115 DTAC Blowups!

[GLC]

I encountered a string of Blowups in a recent county F class championship,UK),1000yrds) I was shooting my new 6x284 with Krieger 7.5twist barrel, Nesika K action Mcmillan F class stock... approx 50-60 rnds through it.
Dtacs were running 3240 with N560, no pressure signs at all..
Load was developed at 100yrds with regular 1/4" groups if I did my bit.

First shot downrange did not indicate on the Target 2nd shot was a 4 which I adjusted for and 3rd shot,1st to count) another miss, by this time I was scratching my head as I had already lost any chance of a decent placing! 2nd to count was a v bull on the tight target with a further 4 consectutively, then the next shot was a witnessed blow up by my fellow competitors,300 yrds down range) and the same with the next 3 shots, final string were hits, but we think some jacket material made it!

My question is: has anybody else encountered this? I thought the Dtac,with Sierra jackets) would easily hold together at this velocity, is it the 7.5 twist that is causing it? would a single base powder be the answer?

Very frustrating!
This makes this cartridge pointless as I can get a .243 going 3200 quite easily with this bullet.

I know of some people running these bullets at 3300 without any problems.

I have e-mailed Superior shooting, but as too date I have not had a reply.

Appreciate any input.

P.s I borescoped the barrel which looks like a copper mine!!!

Thanks

Gary

 

[mikecr]

The cartridge truly IS pointless...
Why not a 6BMG? With that you could have even more heat/cal, and with 7.5tw you could blow up all but solids!
For a couple hundred rounds anyway..

Seriously, you can have the next barrel,one in 8-8.5tw) chambered in something practical, and never run into bullet issues.
Or you might barely get by with your current setup with a treatment of Tubbs FinalFinsh and then tungsten coating your bullets.
Or go to solids,good luck with that).

 

[rstreich]

I think you need to find out why you have all of the copper in your barrel. Clean it out and then borescope it. I've got an 8-twist Krieger on my 6-284 and I haven't seen even a hint of copper when I clean.

I don't see how a change of powder would keep your bullets together, but with that combination, I'd consider a slower powder like Reloder 25 or N-165 instead of N-560.

Mike, Jeez, I'm sure we're both sorry to offend your sensibilities, but, personally, I'm looking forward to working a PD town next Spring with 87 V-Maxes at 3600 FPS.

 

[rcw3]

Appreciate any input. . . .

I borescoped the barrel which looks like a copper mine!!!

Gary

Velocity seems to be the focus and bullets are blowing up. Hmmm, let's see, how about give your barrel a good cleaning to get rid of the copper and slow things down to reasonable levels. I find it hard to believe you can get consistent accuracy up there anyway.

Robert Whitley

 

[Bob3700]

Gary,

That cartridge is certainly going to make a 243 look like it has very long bbl life. I can't imagine that a steady diet of 107-115 gr bullets would give a bbl life of even 1K rounds.

That said. You need to get the copper out and then treat the throat of that bbl to make it smooth. That probably means JB and some Bore Tech Eliminator. Use those until your borescope says things are clean.

Now take an honest look at the throat area of the bbl. Does it have the "lake bed cracking" effect? If so, that is the source of your problems. The cracking is deteriorating your jackets before the bullet even gets out of the bbl. If it is not too bad, you can smooth the throat and the first 6-8 inches of bbl with some lapping compound. Try some JB Bore Brite. That will help to smooth the bbl a bit.

Also, try some coated bullets. All those things will help, but if the bbl is too far gone, replacement is the only sure fire cure.


Bob

 

[jb1000br]

If you smooth it up too much it will collect MORE copper...

too fast and fast twist have a propensity to blow up bullets...not all barrels treat the bullets the same...some just tear up bullets...ask Randy Robinette.

JB

 

[Forum Boss]

Yes, be mindful of what Jason says--"mirror finish" bores WILL grab more copper.

--It may not be "flavor of the month", but the 6-284 has much to recommend it. Skip Talbot and Randy Dierks set many records with this cartridge not too long ago, and I've personally seen a 6-284 spank everything at my own club's 600-yard varmint match. It IS an overbore cartridge, so barrel life will be short. But the 6-284 can deliver some impressive ballistics, even with a 107 SMK. Dierks 6-284 held the NBRSA light class 1000 yard 6-target aggregate world record of 7.162" for 3 years.

GLC--Note that Dierks was shooting a 107 SMK at 3450 fps in a 1:8.4" twist barrel. Faster than your load, but a slower twist barrel.

Link: http://www.dierksind.com/6mm-284.html

--Q: What is the barrel bore diameter? If this is a 0.236 Krieger, you might want to move to a bullet that's a bit narrower in diameter. An anti-friction coating might be in order for the DTACs if you stick with them.

 

[GLC]

Guys Thanks for the replies:

Firstly in response to Mike CR:
The 6x284 was a project to see if I could get a 6mm cartridge to compete with the big 7's at a competitive level, which at the current velocity it does,,apart from not hitting the target! I am fully aware of barrel life and I actually shoot a 6x47SM for some comps, this is an excellent cartridge and it will compete with 6.5x284 or a steady or readable day, just not with fish-tales or good old English weather!

I am lucky enough to have several competition rifles in different calibers so it's a free country,just about in the UK!)

Saying that solid bullets are now banned in competition,don't ask) so cannot go down that route.


Re Copper:

Most of the is located in the last 12 inches of the barrel, I am presuming this is where "something" happened to the bullet, Barrel is new, no more than 50 Rnds and was spotless before the start of the shoot, rifle was chambered and assembled by one of the best gunsmiths in the country so I doubt if there is anything sinister in the chamber,,i looked but my eye is relatively untrained in for this sort of thing)

Another forum member sent me a PM and he also mentioned if my barrel was tight it may cause excess friction with the DTAC with is slightly oversize? I am beginning to think this may be it, coupled with a fast 7.5 twist......, I could try the Berger BUT the reason I avoided it was reports that it would vaporise.
NO fireacracking at all in the throat, as I said this barrel is new,a very minimal amount of copper past the throat.

One other thing I forgot to mention, I have pointed the bullets, do you reckon this may have had any influence on the jacket's integrity?

I recently had a very bad experience with JB paste and do not want to go down that route.


So..

I will find out the bore OD and report back, I will also back the load off a grain or so,3200) and try it, I will also try the boron Dtac's.

I appreciate all of your comments, they have been very helpful,,maybe when my barrel goes south I will go for an 8 twist instead!!)

Regards

Gary

 

[BlackHawk95]

Hello, Ive fought the same problem with a .243AI ever since I got the gun. I have a Lilja 7 twist barrel. And Ive shot just about every bullet trying to find one that doesnt vaporize. On my gun however, whenever the velocity got over 2950fps i would start loosing bullets. The berger 115 VLD's were the worst. The DTAC 115 did better but after about 25 rounds it would start happening again. The last bullet I tried was the 105 Scenar. I did have better luck with it. As it is smaller in diameter. I am however replacing that barrel with a 8 twist Broughton 5C.

Good luck,

Jerid

 

[rstreich]

Re Copper:

Most of the is located in the last 12 inches of the barrel, I am presuming this is where "something" happened to the bullet

That's just where the copper condensed, and not necessarily where the damage occurred.

Another forum member sent me a PM and he also mentioned if my barrel was tight it may cause excess friction with the DTAC with is slightly oversize? I am beginning to think this may be it, coupled with a fast 7.5 twist......, I could try the Berger BUT the reason I avoided it was reports that it would vaporise.

Could be that even the Sierra jackets given the bullet-to-bore dimensions and high RPM put you over the edge with that bullet. How does your barrel compare to those that you say are pushing their DTACs at this speed without any problems?

I wouldn't bother trying the Bergers or any other J4 jacketed bullet, at least until they come out with one of the thick-jacketed versions for the 6mm.

Paul,Moderator) made a good suggestion: a skinnier bullet. If you don't mind giving up the extra BC, you could try the 105 Scenars. I have a 31", .236 bore, 8-twist Krieger and I'm headed to the range today to try a load of RL-22 that runs the Scenars at around 3400 I think it is. I ran them all the way up to 3500+ with both RL-22 and RL-25 with no indications that anything was coming apart. I was on a 300-meter range, but I think if they were going to come apart, they would have done so before the 300-meter mark.

One other thing I forgot to mention, I have pointed the bullets, do you reckon this may have had any influence on the jacket's integrity?

Could be, but unless you put a lot of pressure on the bullet, I wouldn't think this would show up in the bore, i.e., all of the copper in your barrel.

 

[GLC]

Rstreich,

thanks for your info, understood about the copper location.
Left it to soak with Wipe-out overnight, and it came out liquid blue! I am sure another 3 of 4 applications then a brush with copper solvent will be required.
One of the UK BR guys,Vince Bottomley) a contributor to this site, is shooting a 6x284 with DTAC's BUT he is using a 8.5 twist with good success, he has had it up to 3300 with no problems but settled on 3250, not sure if he has a tight barrel. Also good friend of mine has an identical rifle with the same Krieger 7.5 twist and cambering,same reamer) but I do not know how "tight" his barrel is, he is using retumbo and settled on 3220, so this will be my next test to drop the velocity.

I considered the smaller bullets but we have 3275FPS as range safety limits for F class so I would think a DTAC at 3150-3200 would still be better than a 105 or 107 @ 3250-3300?,which is already over the limits)

I will try the coated DTAC's on Wed next week, I have a 1000yrd lane booked with electronic target at Bisley, I will also try a grain less. Still no response from David Tubb's company......

I did all my original testing in a 100yrd Tunnel so I am sure I would have no indication of problems.... until shot at distance so your testing at 300 Meters is perfect.

Again thanks for your suggestions, I will post my findings, so hopefully other do not encounter the same frustration!

 

[mikecr]

If your in it for the learnin, there is an opportunity here.

You say your friend settled on 3220fps with a SIMILAR barrel.
Can you MEASURE the twist rate of both barrels?
Do you have high confidence in his velocity measurements?

Here's the thing; There is nothing wrong with DTAC bullets.
People shoot em all the time without issues. And even though there are recommendations out there for 7.5tw, they don't need it.

Now this is how I would have predicted, up front, that you could run into this. And you can validate or ignore it, up to you.

Bullet RPM Limit =,0.308-YourCAL)*238+290
Your RPM's =,720*VEL/TWIST)/1000

So your RPM limit is ~305Krpms
Your friends 3220fps & 7.5tw produces 309Krpm, but if his twist is actually 7.6 he would be just at the limit at 305Krpm. If even slightly slower still, he would be fine.
This may be the case...

Obviously there is more to it, more real factors, but I haven't seen many contradictions to this test.
If it holds here, YOUR blowups will stop at ~3150fps.

Good luck either way

 

[GLC]

Mike CR

Great info, My friend is going to shoot his 6x284 alongside mine, his velocities are credible as taken with the same chrono in the tunnel range.
I will measure both twists and try to work on your formula, I totally agree that a 7.5 twist is not necesscary.
Watch this space after Wednesday!
I am going to solve this one way or another...

I am led to believe the 7.5 twist Kriegers we both have are from the same batch and order, but one could be easily out.
Again as ever I appreciate all your help and comments

With regards
Gary

 

[rstreich]

I considered the smaller bullets but we have 3275FPS as range safety limits for F class so I would think a DTAC at 3150-3200 would still be better than a 105 or 107 @ 3250-3300?,which is already over the limits)

Yeah, with the range limits, it'd be better to try and just slow it down.

Bullet RPM Limit =,0.308-YourCAL)*238+290

Mike,

Where did this calculation come from? It seems a bit arbitrary. In load testing, I've pushed 87 V-Max bullets all the way up to 3800 without losing any in an 8-twist. Today I was only shooting 100 yard groups with them at 3550+ with good accuracy. Both are well over your RPM limit.

 

[mikecr]

It's a rule of thumb I put together 10+yrs ago. A best effort that fairly well predicted the blow-up point with wildcat cartridges pushing heaviest/cal bullets. Combinations such as the 22/243 middlestead and 90JLKs..6/06 & 105VLDs...30/CheyTac & 240smks...

But as with all rules of thumb, it's use is limited to a narrow range because it doesn't actually represent the truth,alot like Greenhill).
In this case the narrow range is HEAVIEST per CAL, as this is where I had plenty of blow-ups to define.
Few lighter bullets,like 95gr and below) blow-up because they generate less in rotational forces, for less time, and usually with less heat.

I'm pretty sure Sectional Density should be included in the rule, but for now, I just keep my eyes open for any blow-ups discussed. Maybe someone will talk about blowing up lighter bullets. This will be rare, because there is no reason to end up there. The twist requirements are lower, and the energy required would always be better used for higher BC bullets.
And without the data, it's pretty hard to predict anything.

It takes time to nail something like this, and it couldn't be done at all if it weren't for forums with plenty of open discussion. Just try & think of any other way it could be done..
Who would do it?
Well I'm trying

 

[BlackHawk95]

Mike. The troublesome Lilja barrel Ive got in .243AI blows up bullets in this fasion, 115gr DTAC at 2975fps, Lapua 105gr Scenar 3100 FPS and a 95gr Sierra M/K at 3200 FPS. Its a 7 twist barrel. Whats funny about my barrel is it seems to shoot the 95gr bullets better than the 105 or 115gr!

The last time I shot the gun it was all over the place! I couldnt get it to group under an inch. Whats strange is, right after I got the gun back from the gunsmith, I shot a few groups with the 105 Scenar and they shot pretty well for not really trying. I then layed the gun down for a couple weeks and came back to it and I couldnt get it to shoot at all. Whats up with that? So Im having an 8 twist Broughton put on chambered in 6mm BR.

Ive got another Broughton 8 twist on order to try again in the .243 AI. I hope it goes better.

Jerid

 

[GLC]

Update:

Tested the rifle today @ 1000yrds:

EVERY bullet fired from 3087-3200 vapourised, including the boron coated,not one got to the target), my friends did not miss a beat, @ 3220-3230fps did not measure the two twists, the rifle is going back tomorrow for some professional advice!,I have a sneaky feeling my twist is fasting than ordered.

Will report the final conclusion.
regards

Gary

 

[BlackHawk95]

Gary, Im really interested to see what is said about your barrel. It sounds as if you have the same problem I have. However your barrel seems to be able to shoot a little faster than mine before the bullets blow.

Please, keep us informed

Jerid

 

[GLC]

Will Do Jerid!

 

[GLC]

Hi Lynn,

thanks for the info, very interesting, however it still doesn't explain while a virtually identical barrel with,alledgedly) the same twist shot in the same conditions with the same load had no problems whatsoever, this barrel was clean as well?

,our seating depth's were within 20 thou of each other as well and both of our DTACS were pointed.

I am going to send the rifle back to the Gunsmith for his opinion and depending on the findings,and if I get it back unaltered I will try as you suggest)

THANKYOU!

with regards

Gary