Some of this information is located on a bullet-testing related thread, but I thought it deserved its own thread to talk abou the load testing and gain twist barrel.
My 338 Lapua Magnum Improved rifle was designed from the ground up to shoot some of Noel Carlson's ultra high BC lathe-turned, banded solid bullets out to and hopefully beyond 2,000 yards at my home desert range in Nevada, elevation 2,100 ft. The rifle has a 32" (not including break) Bartlein gain twist barrel (starts at a 12" twist at throat and exits the muzzle at roughly a 5.4").
Here is a picture of a dummy round using blown out brass from the same reamer and one of the 338 7 caliber long ZA bullets seated to roughly proper length. The tip of the ZA has been flattened by my gunsmith to prevent injury. . . as when I shook it out of the box it hit my palm point first and drew blood...nasty and TRUE.
Here she is on the left, next to the 7WSM and 284 Winchester (with 180 Berger's seated to their shooting length in my 7WSM and in my brother's .284 Winchester. On the left is a piece of new 338LM brass and a hydraulically blown-out case to my improved reamer design.
The 338 total length is right at 4.135 inches
This is what the rifle looks like. Manners T2-A in Coyote Brown, Barnard PL 338 single-shot action, 32" Bartlein gain-twist barrel, March 6-60X scope w/ tactical ranging reticle, and a removable Badger FTE break....(for the AAC Titan Suppressor that is on order for it). The rounds shown with it are the 338 LM Improved with 7 Caliber ZA, my 7WSM with Berger 180, and a .308 with 170 Lapua Lockbase.
Range Day-1 I ran the first 8 rounds through the rifle. The bullets were GS Custom 295 grain SP lathe turned solids. I was just trying to get a feel for the rifle and see if I could find the pressure limits with H. Retumbo powder (suggested to me by Greg Tannel). The Temp was a bit spicy, 92 degrees in the shade when I started and 96 when I stopped (as in from 7:45 am to 8:45 am).
I loaded up 2 rounds with 95 grains, 2X 96, 1X 97, 1X 98, 1X 99, and 1X 100 grains of H. Retumbo. To my surprise, I never found a cratered primer or ejector mark on the brass; no pressure signs at all. So 100 grains is well within the pressure limits of this particular firearm; the 100 Grains was producing 2,980 fps. I wanted to push it a touch faster to see if 102 was near the pressure limit and what it mean in terms of velocity. My original goal was to get close to 2,950 fps for my accuracy load with the 300 grain bullets.
Amazingly, the last 4 shots, 1X each at 97, 98, 99, and 100 grains, formed a nice little .75" outside edge to outside edge roughly 1/2 MOA clover-leaf group on the target. The slowest load (95 grains) produced a velocity of 2,840 fps; the highest (100 Grains) produced 2,980 fps.
How confident am I in those velocity numbers - VERY. I recently picked up a second CED Millennium chronograph, and had them set up end-to-end so both could measure the same shot. They were producing different numbers for each shot (to be expected), but the velocity differential was consistent shot to shot. The numbers shown above are the average of the two chrono results.
Range Day -2. This day was even hotter than Day 1. Temps ran from 96 to 102 in t he shade, so velocities were also higher. Again, no pressure signs at all. The loads with these bullets are running with low pressure and pretty uniform velocity (pretty tight ES and SD). I have gotten a uniform (only 3 shot groups so far) roughly 1/2 MOA groups at 100 yards, for ALL loads between 98.5 grains of Retumbo to 100 grains of Retumbo (1/2 grain increments). Every one of the 1/2 grain groups measured outside edge to outside edge .75" or slightly less.
Sadly my load development has stopped for the short term until I can get more of the GS Custom 295 SP bullets. I only purchased 50 and that has not been enough to provide me with definitive load development results. I am satisfied that my 1/2 MOA results are solid, but there might be a better load out there and if so it would be nice to find it. At very long ranges I would prefer 1/4 MOA to 1/2 MOA any time. Granted, the shooter makes up for a ton more accuracy at the target then that, but it's a confidence thing to me. (I am holding by my last 18 bullets to shoot steel out to 1,800 yard with my brother and a friend from Houston.)
I will post more from my Desert shooting trip and loading results with the rifle as they are known.