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Well, Went out this morning and did some more range testing with the 338 AM. I had tested the 350 gr ULD RBBTs previously and ran into the same primer cratering as in previous tests.
Well, after discussing things a bit more with Black Diamond 408, I decided to replace the very long striker spring in the BAT receiver with a very stout 32 lb Rem 700 spring. This required a spacer to be machined that would take up the extra space on the long BAT firing pin so it would work with the Rem 700 spring.
I had tested this setup with the 350 gr ULD RBBT and the primer cratering was significantly less then before, a dramatic improvement over the factory spring.
For this test I had the spacer on the front of the firing pin assembly which added its weight to the moving striker assembly.
For the test today I put the spacer on the rear of the assembly near the cocking piece. In this location is was not included in the moving striker assembly and lock time should have been decreased significantly as the spacer was made from stainless steel.
Loaded up some test loads using 151.0 gr H-50BMG Under the 300 gr SMK and seated them to just touch the lands at an over all length of 4.135".
First shot down a clean oily barrel showed significant cratering. Second shot pierced the primer as did the third shot. The average velocity was 3558 fps for the three shots and the group at only 100 yards was in the .2"s.
Still I was baffled by the return of the cratering on the primer. I set there and ran things over in my head and then got an idea to try. I ran back to the shop and pulled the bolt apart. Put the spacer back on the front end of the firing pin so it would again be part of the moving striker assembly and then ran back out to test the last three test shots I had loaded.
As with the 350 gr ULD RBBTs, there was still cratering but dramatically less then there was with the spacer at the rear of the striker assembly. Fired the last three shots and cut another one hole cloverleaf in the .3"s with an average velocity for those three at 3554 fps with an Extreme spread of 8 fps.
There was cratering but no danger of piercing at all.
This is however to warm of a load with this powder. Primer pockets were just beginning to loosen at this point and there was more of an extractor mark then I like to see. I think I will drop her down to 149.0 gr and call that max with H-50BMG. Velocity will be right at the 3500 fps level.
So why did the location of the spacer make any differnce at all in the primer cratering, well, in my opinion, its a simple matter of firing pin momentum. With the spacer at the rear of the assembly, its weight is not included in the moving striker assembly.
While the striker assembly may be faster, it will have less momentum when it strikes the primer allowing the chamber pressure to more easily push the firing pin back resulting in pierced primers at lower pressures.
With the spacer positioned on the forward end of the firing pin, its weight will be included in the moving striker assembly. While the firing pin velocity may be less, energy will increase as will momentum which seems to be enough to resist the chamber pressure forcing the firing pin back and limiting cratering.
Lock time does not seem any different although it has to be slightly slower but with the very stout 32 lb spring, I would bet its as fast as a standard Rem 700 which by most standards is still pretty fast.
I was a bit suprised at the results but it does make sense.
With the spacer in the correct location, +3500 fps is practical with the 338 AM and a 300 gr SMK. Bolt lift and extraction are effortless to say the least with even the 151.0 gr load level. Dropping the charge down to 149.0 gr will just add case life with no real drop in performance at long range.
With those six shots I got her zeroed at 100 yards so now I can take her out and test at long range. Will do so this week and post the results at long range.
Just thought some of you may be intereted in this test.
Have a good weekend,
Kirby Allen(50)
Well, after discussing things a bit more with Black Diamond 408, I decided to replace the very long striker spring in the BAT receiver with a very stout 32 lb Rem 700 spring. This required a spacer to be machined that would take up the extra space on the long BAT firing pin so it would work with the Rem 700 spring.
I had tested this setup with the 350 gr ULD RBBT and the primer cratering was significantly less then before, a dramatic improvement over the factory spring.
For this test I had the spacer on the front of the firing pin assembly which added its weight to the moving striker assembly.
For the test today I put the spacer on the rear of the assembly near the cocking piece. In this location is was not included in the moving striker assembly and lock time should have been decreased significantly as the spacer was made from stainless steel.
Loaded up some test loads using 151.0 gr H-50BMG Under the 300 gr SMK and seated them to just touch the lands at an over all length of 4.135".
First shot down a clean oily barrel showed significant cratering. Second shot pierced the primer as did the third shot. The average velocity was 3558 fps for the three shots and the group at only 100 yards was in the .2"s.
Still I was baffled by the return of the cratering on the primer. I set there and ran things over in my head and then got an idea to try. I ran back to the shop and pulled the bolt apart. Put the spacer back on the front end of the firing pin so it would again be part of the moving striker assembly and then ran back out to test the last three test shots I had loaded.
As with the 350 gr ULD RBBTs, there was still cratering but dramatically less then there was with the spacer at the rear of the striker assembly. Fired the last three shots and cut another one hole cloverleaf in the .3"s with an average velocity for those three at 3554 fps with an Extreme spread of 8 fps.
There was cratering but no danger of piercing at all.
This is however to warm of a load with this powder. Primer pockets were just beginning to loosen at this point and there was more of an extractor mark then I like to see. I think I will drop her down to 149.0 gr and call that max with H-50BMG. Velocity will be right at the 3500 fps level.
So why did the location of the spacer make any differnce at all in the primer cratering, well, in my opinion, its a simple matter of firing pin momentum. With the spacer at the rear of the assembly, its weight is not included in the moving striker assembly.
While the striker assembly may be faster, it will have less momentum when it strikes the primer allowing the chamber pressure to more easily push the firing pin back resulting in pierced primers at lower pressures.
With the spacer positioned on the forward end of the firing pin, its weight will be included in the moving striker assembly. While the firing pin velocity may be less, energy will increase as will momentum which seems to be enough to resist the chamber pressure forcing the firing pin back and limiting cratering.
Lock time does not seem any different although it has to be slightly slower but with the very stout 32 lb spring, I would bet its as fast as a standard Rem 700 which by most standards is still pretty fast.
I was a bit suprised at the results but it does make sense.
With the spacer in the correct location, +3500 fps is practical with the 338 AM and a 300 gr SMK. Bolt lift and extraction are effortless to say the least with even the 151.0 gr load level. Dropping the charge down to 149.0 gr will just add case life with no real drop in performance at long range.
With those six shots I got her zeroed at 100 yards so now I can take her out and test at long range. Will do so this week and post the results at long range.
Just thought some of you may be intereted in this test.
Have a good weekend,
Kirby Allen(50)
