I have a 26in bartlein 1-7.5 twist chambered in 6 Creed on a impact 737. It was a great shooter with around 0.3 moa groups and SD's around 4 for at 2945 fps for about 600 round. Since then I have a big increases in velocity with about every 80 - 100 rounds. It will just anywhere from 35 to 40 fps and my SD's have slowly moved up to around 11. Now my velocities are up around 3080 fps and if i try to change my load at all i get pressure signs even though i am near a minimum charge of powder. Also I have to rezero every couple of weeks. My groups are still around 0.5 moa at 100 yards. I have tried different brass and powder and primers with no change. I have close to 1600 rounds on this barrel. I am wondering if I should pull it off and get a new barrel. My buddy has the same spec barrel as me on a impact and his will shoot lights out with what ever he throws in it. If someone has a recommendation for something else please let me know.
Unusual velocity changes.
- Thread starter PB6creed
- Start date
1600 rounds is about where I would expect the barrel to drop off. But the velocity numbers don’t normally go up at end of life. The other things like pressure and you saying you are at minimum loads. Have you checked you powder thrower against a different scale?
I'd scope the bore to see if you have a carbon ring or other buildup. Also see how the throat is looking.
Cleaning regimen? I would be borescoping for carbon ring and setting barrel back to steel or minimal fouling and checking again.
Yesterday I shot 10 rounds, average MV 2628, es 18.9, sd 6.4. Last week I shot 28 shots, average MV 2798.7, es 27.2, SD 7.2. Same rifle same load. The rife shot very good groups yesterday and seemed to shoot very good on steal last week. Xero C1 Garmin. Different location. What is the most likely explanation for the huge change in MV? I have no idea whats going on. Thanks for your input.
What powder are you using and was the temperature very different between range sessions? Some powders are more temp sensitive than others.
the data matches the slower velocity. The faster velocity is off the chart. 139 grain scenar and 39.8 grains H4340. I shot out to 1000 yards and was high when using the fast velocities in my ballistics calculator. I've had carbon rings before but never seen one go away without world class removal efforts
Others on here gave you some solid leads to follow. Since your reloading, I'm gonna go out on a limb here and say to check your case lengths and make sure you don't have some that have stretched out a bit too long due to multiple reloadings. I know it's a longshot (no pun intended) but I've seen it happen.
Might be worth starting your own post man. I almost put your experience in with the OP, and was confused for a moment until I realized you jumped on his post with your own questions.
You'll also want you add in your cartridge, cleaning regimen, and round count when you ask.
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I'm going with the others on the development of a carbon ring. Symptoms of that are usually along the lines of what you're seeing (velocity/pressure increases).
This is where borescoping your barrel/chamber is worth the small investment in the equipment. Until you confirm that you have or don't have the ring, you should hold off on making big changes to reloading equipment and components.
Also, 1,600 rounds on a 6 CM is right at the end of most barrels' lives (and many give out before that). When your SD opens up, velocity starts to drop (what you may not see with that ring), and accuracy starts to come apart...you've toasted it. It wouldn't be bad to get your new barrel spun up now so you can replace your old one without any down time.
I'd invest in a bore scope and see what's going on. As others mentioned you could have a carbon ring.
