Twist rates for shorter barrels

[buggsb]

In my particular application a shorter rifle is highly advantageous. My current rifles are using 16" and 18" barrels.

At SHOT I spent some time looking at the Desert Tactical rifles and noticed that their compact model in .308 is using a significantly faster twist (1:8 i believe)

Any thoughts here regarding the necessity/advantages of faster twist rates in shorter 16-18" tubes?

Thanks in advance for any help you guys offer!

 

[mwroseberry]

Re: Twist rates for shorter barrels

Helps stabolize the bullet since you have less barrel to stabolize in.

 

[Rob01]

Re: Twist rates for shorter barrels

There is no reason for an 8 twist on a .308 unless you plan on shooting 240 SMKs subsonic all the time. A 10 twist on either of those barrel length will work fine.

Not sure where you saw 8 twist on DTA but their 308 rifles come with 11 twist barrels. Even the 16" Covert. They might have had a special build with a 8 twist but I don't think it's standard.

Maybe we can get someone from DTA chime in as they are here on the Hide.

 

[rcw3]

Re: Twist rates for shorter barrels

Typically a slightly faster twist may be viewed as desirable only because velocity may be slower with a short barrel and the faster twist may help with bullet stabilization - but typically it's only a half inch or so faster (i.e. like instead of an 8.5 twist for a 6.5 mm barrel with the 140-142 gr. bullets, they run with an 8" twist).
Robert

 

[gstaylorg]

Re: Twist rates for shorter barrels

I have one of the Coverts. It has a 16" 8-twist pipe and, although the DTA website still states a 1 in 11" twist, I believe the 1 in 8" will be the standard. Although originally designed with heavy (>220 gr) subsonic loads in mind, the 16" 8-twist has turned out to also be remarkably accurate with lighter (shorter) loads as compared to the subsonics (see Fredo's post ~1/3 down the page at the link below). Mine seems to favor 175 and 190 SMKs over the 168/155/150 loads I've tested.

http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=1597047&page=13

 

[Rob01]

Re: Twist rates for shorter barrels

They must have swapped over and didn't update the site.

I am betting it has more to do with heavy subsonics than length as I know POF has 12" and 9" .308s and they run 10 twist barrels on them. They have shot them to 1000 also.

What velocities are you getting with your Covert and the 175/190s?

 

[BlackOps Tech]

Re: Twist rates for shorter barrels

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Rob01</div><div class="ubbcode-body">I am betting it has more to do with heavy subsonics than length </div></div>

I agree. The only .30 cal I currently use an 8 twist on is the CRUX, which is a 16" rifle chambered for the .300BLK. It's primary duty is to launch the 220gr subsonic loads. My other 16 and 18" .30 cal barrels are 10 twist. I haven't seen a need to run the faster twist with the 168 and 175 gr loads.

Having said that, testing has shown for the .300BLK that shooting the 123 gr standard load (simulates 7.62X39 ballistics) in a 16", 8 twist barrel has revealed very good results. I believe the 168 and 175 grain projos out of the shorter 8 twist barrel will work very well also. I intend to test handloads out of the .300BLK using the 168 and 175 SMK.

 

[gstaylorg]

Re: Twist rates for shorter barrels

As I mentioned, my understanding is that subsonic loads were a big part of the original reason for the 8-twist, but it has turned out to also work very well with more common match-type loads.

I am losing less than 100 fps on average with the 16" 8-twist barrel when compared side-by-side with the 22" 11-twist using several factory loads. For example, the factory MV value for Black Hills and FGMM 175s is 2600 fps. At ~45 degrees F, the 22" 11-twist came in very close to that (2630 and 2590 fps, respectively) and the 16" 8-twist was running just under 100 fps slower (2540 and 2505 fps, respectively) (link below). Trajectories calculated at JBM using these MVs have been spot on out to 600 yd, so I think the #s are fairly accurate. As I understand it, these data are also consistent with the results obtained by sr90 and his folks at DTA.

For the complete MV data tables, see Book I of the Covert Trilogy, ~1/3 down the page: http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=1597047&page=16

I am currently testing several heavier (longer) loads for MV and accuracy in both barrels (including the SMK 190s) and the results will be covered in Book II of the Covert Trilogy (LOL) when I have finished collecting the data.

 

[buggsb]

Re: Twist rates for shorter barrels

Thanks guys, great stuff.

I hadn't really looked at the website much, but the brochures they had at SHOT correctly show 1:8 twist on the 16" .308 Covert barrel.

I really appreciate the input. What you are saying makes sense relative to the heavier subsonic bullet weights.

It's interesting to see gstaylorg that you are getting good results with the 175's also. I'm pretty hot to get one of those DTA's to spend some quality time with (what the heck, I only need one kidney anyway, right?)

My current tube is a chopped down 5R Milspec and I'm getting acceptable accuracy with 168's from the 1:11.25 twist at 16". From a velocity standpoint my data is consistent with gstalorg's in that I've only seen a loss of approximately 125 fps from 24" to 16" . If I was to do another one from scratch though, I'd probably consider the slightly faster 1:10 as you guys suggest.

 

[gstaylorg]

Re: Twist rates for shorter barrels

I had the same thought and that's why I ended up with both the 16" 8-twist and 22" 11-twist barrels. Figured between the two I'd should be GTG with a range of different loads. So far though, I've spent most of my time behind "Shorty" while "Stretch" has watched from the range bag.

BTW: kidneys are highly overrated (LOL).