7 twist 6.5CM in 20” AR?

[bobke]

Have divested myself of .308 AR rifle and most of my supplies to focus on 6.5 cal AR’s and have question regarding advice of use of 7.0 twist for a new 20” +2 gas Krieger barrel. All of my experience with current inventory of 6.5GR, 6.5x47 and 6.5CM in either bolt or gas guns use 8-8.75 twist barrels, all with very good results. Advice from well known smith was to go 7 twist because of lower expected velocity from shorter barrel, in order to get proper stability from bullets from 136-147gr. And the Krieger was available from stock.
From actual experience, is there a downside to 7 twist, is there a negative effect with lighter bullets in the 120-130 range and with a Superlative adj gas block, what port size would be recommended, also using JP SCS, with about 20% of use suppressed. Have seen very little anywhere regarding use of 7 twist in 6.5CM and wonder if this is best advice or just stick with 8? Nothing against Krieger at all-have both Kriegers and Bartleins, just more concerned about dropping $1k on a barrel with more limited application. Have looked a stability formulas and doesn’t appear to over stabilize, but there is a reason most are 8 twist.
Experienced advice greatly appreciated, as I still have time to revise direction. Thx.

 

[CrabsandFootball]

I would stick with 8 twist. You arent going to be able to load to mag length the longer bullets that will benefit from a faster twist.

The 130 berger AR is the perfect round for a semi auto 6.5C IMO.

 

[BytorJr]

I have a 1/7.5" twist and my upper is having all sorts of issues. May as well be patterning a shotgun. I don't know if it's the twist, the barrel (Bartlein) or the barrel nut, but for the 1600 bucks I blew on it, it should be shooting sub-MOA. Sadly, my belief is I'll be stuck with this with no help from maker. Stick with 1/8".

 

[bobke]

Bytor
Any more details on your build? Barrel length, gas length, gas port diameter, buffer system? Above comments and the fact that Bartlein gain twist barrels don’t even get down to a straight 7.0 twist has me concerned if I’m being sold a slow moving barrel selection, or if there’s real validity to concept. Very well respected smith, so allowing room for discussion, but absence of any volume of builds with 7 has me in a quandary. Thx for comments.

 

[Steel head]

Unless you’re planning on just Shooting 150’s get a 1-8.
130’s are the hot ticket for ar’s

 

[bobke]

Have large stash of Scenars from 120-139, as well as 140 AMax and 129 ABLR’s, and am seeing the wisdom of staying with 8 for this build. Anything much heavier doesn’t really fall into my scope of normal usage.

 

[Steel head]

Have large stash of Scenars from 120-139, as well as 140 AMax and 129 ABLR’s, and am seeing the wisdom of staying with 8 for this build. Anything much heavier doesn’t really fall into my scope of normal usage.

I repurposed my 260 to 0-1000 yards and went with 130’s.
Flat and fast with a bit less recoil.

 

[BytorJr]

Bytor
Any more details on your build? Barrel length, gas length, gas port diameter, buffer system? Above comments and the fact that Bartlein gain twist barrels don’t even get down to a straight 7.0 twist has me concerned if I’m being sold a slow moving barrel selection, or if there’s real validity to concept. Very well respected smith, so allowing room for discussion, but absence of any volume of builds with 7 has me in a quandary. Thx for comments.

22" barrel. Adjustable gas block, have no idea about gas port diameter. Buffer is whatever resides in a Magpul PRS stock, Bartlein barrel.

 

[CNC-Dude]

22" barrel. Adjustable gas block, have no idea about gas port diameter. Buffer is whatever resides in a Magpul PRS stock, Bartlein barrel.

Did you ever check to see if the barrel nut has still tight?

 

[Lowlight]

I have an 18" 260 that is a 7 twist and it works very well, it's a bolt gun not a semi, but it performs fine

Since the MV is down the 7 twist will balance it out and work okay, bit more drift with it, but there is no issue going shorter and increasing the spin.

 

[LRRPF52]

I have a 1/7.5" twist and my upper is having all sorts of issues. May as well be patterning a shotgun. I don't know if it's the twist, the barrel (Bartlein) or the barrel nut, but for the 1600 bucks I blew on it, it should be shooting sub-MOA. Sadly, my belief is I'll be stuck with this with no help from maker. Stick with 1/8".

Who did the profiling, chambering, and barrel extension work on your Bartlein?

I’ve been shooting 130gr VLD in my GAP-built Bartelin .260 Rem, 22”. It’s a laser. Bug-holes at 100yds, sub-half MOA 5rd strings at 500yds, sub-MOA out to 1000yds.

My first load development with 139gr Scenar started with really tight 5rd groups, which just kept shrinking as I went up in charge weight.

I prefer the 130gr Berger for LR since it’s so flat, doesn’t stress the gas system like 140s and higher do, and I can get it going fast into what I would expect from a .260 Rem 22” pipe.

The tightest twist on AA 6.5 Grendel pipes is 1/7.5” for 18”and shorter after Bill A. did the stability factor analysis and testing with them. You should be able to use 1/8.75 and tighter common 6.5mm barrel twist rates with a 20” 6.5CM. I don’t see how 1/7 twist hurts you though. Berger twist rate calculator doesn’t indicate any problems with it. You’re not shooting anything into the mid 3000fps range, so bullets should hold together well.

A 9” twist plays out as marginally-stable for the 130gr VLD even at 2850fps (that’s fast even for 22” gas gun). Once you go to 1/8.75” twist, it becomes stable. 130gr VLD at 2650fps with 1/8.75 twist becomes ever slightly marginal, but not enough in practical difference (G7 goes from .288 to .287-pole vaulting over mouse turds).

I ran those to get an idea of how much the twist rate and different velocities were showing up, resolution-wise. 1/7 would be good for shorter barrels, but even an 8 twist keeps the 130 VLD optimized at 2350fps mv.

I wouldn’t go looser than an 8 twist for a 20” 6.5CM gasser for these reasons. Unless I’m missing something, I don’t see how the 7 is bad.

 

[bobke]

Appreciate the qualified comments. Don’t see much traffic from those using 7 twist, but after comments here and looking at Berger stability calculator and another from a reference to a Miller calculator, seems like a very viable proposition, though outside of my current experience to date. So...never too late to learn a new trick.
Any suggestions regarding gas port diameter, given will be 20”, +2 gas and a Superlative adj gas block? My other CM’s are rifle gas 21 and 22” barrels, so missing a reference there, as well.
Thanks for counsel and guidance guys. Once again, proof of the real value of the assembled wisdom here.

 

[Jakeweb09]

I have no experience with this personally especially on a AR platform but I’ve spoken with my smith on other short barrel builds and he’s mentioned this (on bolt platforms) but just mild tweaks. In my instance I had a 7 Saum built with a 26” barrel and shooting 180 vlds and we talked about building it’s twin with a 18-20” bbl. and he said that the 8.4 twist in my barrel is perfect but if going down in length to move to a 8 twist as it’ll help stabilize since it’ll be moving slower out of the barrel and the faster twist should get it to a similar rpm out of less barrel. In your case I think stick with 8 twist since the 130s seem to be the best bullet weight for the AR. but when shooting heavies then it holds merit

 

[Lowlight]

Just an FYI,

The military went to 7 twist 260 after seeing a short one, they first played with a 14" one then when they built it they went 24" on the barrel but kept the 7 twist,

it did create some issues so they switched to the Lapua bullet to thicken the jacket up a bit.

You can have speed or twist but when you start going fast twist and high speed you run into problems, even if the calculators say the stability is okay, the jacket might not be.

 

[bobke]

Thanks, Frank. Do you have a sense of what velocity bullets with less substantial jackets were causing loss of accuracy or issues down range? Most of my 6.5 stash for CM is Lapua in reserve, with some 140 AMax for every day use and 129 ABLR’s for hunting. Run AMax around 2680-2700 and ABLR’s around 2780-2800 in 21-22” barrels with 8 twist without really pushing, and am figuring to lose about 40-50fps when going 20”. Feel like I’ll be fine, but you make a valid point regarding pushing both twist and velocity at the same time. Thanks.

 

[Slevin182]

I would stick with 8 twist. You arent going to be able to load to mag length the longer bullets that will benefit from a faster twist

+1