6.5 in shorter barrels

[Mban2]

I am looking to pick up a large frame AR and am wanting something more mobile than my bolt gun. I shoot .260 but for the sake of generalities, let’s just say .260rem or 6.5 answers will apply.
My question is: At what point does the shorter barrel length make the difference between a 6.5 bullet and a .308 bullet negligible?

i want to chamber the rifle in .260 but am concerned that an 18”ish .260 won’t be worthit over an 18” .308.

what are your thoughts?

 

[stello1001]

How could an 18" 260 not "be worth it over an 18" .308" according to you? The 260 is still pushing a better BC bullet than the 308 whatever the barrel length is. If you compare a 14" .260 to a 14" 308 or a 30" 260 to a 30" 308 the 260 is still going to push the better BC bullets. Obviously my examples are a bit extreme but you get my point.

 

[Mban2]

Also, what is the general consensus on the most effective short barrel for a .260 or 6.5 creed? Looking to make it effective out to 8-900

 

[Mban2]

How could an 18" 260 not "be worth it over an 18" .308" according to you? The 260 is still pushing a better BC bullet than the 308 whatever the barrel length is. If you compare a 14" .260 to a 14" 308 or a 30" 260 to a 30" 308 the 260 is still going to push the better BC bullets. Obviously my examples are a bit extreme but you get my point.

Good point, but I am wondering if there is a velocity threshold that people try to meet in order to start stacking the advantages of the 6.5 bullet when compared to the .308. Maybe I’m thinking about this incorrectly

 

[ma smith]

My question is: At what point does the shorter barrel length make the difference between a 6.5 bullet and a .308 bullet negligible?

For a sufficiently general answer, IMHO you need to weight the various tradeoffs of the two rounds. I don't think it all comes down to in-flight ballistics. You have terminal and inermediate barrier considerations as well as recoil management and muzzle signatures...etc

 

[Nimothy]

Good point, but I am wondering if there is a velocity threshold that people try to meet in order to start stacking the advantages of the 6.5 bullet when compared to the .308. Maybe I’m thinking about this incorrectly

I think the velocity shortcomings effect both or any caliber for that matter. I had a 18” 6.5cm on an aero upper that still made low 2700’s with 130’s fwiw. Those loads weren’t hotrodded I’m pretty sure it was a eld with around 43-43.5 grains of 4350

 

[Gtscotty]

I just got an 18" 6.5 CM, and have been hunting with my 20" 6.5 for a while. Here are the velocities I get with the same loads (developed for the 20") in both:

20"
127gr LRX @ ~2,850 fps
142gr ABLR @ 2,734 fps

18"
127gr LRX @ 2,742 fps
142gr ABLR @ 2,680 fps

Here are some of the working loads I've used in my previous .308s for comparison. For most uses, I'll take the 6.5.

20" .308 Win:
178gr ELDx @ 2,595 fps

18.5" .308 Win:
150gr IL @ ~2,750 fps

16" .308 Win:
168gr BT @ 2,600 fps
178gr Amax @ 2,510

 

[Abbotm2]

My 18” 6.5cm 140gr had the same dope as my 24” 308 175gr, around 11 mil to 1k

 

[drewthebrave]

I think the biggest limiting factor will be the pressure at the gas port in a 6.5 or .260 barrel. I'd be comfortable going with an 18" or even 16" barrel as long as it has a rifle-length gas system. I'd hesitate to run a mid-length gas system in any 6.5/.260 barrel unless you have an adjustable gas block or carrier to reduce the violent cycling of the bolt.

Either way, the more aerodynamic bullets that you can use in 6.5/.260 calibers will be more aerodynamic than comparable .308 bullets regardless of how long your barrel is. But if you want a really handy AR, I'd consider a 12"-16" AR15 in 6.5 Grendel or 6mm ARC over a large frame gun. It just depends how much extra you're willing to pay for mobility on the large frame platforms.

For what it's worth, my 18" 6.5 Creedmoor comes in at about 12 pounds all kitted out with a full mag & 2.5-15x Ares BTR, whereas my 18" 6.5 Grendel is only 9 pounds with a mag & 3-15x SWFA. With a lighter barrel and optic, I could probably get the Creedmoor down to 10-11 lbs, but the Grendel could easily be under 8 with a lighter scope. Since Grendel covers the intermediate role so well, I don't mind the extra weight on the big frame gun to tame the recoil a bit better. With a 12" Grendel (or 6ARC), you save even more weight & size, while still maintaining the ability to hunt within ~300 yards and shoot targets out to 600+ yards very effectively.

For some people, a "mobile" gun could be 13 pounds, while others might not be happy with anything weighing more than 8 lbs. Just define the role you want it to fill, and build around those parameters. For me, the 6.5 Grendel hits the sweet spot, but the 6.5 Creed/.260 Rem definitely extend your range if you have the cash to spend -- and don't mind the extra weight.