6XC - 155gn DTAC disintegrating.

[JohanH]

Hi all,
I’m having issues with my 115gn DTAC projectiles (coated with “nose ring”) not making it to the target … visibly disintegrating in flight.
I’m running the projectiles through a 31” Pac-Nor barrel with 1:7 twist at 3,048fps.
The first 5 or 6 shots go fine and then as the barrel warms the failure rate is as high as (or higher than) 50%.
Yesterday I terminated my second string in the shoot - I was going well until shot #5 and then the next 4 disintegrated.
Anyone else out there seeing similar issues?
I’m thinking the velocity/barrel length/twist rate combination may not be compatible - planning to drop the muzzle velocity by 100fps. Any thoughts.
Cheers.

 

[Ranger188]

Your bullet RPM's are a bit high at, 313,508
The "nose ring" might be a weak point once you get that high.
How many rounds on this barrel?
Don't know if this is a .236 or .237 bore. A .236 and your other things might add to a bullet
jacket stress.
Hard carbon that's not cleaned out will also help rip a bullet jacket making it weaker also.

 

[JohanH]

Hi Ranger,
Barrel has only done 300 rounds.
I’m a very regular and fastidious barrel cleaner so, whilst I’ve not run a bore scope down the barrel, I can’t imagine its carbon build up?
Will check the bore.
Thanks for the input!

 

[JohanH]

Your bullet RPM's are a bit high at, 313,508
The "nose ring" might be a weak point once you get that high.
How many rounds on this barrel?
Don't know if this is a .236 or .237 bore. A .236 and your other things might add to a bullet
jacket stress.
Hard carbon that's not cleaned out will also help rip a bullet jacket making it weaker also.

Based on Pac-Nor specs the bore is .236 on the lands …

 

[Walt Krafft]

Back off the speed to 2850-2900 and shoot slower. Barrel heat is your main problem.

 

[rwj]

Back off the speed to 2850-2900 and shoot slower. Barrel heat is your main problem.

Not only heat, the bullet spin rate is approximately 313,000 RPM at that combination of barrel twist rate and muzzle velocity! I don’t know the RPM limit for DTAC bullets, but suspect you’ve exceeded it!

Slowing muzzle velocity to 2900 FPS is a great suggestion and will also lower the bullet spin rate to approximately 298,000 RPM. Based on personal experiences, I use a limit of 300,000 RPM for 6mm bullets…

 

[Bill Norris]

115 DTACs in my 6BRX with a 1 - 7 tw. @ 2850 to 2900 FPS works well. Accuracy just not quite as good as Barts 103 & 105s.

 

[AckleymanII]

I have had exceptional luck with Sierra 107s & Berger 108s vs 115s, and Berger 105 VLD-h at 3150 fps out of a 30" .237 bore.

Johan, I think that the rotational torque on the base of the bullet may be caving in the Boat tail...just a guess.

I shot the Sierra 107s at 3250 in a 30" bbl using R#17 with Rem 7 1/2 primers in Lapua Palma brass. This was the accuracy node with R#17, tiny, tiny groups. Again, bore dia is .2370, measured. RASS Tuner brake used.

 

[STOMP442]

Slightly thicker jacket on a Berger Target bullet may eliminate your problem as well, and you should be able to keep the velocity up. I feel like the 105 Hybrid gives me the best of both worlds, high velocity and good BC. The 109 can be a bit tricky to tune but, can be an exceptional option as well if you find it's happy place.

 

[jelenko]

To add to what has already been said.

Berger did a study on what was causing their bullets to come apart.
There were several contributing factors but the immediate cause was some of the lead melting from heat going down the barrel. Once some of the lead was softened enough, centrifugal force caused the softened lead to burst out of the jacket. The higher the RPM's, the less the lead had to be melted.

 

[JohanH]

Thanks for all of your responses - much appreciated.
The general consensus appears to be to slow the projectile down - so I’ll start with a 100fps drop and see what happens. I’ll keep you all posted.

 

[Fuj]

I'd be running an 8.25 twist rate or go to an 8.5 if you have more
room for speed. As of last I heard, Sierra was still making these
for Tubbs, Try some non ringed bullets if they are available.