rufous
Well-Known Member
I wanted to let you all know the results of my barrel set back and rechamber. The barrel is a Lothar Walther and it was/is chambered for 300 Win Mag. The original gunsmith who installed it gave it a longer throat than I wanted (it was 0.300" long and was parallel at 0.3085" diameter). The action is a Winchester Model 70 Classic Stainless with the pre-64 type action. It has a 3.6" magazine box. It has a synthetic Rimrock sporter stock and a Swarovski 3-9 AV scope and weighs 9.3#. It is my climb the mountains big game rifle. When I sent the rifle to the smith who installed the new Walther barrel I sent him a dummy round to set the throat length but he made it too long. It shot well initially with the 220 SMK and the 200 Partition but after about 1000 rounds the throat had lengthened and accuracy was not that good with that much jump to the lands. So I sent it to Greg Tannel to have him set the barrel back and rethread and rechamber. He did an excellent job, giving it a neck diameter of 0.002" over loaded round neck diameter, a 0.3083" parallel throat of the specified length of my dummy round. I wanted to shoot the new 200 grain Nosler Accubond bullet.
When I got it back from Gre-Tan Rifles I started with H1000 but then tried RL25. In the previous chamber with the longer throat I was using 79 grains of RL25 with the 200 Partition for 3030 fps but this new chamber with the shorter throat requires only 74 grains of RL25 with the 200 Accubond to get 3030 fps. In my 7mm Rem Mag the 160 Partition and 160 Accubond use the same amount of powder for the same velocity so the difference in the 300 Mag must just be the tighter chamber.
Anyway I started with 76.5 grains of RL25 based on the fact that 78 grains of H1000 blew a primer. Accuracy was not great so I backed down to 75.5 grains. I started on the lands but 0.050" off the lands has proven to be the best seating depth. Then I tried 75 grains of RL25 with the 200 Accubond at 0.050" off the lands. I shot 4 shots at 250 yards and they landed in a 1.87" group. Then 3 shots at 400 were in 2.46". Then I shot 5 shots at 500 yards and they were in 4.93". Velocity from my 25" barrel is 3070 fps for 13 shots recorded. This load is a bit hot even though the powder charge is low. Incidentally my Swarovski 3-9 AV scope's clicks have a value of 0.28"/ click, not 0.25". It only took 30 clicks up at 600 from an initial zero at 250 instead of the 34 clicks that it should have taken (based on the ballistic coefficient and velocity and elevation).
I then tried 74 grains of RL25 with the 200 Accubond seated 0.050" off the lands. I have shot this bullet in this chamber with 76.5 grains and did not blow the primer out like I did with 78 grains of H1000 so while 75 grains of RL25 is fast it does not seem dangerous. Anyway I was concerned with the high velocity of 3070 fps with the 75 grain charge and since 75 grains looked to be more accurate than 75.5 grains I thought I should try 74 grains. First group at 250 was 1.02". Second group at 250 was 1.72" with all 6 in 1.72" (5 of those were in 1.02"). There was only about 0.7" of vertical for all 6 shots at 250 yards. First group at 400 yards was 3.82" (one shot well to the right of the other two with only 0.75" of vertical). Second group at 400 was 2.11" and it landed in the middle of the first group so I had 6 in 3.82" at 400. Velocity was 3030 fps. I have my load!
Finally I went out to make sure that my rifle was zeroed from field shooting positions, not just from the bench. I shot the 200 Accubond with 74 grains of RL25 load at 250 yards from the prone position using my Kramer Snipepod and sling. Group was perfectly centered and measured 1.7". Then I tried a group from the sitting position with my taller Kramer Snipepod and amazingly shot a 0.76" group that landed in the middle of the first group such that all 6 shots were in 1.7" at 250 yards. So now I have fired 6 groups of 3 shots each (4 at 250 and 2 at 400) with the 200 Accubond seated to 0.050" off the lands and 74 grains of RL25. Average group size is 0.594 MOA. I love this rifle. I first sent this rifle off to Arnold Arms in December of 1998 after wearing out the factory barrel and 4 barrels later gave up on them (none of them shot accurately). Then I sent it to a reputable 'smith but he gave it a longer throat than I wanted. Finally it is perfect after 5 agonizing years. Rufous.
When I got it back from Gre-Tan Rifles I started with H1000 but then tried RL25. In the previous chamber with the longer throat I was using 79 grains of RL25 with the 200 Partition for 3030 fps but this new chamber with the shorter throat requires only 74 grains of RL25 with the 200 Accubond to get 3030 fps. In my 7mm Rem Mag the 160 Partition and 160 Accubond use the same amount of powder for the same velocity so the difference in the 300 Mag must just be the tighter chamber.
Anyway I started with 76.5 grains of RL25 based on the fact that 78 grains of H1000 blew a primer. Accuracy was not great so I backed down to 75.5 grains. I started on the lands but 0.050" off the lands has proven to be the best seating depth. Then I tried 75 grains of RL25 with the 200 Accubond at 0.050" off the lands. I shot 4 shots at 250 yards and they landed in a 1.87" group. Then 3 shots at 400 were in 2.46". Then I shot 5 shots at 500 yards and they were in 4.93". Velocity from my 25" barrel is 3070 fps for 13 shots recorded. This load is a bit hot even though the powder charge is low. Incidentally my Swarovski 3-9 AV scope's clicks have a value of 0.28"/ click, not 0.25". It only took 30 clicks up at 600 from an initial zero at 250 instead of the 34 clicks that it should have taken (based on the ballistic coefficient and velocity and elevation).
I then tried 74 grains of RL25 with the 200 Accubond seated 0.050" off the lands. I have shot this bullet in this chamber with 76.5 grains and did not blow the primer out like I did with 78 grains of H1000 so while 75 grains of RL25 is fast it does not seem dangerous. Anyway I was concerned with the high velocity of 3070 fps with the 75 grain charge and since 75 grains looked to be more accurate than 75.5 grains I thought I should try 74 grains. First group at 250 was 1.02". Second group at 250 was 1.72" with all 6 in 1.72" (5 of those were in 1.02"). There was only about 0.7" of vertical for all 6 shots at 250 yards. First group at 400 yards was 3.82" (one shot well to the right of the other two with only 0.75" of vertical). Second group at 400 was 2.11" and it landed in the middle of the first group so I had 6 in 3.82" at 400. Velocity was 3030 fps. I have my load!
Finally I went out to make sure that my rifle was zeroed from field shooting positions, not just from the bench. I shot the 200 Accubond with 74 grains of RL25 load at 250 yards from the prone position using my Kramer Snipepod and sling. Group was perfectly centered and measured 1.7". Then I tried a group from the sitting position with my taller Kramer Snipepod and amazingly shot a 0.76" group that landed in the middle of the first group such that all 6 shots were in 1.7" at 250 yards. So now I have fired 6 groups of 3 shots each (4 at 250 and 2 at 400) with the 200 Accubond seated to 0.050" off the lands and 74 grains of RL25. Average group size is 0.594 MOA. I love this rifle. I first sent this rifle off to Arnold Arms in December of 1998 after wearing out the factory barrel and 4 barrels later gave up on them (none of them shot accurately). Then I sent it to a reputable 'smith but he gave it a longer throat than I wanted. Finally it is perfect after 5 agonizing years. Rufous.
