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-   -   308 AR barrel length? (http://www.firearmstalk.com/forums/f20/308-ar-barrel-length-88335/)

Sgt_Skrb_25 04-07-2013 06:54 AM

308 AR barrel length?
 
I am going to start on a 308 build, I wanted to get some feedback from you guys about length and twist of the barrel. I want to reach 1K yards with it. I know that type of bullet and grain is a variable, but I want to get the right length of barrel. I've read that you need at least 20" to even get close. But velocity is an issue with that short of a barrel. I was thinking of going either 22",24" or 26", just not sure which yet.

Jagermeister 04-07-2013 09:44 AM

My CZ-550 in .308 is rated to be accurate 1000 yards, and the barrel length is almost 24". I would go with the minimum needed for carrying/mobility purposes. 24" in my opinion. Now, the military battles with M4s that have 22 and 24 inch barrels, but shoots a smaller round. M1 Gerand is 24" (30-06), and some awesome long range kills have been recorded with this gun. I would go with a 26" if you are surpassing the 1000 yard range, but I do not see why you would be doing this. Not sure why you would be shooting at a 1000 yards either. Yet, as they say, non ya.

SSGN_Doc 04-07-2013 04:58 PM

Military m4s have 14.5" barrels.

The .308 will still reach 1000 yards from a 16" barrel. What you need to specify is what velocity you hope to still have at that range.
Accuracy is not a result of barrel length but velocity is. Velocity can contribute to accuracy some by keeping the bullet from crossing the transonic threshold before arriving at the target and possibly destabilizing the bullet.


British AR based .308 (7.62x51 NATO) rifles have a 20 or 22" barrel. You won't see much advantage going longer than 24" because the powder capacity of the round would be running out of burn much past that unless you want to handload exclusively and pick some slow burning powders.

Then you have to ask how portable you want the platform. Then arrive at a balance. ARs with long barrels get front heavy pretty fast because of the long action. How many 1000 yard shots are you likely to take? What is the average range and long range distance you are most likely to shoot most often? Will the rifle be field carried much or just fired on a range?

Just some things to keep in mind.

Jagermeister 04-07-2013 05:49 PM

Quote:

Originally Posted by SSGN_Doc (Post 1206359)
Military m4s have 14.5" barrels.

The .308 will still reach 1000 yards from a 16" barrel. What you need to specify is what velocity you hope to still have at that range.
Accuracy is not a result of barrel length but velocity is. Velocity can contribute to accuracy some by keeping the bullet from crossing the transonic threshold before arriving at the target and possibly destabilizing the bullet.


British AR based .308 (7.62x51 NATO) rifles have a 20 or 22" barrel. You won't see much advantage going longer than 24" because the powder capacity of the round would be running out of burn much past that unless you want to handload exclusively and pick some slow burning powders.

Then you have to ask how portable you want the platform. Then arrive at a balance. ARs with long barrels get front heavy pretty fast because of the long action. How many 1000 yard shots are you likely to take? What is the average range and long range distance you are most likely to shoot most often? Will the rifle be field carried much or just fired on a range?

Just some things to keep in mind.

Thanks for showing my mistake. Trying to convert to inches has screwed me up. Living in Germany way to long. I retyped in the conversion calculator numbers for my CZ and the M1. They are correct. What the hell happened with the M4 length conversion. ****!!! Maybe I just mistyped. All my ballistics knowledge, bullet/shell measurements and barrel length knowledge is European standards. My firearms testing and hunting classes only utilized the metric system. I also need to know what the temperature is in celcius, or distance is in kilos to relate in any circumstance. :o My experience on the 1000 meter range shows 24 inches to be optimal. My friend shoots with a CZ with shorter barrel then mine. I think his type of rifle is called a "scout" rifle in the States? I know CZ calls my synthetic an American. His groupings are wider then mine. I also tried his .308 with the same results. AR competition/match rifles have long barrels in the comps I see over here. Nice, but would not want to carry around. Here is the shortest comp rifle I could find over here for those distances: http://www.frankonia.de/710241/352520/productdetail.html?pagingSize=32&lastSelected=f_s_ marke&sortOption=f_n_sortmatrix_70_30_30_30&f_s_ma rke=Pro+Tuning%7CHeckler+--+Koch&page=0&displayType=gallery&articleNumber=175 661&navCategoryId=6748
It is 45.7cm which is barely under 18 inches. Still, I must admit I am comparing long range hunting rifles with AR rifles. Plus, the furthest I shoot ARs is a hundred meters at the range. So, I will concede to your advice. You had a good point about what it will be used for, I was thinking the same thing. I would also want a shorter barrel unless it is for long distance propped shooting like the comps over here. Cheers!

AgentTikki 04-07-2013 06:08 PM

I think Docs got it right. Intended use is gonna govern your build moreso and you'll have to be a lot more carefull in your compenets to meet your goal. The 308 gonna be able to out that far but honestly I don't think its gonna be the right tool for your goals.

Just because a swiss army knife can carve a roast, doesn't mean you should use it. Right?

I think maybe looking at the ballisitics of the 308 and other similar rounds should govern your build.

SSGN_Doc 04-07-2013 06:19 PM

1000 yard ranges are kind of rare in my part of the country. 600 yds is the max distance I shoot in service rifle competitions with a 20" barreled AR15 in 5.56.

I have a Sako in .30-06 which has a 22" barrel. It will reach out and group well at 1000 yds.

If the rifle is for competition use the long barrels don't matter as much with regard to portability.

I'm doing a .308 AR build but will not be going over 20" because the loss of portability for the minimal velocity gain is just not worth the extra length.

Sgt_Skrb_25 04-08-2013 02:51 AM

Gents, I like the input. I guess I don't really have a purpose for it, just want some range. I think 22" might be good.

JonM 04-08-2013 04:04 PM

24" is the minimum length needed in 308 to keep a bullet supersonic past 1000 yard mark. Once a bullet transitions from super sonic to subsonic speeds that is thought of as its max accurate range. Accuracy goes to hell after that occurs.

If a target is at 1000 yards your pretty much wasting ammo and plinking unlessyou have a lot of training to do that and even those folks dont talk about how many times they miss at that range.

Now what do you reallllly want that ar10 type for??

sweeper22 04-08-2013 04:08 PM

All I can speak to is my own needs and preferences.

If you want this to be a potentially mobile and versatile rifle, with some CQ capabilities, I'd suggest an 18" setup. That'll shoot "well enough" out to 1000yds while still being light and maneuverable. I've actually been looking at the DPMS Recon 308 (16") for this type of rifle myself. Panther Arms (DPMS) actually offers a pretty solid 308 for the money, and I'd expect most 16-18" AR-10s to perform about as a Springfield M1A would.

If you want to hunker down in one spot and shoot the tightest groups possible at north of a half mile...get a longer barrel, plan on spending $2500-3500 on your AR-10, and then an extra $1-2K for optics, and major preference to stainless/bull barrels. So you're looking at a $4K minimun for this type of project, give or take.

It really comes down to what you expect the rifle to do within your budget. If you want the latter but lack the budget, I'd suggest going the bolt gun route. I certainly won't be setting up a $4-6K AR-10 anytime soon. So I would lean toward either a DPMS, M1A, or bolt rifle to give me something approaching my desired 308 results for under $2K.

Regardless of your setup, I'd imagine the desired twist is always going to fall between 1:10 and 1:12. And 168gr match ammo will maximize the grouping potential of almost any barrel in this range.

Sgt_Skrb_25 04-08-2013 04:57 PM

I just want a long range platform. I really love the AR platform, and not really a bolt gun platform guy. I have other weapons for CQB and other purposes. I figure if I'm building another AR , I want to get some range. I comfortable shooting up to 500-600 meters now, but would like to go longer. Money is not that much of an issue, I expected to invest 3-4k. I guess it's just a personal challenge to build a rifle that can do that, and then learn to shoot that range. I have friends who were snipers and like to learn from them. I just don't have the rifle that can do that yet.


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