels to .00005" (fifty millionths of an inch, or half a tenth). I have seen it done. And even very small increments can make a difference. That's why everyone always says to start your load a little low and work up on it.
