A-tip Blow Ups

[Tumbleweed]

Hey all. I've had three confirmed mid air blow ups of 250 A-tips out of my RUM. The barrel is a 34" Bartlein 5r 400MOD 1:9 to 1:8 transitional twist with 400 rounds on it. It also has a DE +P throat that is all but blown out... so lots of jump before making rifling contact.

The A-tips are running 3160fps.
First time was shooting at a 250 yard target where the bullet never made it and the Labradar gave no reading. The last two times have been at ELR where there was no impact and I had Labradar readings out to about 50 yards before they blew up. One was caught on video with a piece of shiny jacket glint exiting out to the right.

Curious if guys have ran them faster with good results or if this is right at the threshold of what these bullets will put up with. It also could be that the bullets don't like 1 1/8" of jump before they slam into the rifling. Accuracy is still good at .3 to .4 at 100. Last weekend I shot a 4 shot group at 2,093 yards that held 7" of total vertical dispersion. So, when they're not blowing up, they're performing

 

[B23]

When you say "throat that is all but blown out" I'm assuming that means excessive throat erosion? A rough throat can wreak havoc on bullets.

Also, even with a 34in pipe and Shawn's +P throat, 3160 is blazing fast for a 250gr in a 300 RUM so that combined with a potentially rough throat could be causing your bullets to self destruct.

On a side note, Bartlein's 400MODBB barrel steel is supposed to last a fair bit longer than regular SS, I'm curious, have you found this to be the case or not so much? What is your round count in that rifle?

 

[Tumbleweed]

When you say "throat that is all but blown out" I'm assuming that means excessive throat erosion? A rough throat can wreak havoc on bullets.

Also, even with a 34in pipe and Shawn's +P throat, 3160 is blazing fast for a 250gr in a 300 RUM so that combined with a potentially rough throat could be causing your bullets to self destruct.

On a side note, Bartlein's 400MODBB barrel steel is supposed to last a fair bit longer than regular SS, I'm curious, have you found this to be the case or not so much? What is your round count in that rifle?

So it's the +P shallow land section of the throat that is gone. It's not a very rough throat yet. I don't see a "rough throat" being a contributor at this round count. Yes, speed is good but it's not crazy pressure, I still have normal extraction using ADG brass. Reloder 50 is performing well in this long barrel. Too early to tell on the 400 MOD yet, I bought it in the hopes of extended barrel life. I am hearing reports/concerns that the barrel life may not be as good due to the material being too hard. I guess time will tell.

 

[EsotericJunkie]

3160 x 720/8=284,400 rpm. You can expect blowups at that RPM with very long bullets. The jacket gets thin as it is extruded longer and longer. You might want to back the velocity down to 3000fps which will yield 270,000 RPM.

EJ

 

[Tumbleweed]

3160 x 720/8=284,400 rpm. You can expect blowups at that RPM with very long bullets. The jacket gets thin as it is extruded longer and longer. You might want to back the velocity down to 3000fps which will yield 270,000 RPM.

EJ

Great point! Hadn't really heard what Hornady's RPM threshold was on these. You very well could be right

 

[Lawnboi]

What’s your bore/groove diameter? Seems like that’s a contributing factor in some other calibers when it comes to blow ups.

I’m shooting 110s at 2990 in a 7 twist. I’m 600 in without any blow ups yet. That’s in a .237 barrel, 6 groove, with cleaning every match (~160 rounds)

 

[Tumbleweed]

What’s your bore/groove diameter? Seems like that’s a contributing factor in some other calibers when it comes to blow ups.

I’m shooting 110s at 2990 in a 7 twist. I’m 600 in without any blow ups yet. That’s in a .237 barrel, 6 groove, with cleaning every match (~160 rounds)

Dimensions are standard SAAMI. .300" land .308" groove. This is a 5r which in theory should be easier on jackets. Part of my reasoning doing a transitional twist was also to put less stress on the jackets with a gradual increase of bullet RPM in the bore. Of course once it's in the air, that may not matter.

 

[Tumbleweed]

Thought I'd give an update. I've continued to get 1 out of 10 or so bullets that come apart mid air. Today I backed the load down to 3025 fps to see if they would hold up. I had two more blow ups that did not make it to my 100 yard target verified by incomplete yardage speed readings on the Labradar. My theory is that with this +P throat that is now gone, the jump and consequential slam into the rifling is too much for the Atips to handle. Back to the 245's looks like

 

[Harvey_NW]

It also could be that the bullets don't like 1 1/8" of jump before they slam into the rifling.

An inch and an eighth of jump?