I'm running a 300 Norma Alex wheeler improved. Last season I was shooting 230 atips at 3195 and maybe would loose 1 out of 50 but was a super accurate load at distance. Today went back after taking the whole gun apart for a thorough cleaning and inspection. Same load same varified speeds but these things were just blowing up 80% of the time. My twist is 8.5
When running the 245 bergers at 3000 I had no issue same as last season. Any thoughts or recommendations?
Calculated rpm is 270,000 and stability factor of 1.73
How many rounds on the barrel and how long is it?
About 10 years ago, I was testing the cost/benefits of overbore ratio and pressure for ELR cartridges. I was using the 338 Lapua case as the parent because it was known for consistent quality and ability to handle pressure. The 338 Lapua was also a poster child for improving because it picks up significant capacity. I ran the overbore ratio up by improving cases and necking down. None of them were intended to be a final solution, just provide the information for a better choice.
I started with a straight neck down 7/338L. It's capacity was pretty close to a 7/300Norma. That went pretty well so I did 30/338LI, then 7/338LI. I was using modified Stevens 200 actions in XLR chassis as mules to keep costs low enough to run a few of them at a time. The baseline they were compared to was a 7/270wsm set up the same way.
The 7/338LI had a spectacular run on a 32"x36" 2300 yard target with about 75 rounds on the barrel. Frustration started soon after. Vertical spread opened up and nocalls increased. I was shooting with 3 others and one of them happened to be off his scope during a string of nocalls. He suggested the bullets were bursting. They were. I started videoing strings on a 1200 yard target where the lighting was often right to catch trace.
Poof Video
The bullets were Berger 195 EOL at 3300 fps from a 32" barrel. The barrel had 250 rounds through it. Checking the 7/338L showed it was doing it as well at 450 rounds.
I did that for a couple years and came away with the following perspectives:
1 - Bullets will put up with a lot of nonsense with a fresh barrel
2 - Throat wear, pressure and rpm all contribute to bullet bursts.
3 - There were several stages of bullet performance deterioration. Working backwards from the obvious:
3.1 - Bullet bursts early in a string, multiple bursts per string.
3.2 - Bullet bursts late in later strings
3.3 - Wounded ducks. The trace would show a distinct kink and the impact would be at a random point inexplicably outside the rest of the group. Sometimes the video would pick up the core and jacket separating.
3.4 - Miserable BC consistency shown by much larger vertical dispersion that really starts to show up by a mile.
3.5 - Degraded BC consistency. Barrel is still useful inside a mile if competition didn't demand extreme levels of precision.
3.6 - The trued firing solutions will show the BC trending downward.
4 - By the time the barrel bursts its first bullet, it hasn't been much use beyond a mile for a while.
5 - Boutique brass will take more pressure for multiple firings than gives the best performance from jacketed bullets. Even with a fresh barrel.
6 - Depending on your cleaning process, the velocity of a given load may increase over the entire useful life of the barrel. Alternatively, adding abrasives to the cleaning process can control this. The problem is the BC consistency goes away even faster.
This is smash and grab science at best. Nothing was proved. I'll chamber a fresh barrel and waste the last 100 rounds of a 500 round barrel before I waste another 100 rounds of ammunition. Do the math.
