Rifle shooter, we were in target-rich environments, ground squirrels, p. dogs, and jackrabbits. Barrels were considered expendable, and we did not give a tinkers darn about shooting the throat out of a barrel. As a general rule, we had the barrels set back in the realm of 1800 rounds, and at that point, the leade was not shot out, but getting on the long side where the bullet was not in the neck enough to give consistent grip and ignition characteristics.
Our old German gunsmith, Joe Wagner taught us how to measure not only throat length but throat dia. As the barrel is fired, the leade grows in length and dia. This growth drops velocity, so as a barrel is fired with the growth, chamber pressure drops and accuracy decreases. There is a "window" where accuracy slowly decreases to a point where you are not shooting 3/8" groups anymore, which was our goal. On the barrel set back, the bore dia can be larger, and the gunsmith has to have a reamer pilot of that dia so the reamer is not rattling around in the chamber creating an egg-shaped chamber. The key on a barrel setback is to get the new throat dia the same as the dia at the muzzle. High-quality barrels usually have uniform barrel dia to 0.0001, and this should be checked on every new barrel blank with the bore dia stamped on the outside of the barrel. A tight barrel will peak pressure sooner and a loose barrel will be able to see some pretty high velocities. IN 6mm you also buy barrels in .236 or .237 bore dia. I prefer a .236 for short-range bench rest and a .237 for all varmint rifles and long-range rifles. Shooters are getting a LOT more informed on the details of chamber dimensions, and throat geometry including freebore dia and length, and Web dia.
On a final note, after you have tuned your load on your new barrel or gun, if you chronograph that load, you have learned the frequency where that particular barrel likes to resonate. So, as the throat grows in dimension both width and length, you loose velocity and at some point, the barrel is not resonating the same or is at a different "node".
So you have several options and it is best to use both. First, maintain that distance from the lands that the bullet liked when it was new. Second, maintain the velocity that the barrel was shooting at when you first tuned the load. This is just dumb butt simple...works every time.
My brother uses a different approach that he can afford. He seats the bullet just about on top of the powder. He works up a load that shoots a bug hole at this seating depth which is usually pretty deep. As the leade grows, he does not adjust the seating depth much. When the accuracy is gone, he discards the barrel or rifle. This method keeps his gunsmith very happy. My brother has no patience for my method...none. I Watched him tune a Rem long range 700 factory in 25/06 shooting the 115g Bergers at 2" groups at 500 yards, same thing with a Rem 700 Varmint in 308 shooting the 155g Berger VLD hunting. Brother has the money to just replace barrels as needed, and I am forced to get the most out of what I have.
My approach and brothers achieve the same results, he just goes through barrels more than I do. I like chasing the accuracy and details there in, brother has no patience to do this kind of thing. He really liked the concept of increasing the powder charge as the leade dia and length grew...he would do that because it was easy/quick.
Our old German gunsmith, Joe Wagner taught us how to measure not only throat length but throat dia. As the barrel is fired, the leade grows in length and dia. This growth drops velocity, so as a barrel is fired with the growth, chamber pressure drops and accuracy decreases. There is a "window" where accuracy slowly decreases to a point where you are not shooting 3/8" groups anymore, which was our goal. On the barrel set back, the bore dia can be larger, and the gunsmith has to have a reamer pilot of that dia so the reamer is not rattling around in the chamber creating an egg-shaped chamber. The key on a barrel setback is to get the new throat dia the same as the dia at the muzzle. High-quality barrels usually have uniform barrel dia to 0.0001, and this should be checked on every new barrel blank with the bore dia stamped on the outside of the barrel. A tight barrel will peak pressure sooner and a loose barrel will be able to see some pretty high velocities. IN 6mm you also buy barrels in .236 or .237 bore dia. I prefer a .236 for short-range bench rest and a .237 for all varmint rifles and long-range rifles. Shooters are getting a LOT more informed on the details of chamber dimensions, and throat geometry including freebore dia and length, and Web dia.
On a final note, after you have tuned your load on your new barrel or gun, if you chronograph that load, you have learned the frequency where that particular barrel likes to resonate. So, as the throat grows in dimension both width and length, you loose velocity and at some point, the barrel is not resonating the same or is at a different "node".
So you have several options and it is best to use both. First, maintain that distance from the lands that the bullet liked when it was new. Second, maintain the velocity that the barrel was shooting at when you first tuned the load. This is just dumb butt simple...works every time.
My brother uses a different approach that he can afford. He seats the bullet just about on top of the powder. He works up a load that shoots a bug hole at this seating depth which is usually pretty deep. As the leade grows, he does not adjust the seating depth much. When the accuracy is gone, he discards the barrel or rifle. This method keeps his gunsmith very happy. My brother has no patience for my method...none. I Watched him tune a Rem long range 700 factory in 25/06 shooting the 115g Bergers at 2" groups at 500 yards, same thing with a Rem 700 Varmint in 308 shooting the 155g Berger VLD hunting. Brother has the money to just replace barrels as needed, and I am forced to get the most out of what I have.
My approach and brothers achieve the same results, he just goes through barrels more than I do. I like chasing the accuracy and details there in, brother has no patience to do this kind of thing. He really liked the concept of increasing the powder charge as the leade dia and length grew...he would do that because it was easy/quick.
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