AR 308 & 6.5CM barrel length / velocities

[Smitty192]

I could use a little help on some barrel options. I am finalizing the last parts of a SP10 project and I need some solid information on velocities on different barrel lengths.

I've got an extra upper for the rifle and I'm wanting to do a 16" 308 and a 6.5CM for the other upper. I'm guessing the 175 SMK should be around 2450 to 2475, based on other rifles I've owned in the past. I had a GAP10 with a 20" barrel that would shoot 175 FGMM right at 2600 fps.

What I don't know is what velocity is possible with a 6.5CM in various lengths. I'm intriqued by the idea of a 18" barrel. I don't know what speed I could get with 147gr or stepping down to some 130gr bullets. Same dilemma with a 22" barrel. I'm not sure if a short barrel 6.5 will be a notible step up over the 308. I have a metric shit ton of reloading supplies for 308. If a short 6.5 is only a marginal improvement in performance over a 308, I might look at a longer barrel 308 for the other upper.

Any help with velocity data and experiences with a short barrel 6.5CM would be greatly appreciated.

 

[bfoosh006]

While not apples to apples...
You might gain some info from these links.

https://rifleshooter.com/2014/12/30...ato-barrel-length-versus-velocity-28-to-16-5/

https://rifleshooter.com/2016/02/6-...el-length-on-velocity-cutting-up-a-creedmoor/
Updated 2019 info...
https://rifleshooter.com/2019/03/6-5-creedmoor-effects-of-barrel-length-on-velocity-2019/

All the barrel cutdown articles.. ( 2 pages )
https://rifleshooter.com/category/barrel-length/
Another interesting article..
https://rifleshooter.com/2016/05/16-6-5-creedmoor-synopsis-of-a-short-rifle/

 

[Smitty192]

Thanks. It’s a good reference point. I assume there will be noticeable velocity drop from bolt to gas guns.

I would like to get 2600 fps from 147 ELD
I don’t know what length barrel could achieve that. I don’t know if that is doable from an 18” barrel. I suspect not.

 

[bfoosh006]

From what I have read... velocity differences between a bolt gun and a gas gun, are minimal.
As an example, here is Molon's test with a AR15 with the gas block turned around to block gas flow ( before and after )
Just a single reference point, but still...

https://www.ar15.com/forums/ar-15/A...s-Compared-to-a-Single-Shot-AR-15/118-769905/
Also, some other interesting info... ( and poke around on both the webpages for other interesting info )

https://precisionrifleblog.com/2019/09/06/6mm-6-5-creedmoor-load-data/

More bolt gun oriented.

 

[Smitty192]

That is interesting. Maybe 5% loss between the two. I was thinking it would be substantially more. Thanks for sharing that. That’s a good reference point.

 

[bfoosh006]

That is interesting. Maybe 5% loss between the two. I was thinking it would be substantially more. Thanks for sharing that. That’s a good reference point.

Remember to account for wind drift for your caliber choice as well as velocity....
There are some really interesting modeling articles out there as well.

A lot of reading... but something to keep in the back of your mind.
Search Google for ... Long range Hit probability, look at some of the images on Goggle.

https://precisionrifleblog.com/2015/06/09/how-much-does-it-matter-overall-summary/


And....
https://precisionrifleblog.com/2015/04/25/how-much-does-cartridge-matter/

 

[Noobilicious]

From what I have read... velocity differences between a bolt gun and a gas gun, are minimal.

Seconded; the difference between bolt and gas is minimal around 15-30 fps in my experience. A bigger difference comes from fast vs slow barrels.

For @Smitty192, here is a good video about short barrel Creeds:


The people interviewed in above video are now running 18 inch barrels. I have also been researching short barrel creeds for my upcoming build, and seems like 16 inch Creed is possible for run-and-gun, 20 inch for DMR, and 18 as a compromise between both.

 

[whatsupdoc]

Same ammo lot#
Hornady American Gunner 6.5 Creed 140 BTHP.

25" Barrel bolt action = 2701 FPS
20" Barrel Gas gun = 2527 FPS (Rifle Gas)
22" Barrel Gas gun = 2636 FPS (+2 Gas)

 

[Smitty192]

Seconded; the difference between bolt and gas is minimal around 15-30 fps in my experience. A bigger difference comes from fast vs slow barrels.

For @Smitty192, here is a good video about short barrel Creeds:


The people interviewed in above video are now running 18 inch barrels. I have also been researching short barrel creeds for my upcoming build, and seems like 16 inch Creed is possible for run-and-gun, 20 inch for DMR, and 18 as a compromise between both.

Interesting video. A 130 grain bullet might work well in a short barrel.

 

[Smitty192]

Same ammo lot#
Hornady American Gunner 6.5 Creed 140 BTHP.

25" Barrel bolt action = 2701 FPS
20" Barrel Gas gun = 2527 FPS
22" Barrel Gas gun = 2636 FPS

Thanks for the information, really helpful. That is quite a jump in velocity from 20 to 22. Is the 20 inch a rifle length gas and the 22 a +2 gas?

 

[whatsupdoc]

Thanks for the information, really helpful. That is quite a jump in velocity from 20 to 22. Is the 20 inch a rifle length gas and the 22 a +2 gas?

Yes

 

[Jnull]

I just ordered a cf proof barrel for my ar10, glad I went 22” after seeing the velocity difference between 20 and 22

 

[TonyTheTiger]

I'd be willing to bet you could cut the 22" down to 20" and it would still be faster than the existing 20", because that's just how barrels be.

Thanks for the information, really helpful. That is quite a jump in velocity from 20 to 22.

I just ordered a cf proof barrel for my ar10, glad I went 22” after seeing the velocity difference between 20 and 22

 

[Longshot85]

I haven't been on the Hide in about a year.

I just bought some more 147 ELD-M's for reloading yesterday.

I have a 26 inch Criterion barrel bolt gun with 9 inch can that I get 2868fps using IMR4451. Its a hot load but safe in 95F and the rain.

Hopefully I can FINALLY test out my Aero M5 build with a 16 inch Wilson Combat 6.5CM barrel.

There's a lot of reasons why NOT to build a 16 inch AR10 6.5CM.

I built it for hunting whitetail deer and coyotes.

When I get some rounds down the tube and have data from Magneetospeed I'll report back.

 

[Smitty192]

I haven't been on the Hide in about a year.

I just bought some more 147 ELD-M's for reloading yesterday.

I have a 26 inch Criterion barrel bolt gun with 9 inch can that I get 2868fps using IMR4451. Its a hot load but safe in 95F and the rain.

Hopefully I can FINALLY test out my Aero M5 build with a 16 inch Wilson Combat 6.5CM barrel.

There's a lot of reasons why NOT to build a 16 inch AR10 6.5CM.

I built it for hunting whitetail deer and coyotes.

When I get some rounds down the tube and have data from Magneetospeed I'll report back.

16” CM would be interesting. I was looking at those barrels this morning. CSAT special

 

[whatsupdoc]

For a 6.5 creed my experience has been rifle length gas is a no go, primer pockets expanding and blowing primers with
mild loads where +2 gas is so much better. Dont know what a 16" creed barrel will do but it should be interesting.

 

[Longshot85]

I had to wait a while to get mine...ordered in 2020 which was tough on all companies.

I must say that I spoke with several individuals on here in private and was warned from all of them that this is not a length (16 inches) that is ideal.

It is quite different than .308 and simply assuming 16 inches is fine for 6.5CM because it's great in .308 is literal ignorance (which was me before I dug deep)

In the end I decided to proceed down the 16 inch 6.5CM AR10 path.

I'll be shooting handloads only other than some 129 Gr Hornady American Whitetail.

The following are bullets I have:

Old stock 120gr AMAX
Old stock 140gr AMAx
147 ELD-M

 

[Smitty192]

For a 6.5 creed my experience has been rifle length gas is a no go, primer pockets expanding and blowing primers with
mild loads where +2 gas is so much better. Dont know what a 16" creed barrel will do but it should be interesting.

I am curious as to what length would be the shortest you could go and still use a +2 gas system. 18"? Proof has a 20" +2.

 

[Longshot85]

For a 6.5 creed my experience has been rifle length gas is a no go, primer pockets expanding and blowing primers with
mild loads where +2 gas is so much better. Dont know what a 16" creed barrel will do but it should be interesting.

It's facts such as this as to why essentially nobody manufactures 16 inch 6.5CM AR 10 barrels.

I do have a JP adjustable gas block and a JP silent capture spring as well.

I reload, have a magneetospeed and my cans are Form1's so it makes me less leary to tread down this path.

In my opinion it would be foolish to even think about 16 inches on a 6.5cm AR10 if one didn't reload.

I still have potentially set myself to have issues....but.....only time will tell.

I will for sure make a thread on the build and include any data collected.

 

[Longshot85]

For a 6.5 creed my experience has been rifle length gas is a no go, primer pockets expanding and blowing primers with
mild loads where +2 gas is so much better. Dont know what a 16" creed barrel will do but it should be interesting.

Let me add this

I had to buy Wilson Combats gas tube to go with their 16 inch 6.5CM barrel.

I forget what they categorized it as.....but if memory serves me right it's 13.25 inches.

How does a 13.25 inch gas tube compare to what you've tried?

 

[TonyTheTiger]

I forget what they categorized it as.....but if memory serves me right it's 13.25 inches.

How does a 13.25 inch gas tube compare to what you've tried?

That's like a KAC Intermediate. If there was a standardized intermediate it would be the Noveske IIRC which is 2" longer than mid and 1.5" shorter than rifle, KAC is a bit shorter, like 1.5" longer than mid if the measurements I took years ago and my memory are accurate.

 

[TonyTheTiger]

I am curious as to what length would be the shortest you could go and still use a +2 gas system. 18"? Proof has a 20" +2.

I'm no large frame expert, but extrapolating from my 5.56/223 experience with rifle gas 16" and rifle +1" 17" says you could easily do a +2" 18" with CM.

 

[whatsupdoc]

Let me add this

I had to buy Wilson Combats gas tube to go with their 16 inch 6.5CM barrel.

I forget what they categorized it as.....but if memory serves me right it's 13.25 inches.

How does a 13.25 inch gas tube compare to what you've tried?

Rifle tube is 15-1/8" Mid is 11-3/4" so yours is between the two.

When I had the rifle length on the 6.5 it was 100% reliable never a malfunction
"unless" a blown primer jammed the trigger or prevented the bolt going into battery.

Handloading on the milder side was the only solution that alleviated the primer issue, remember the adjustable gas block has to be adjusted so the rifle functions and it is only reducing gas volume not gas pressure. Heavy buffers helped only slightly.

Look you may have no issues at all, now go play with your toy and see how it goes.

 

[MPD142]

140gr Hornady HPBT over 43.5gr Staball

LMT MWS 24” 6.5 Creedmoor 2690fps

T-Box 16” 6.5 Creedmoor Barrel 2482fps

 

[smcfarland]

My 16” 6.5 CM (Faxon Gunner barrel) averages 2510fps with factory 143 ELD-X.
Perfect setup for me shooting pigs out to 400.

 

[richiecotite]

I have the 16” Wilson Combat barrel on my 6.5 CM Aero build. With factory S&B 140 prepandemic, velocity averaged around 2550. Got 2586 from 43.6 gr of StaBall with a 140 gr Barnes match burner

 

[drewthebrave]

I am curious as to what length would be the shortest you could go and still use a +2 gas system. 18"? Proof has a 20" +2.

My Craddock Precision 6.5 Creedmoor 18" has a Rifle +1 gas system and runs great. Recoil impulse reminds me of an 18" Grendel with midlength gas - except the big rifle has much more reciprocating mass. Makes me want to try out a lightweight carrier in the large frame.