Twist question - DTACs at low velocity

[benenglish]

I've searched the board and found plenty of input on what twist to use with the 115-grain 6mm DTAC. However, everything I've seen has been concerned with *rifle* velocities. I'll soon build a 6mm silhouette pistol,haven't decided on a chambering yet) and I plan on launching DTACs at no less than 2200 fps nor more than 2500 fps. I expect to stay consistently in the 2300-2400 fps range.

In that velocity range, what rate of twist should I be looking for? Based on my past experiences,some of them quite bad, though quite educational) I strongly prefer to err on the side of "too-fast" rather than "too-slow" a twist. I'm also aware of the twist rate manufacturing tolerances that can bite you when you run on the edge of stability; that's yet another reason to prefer faster twists to slower ones.

TIA for any help,

Ben in Texas

PS - Any musings on chamberings are also welcome. I sort of assumed, when this project first coalesced in my mind, that I'd chamber for 6BR. Lately, though, I've been looking at the 6mmAR. Any thoughts, especially about attainable velocities from a 15-inch barrel?

 

[Cheechako]

Ben

Call the good guys at Sierra and ask them. They probably did testing at all velocities to establish the BC. 1-800-223-8799.

Ray

 

[mikecr]

I'd be amazed if Sierra did any real stability testing. Why would they?

If you go with 1/7.5" it'll be fine at any velocity or condition.

 

[Cheechako]

mikecr

I hear the same thing about EVERY bullet ever made. Does that mean that NO bulletmaker EVER does testing for BC? I tend to be more positive and assume that Sierra probably did some testing. That bullet was in development for more than two years, including field testing by David Tubb and others, and I find it hard to believe that the tests were for velocity and accuracy only. At a minimum I believe they would have established BC at 100 yards at a certain velocity and calculated the rest. What else are they paying those ballistcians for?

I have heard the same comments about Berger bullets. I know that Walt calculated the BC of many of his bullets based on actual chronograph results. Not all bullets to be sure, but many of his VLD designs. It seems that when a shooter doesn't see the results he expects he immediately questions the maker's claims. It's interesting that you seldom hear a shooter doubting the maker when his tests seem to indicate a BC higher that what is claimed.

BTW, I think that actual results reported by shooters all across the country confirm that an 8 twist is sufficient for the DTAC at rifle velocities. Your recommendation of a 7.5 for the lower pistol velocities is probably a good one.

Anyway Mike, thanks for letting me rant. I'm not disagreeing or arguing with you on this, just venting. I feel better now.

Ray

 

[AlNyhus]

Ben, I've included a link to the JBM Ballistics site. Just input the data and you'll be able to determine what twist you need to properly stabilize an individual bullet. FWIW, a G.S. figure of 1.5 is considered 'perfect' by most ballisticians.

It can seem a bit daunting at first, but if you click on the 'explanation of terms' it clears up pretty quickly. Hope this helps. -Al

JBM Ballistics

 

[mikecr]

Ray, I do believe Sierra. I know Sierra does alot of good testing. But we're not talking about BC. We're talkin stability.
If Sierra does any testing in this area, they're results are certainly a secret. I think they commisioned Henry Childs to do this for them,with the DTAC). They took what they wanted, ignored the rest, and went with what they had.

The fellow asking might do well in 8tw. His profile suggests Texas. I figure 115smk Sg at 2400fps, 80deg & 500'ASL in 8tw = 1.3
Sg of 1.5 under these conditions occurs with a 7.5tw
My estitmates here are based on Henry's measurements and testing, as well as reported by shooters.

We're not really arguing

 

[Wild_Bill]

Hi i would go with a 7 twist as if you are to slow with the twist the 115gr projectiles will not shoot but a little fast and they will still shoot i would go with the 7 twist just for added insurance. I have fired them in a 6mm Dasher 26" barrel and 8 twist. What calibre will you use? A Dasher or BR would be good cases for a 16" pistol barrel as the powder charges are relative small and the powders like H4895 and Varget work well in both aswell.

Cheers Bill
Australia

 

[Allene]

Mikecr
I'm not knowledgeable enough to know what Sierra does or doesn't do but I do know that on most 500 count boxes of bullets, they do print minimum twist for the barrels. I'm also not trying to be argumentative and I enjoy seeing other points of view.
Allen E