AR10 pistol barrel length?

[Golddiggie]

Yup, you read the title right... I'm making an AR10 pistol in 308 Winchester. Why you ask?? Because I'm in Free 'Murica.

I found a barrel manufacturer/site that has options from 7.5"-24".
Configure Your .308 WIN

From what I've discovered in researching, the 1x8.5 twist will give me solid accuracy in the projectile weights I'll be using (lowest planned is 147gr, I have 150gr for reloading and might go with higher weights too).

I've found sources for pretty much the rest of the components I'll need (that I don't already have) including adjustable gas blocks. That way I don't over-gas the action with the shorter barrel. Right now I'm trying to decide between the 10-1/2" and 12-1/2" barrel. Cost is the same as I've configured them (light profile, machined finish, no fluting puts either at $250 plus shipping).

I know these will produce fireballs with each shot. But with the SigTac brace (already have) it should be a hoot to shoot. Plus piss off people on either side (depending on what I put on the end of the barrel). Use the "coming soon" Magpul D50 and it will be a looooong mag dump.

I've found handguards that will work with the 10-1/2" and leave just the muzzle device exposed (which is what I'm looking for so that it closely matches the look of my 300 Blackout pistol). Basically, this will be the bigger brother to my 300 Blackout pistol. I'm not expecting to shoot it beyond 100 yards, but it could have some decent range accuracy to it.

So, question of the day is which length barrel? 10-1/2" or 12-1/2"?? Since the cost will be the same either way. I'm planning to make this purchase by early March to keep the build on track. I plan to purchase components each pay period until I have everything. Expect to Cerakote what's needed sometime in late March or early April and then put it all together. Poll set to close in 14 days.

Giddyup

 

[92G]

some data I yanked from another forum:

150 gr HPBT Win 748 loaded to a chamber pressure of 56.4K PSI WARM load 45 gr NOT a starting load. I chose 748 as the powder as it is most like NATO powder.

26" = 2918 FPS MP= 6.6K PSI ––99.6 % powder burnt -2836 ft/lbs
24" = 2873 FPS MP= 7.4K PSI ––––––––––––––––––––––––––––2750 ft/lbs
22" = 2823 FPS MP= 8.2K PSI ––––––––––––––––––––––––––––2655 ft/lbs
20" = 2766 FPS MP= 9.2K PSI ––98.7 % powder burnt -2548 ft/lbs
18" = 2700 FPS MP= 10.5K PSI ––––––––––––––––––––––––––––2428 ft/lbs
16" = 2623 FPS MP= 12.0K PSI -97% powder burnt ––––2291 ft/lbs
14" = 2531 FPS MP= 14.0K PSI ––––––––––––––––––––––––––2133 ft/lbs
12" = 2417 FPS MP= 16.7K PSI -94% powder burnt ––1946 ft/lbs
10" = 2273 FPS MP= 20.5K PSI ––––––––––––––––––––––––––1721 ft/lbs
8.0"= 2080 FPS MP= 26.0K PSI -87% powder burnt. -1441 ft/lbs
6.0"= 1800 FPS MP= 34.1K PSI -79.4% powder burnt -1080 ft lbs

--

from perspective of ballistics, short barrel LR-308 is pointless. with a 10 inch barrel one can get similar velocities from a 300 blackout (more efficient powder burn, way less recoil and blast). if the goal is function go with at minimum a 16" barrel. if the point is just to be silly then do whatever tickles the feather. whatever you build i would invest in a high quality adjustable gas block, as a short barrel 308 is going to be picky on the gas to run reliably. recoil impulse will be awful.

the SLR gas blocks are excellent. there are other good options too but these are top notch.

AdjustaGas Blocksble Gas Blocks

 

[Golddiggie]

Was already planning on getting the adjustable gas block... SLR is one of the sources I've bookmarked for the build too.

So maybe I'll do the 12.5" barrel then. This is going to be more of a 'fun gun' than anything else. Plus because I can do it. I already have an AR10 rifle (18" barrel) so I'm good there. Plus the Savage Axis rifle for longer range (22" barrel IIRC).

 

[fshalor]

Drat. Now you have me looking up 6.5 creedmor ballistics for sub 14" land.

I agree with 308, little point in doing pistol.

10.5" looks the sweetest!

 

[Golddiggie]

@fshalor The main 'point' of doing this is "because I can". I'm not looking for a 1000 yard shooter with this. Or even a 600 yard shooter. Most of the time I'd use it at sub 200 yards (such as the utility range, or action bays at PF&G, so no more than 100 yards). Plus the noise and fireball coming out the end of it. Shoot at dusk and be seen from orbit.

Another reason is to have something different from almost everyone else. I already have a 300 Blackout AR15 pistol and plan to build a 5.56 NATO upper to mate to it at some point.

Maybe I'll pick up a GoPro and record shooting it on the first overcast day after I've dialed it in. Or late in the day when the flash will show up better. I've already seen how much of a fireball you can get from a 300 Blackout pistol with an 8" barrel loaded on the upper end of the range for powder charge. It was epic. I want to see what happens in the AR10 pistol.

Read through Squib308's post again... Maybe I will do the 10.5" barrel. I'll be reloading 308 Winchester, so I could do up a recipe that gives me the fireball I want, without leaving too much unburned powder. Dial in the adjustable gas block to get it to cycle well and call it done. I'll probably have two powder charges for 308 Winchester then. One for the rifle build, and the other for the pistol build. I could think of worse things.

 

[JRT]

@fshalor The main 'point' of doing this is "because I can". I'm not looking for a 1000 yard shooter with this. Or even a 600 yard shooter. Most of the time I'd use it at sub 200 yards (such as the utility range, or action bays at PF&G, so no more than 100 yards). Plus the noise and fireball coming out the end of it. Shoot at dusk and be seen from orbit.

Another reason is to have something different from almost everyone else. I already have a 300 Blackout AR15 pistol and plan to build a 5.56 NATO upper to mate to it at some point.

Maybe I'll pick up a GoPro and record shooting it on the first overcast day after I've dialed it in. Or late in the day when the flash will show up better. I've already seen how much of a fireball you can get from a 300 Blackout pistol with an 8" barrel loaded on the upper end of the range for powder charge. It was epic. I want to see what happens in the AR10 pistol.

Read through Squib308's post again... Maybe I will do the 10.5" barrel. I'll be reloading 308 Winchester, so I could do up a recipe that gives me the fireball I want, without leaving too much unburned powder. Dial in the adjustable gas block to get it to cycle well and call it done. I'll probably have two powder charges for 308 Winchester then. One for the rifle build, and the other for the pistol build. I could think of worse things.

I SBR'ed my SCAR 17 because why the f*** not. I bought the FN OEM 13 inch barrel from Optics Planet and it worked pretty good. It was certainly picky on ammo but I could find ammo that cycled. Now that I reload I bet I could come up with a perfect round without too much blowback. If you are going to run a silencer use a muzzle brake over a flash hider. The blast chamber has taken a beating from unburned powder but the baffles on the brake give some added protection. Without the can its f***ing loud, like knock old shit down from the ceiling loud.

 

[Golddiggie]

I SBR'ed my SCAR 17 because why the f*** not. I bought the FN OEM 13 inch barrel from Optics Planet and it worked pretty good. It was certainly picky on ammo but I could find ammo that cycled. Now that I reload I bet I could come up with a perfect round without too much blowback. If you are going to run a silencer use a muzzle brake over a flash hider. The blast chamber has taken a beating from unburned powder but the baffles on the brake give some added protection. Without the can its f***ing loud, like knock old shit down from the ceiling loud.

BONUS!!!

I'm already planning to have a brake/comp on it instead of a flash hider. I have a tank brake I got a couple years back when I was going to build the rifle upper. Until AimSurplus had 18" barreled AR10 uppers at a good rate.

Looks like the 12.5" barrel will be a little easier to get other things lined up. Like the handguard for one. Hard to find one that won't partially block the brake side ports. I'll be getting as much of the aluminum parts raw as possible. Or not care what color they are since I'll be hitting it with Cerakote. Might go with Burnt Bronze and then add some 'wear marks' via another airbrush and some black or dark grey. I'll have to see if my old artist airbrush (back from college) will work with Cerakote or not. If not, there's one on Amazon that appears to work well. Not stupid priced either.

 

[JRT]

BONUS!!!

I'm already planning to have a brake/comp on it instead of a flash hider. I have a tank brake I got a couple years back when I was going to build the rifle upper. Until AimSurplus had 18" barreled AR10 uppers at a good rate.

Looks like the 12.5" barrel will be a little easier to get other things lined up. Like the handguard for one. Hard to find one that won't partially block the brake side ports. I'll be getting as much of the aluminum parts raw as possible. Or not care what color they are since I'll be hitting it with Cerakote. Might go with Burnt Bronze and then add some 'wear marks' via another airbrush and some black or dark grey. I'll have to see if my old artist airbrush (back from college) will work with Cerakote or not. If not, there's one on Amazon that appears to work well. Not stupid priced either.

Aren't you fancy! I will have pics in a couple weeks of an SBR I'm building. Rust-Oleum camo paint and netting for a sweet pattern.

 

[Golddiggie]

Sent a message to the barrel manufacturer to find out how long of a handguard will be good. I want it to stop right at the threads, or close to that. Basically, so that it won't block the muzzle brake ports. I do see some 11.7" handguards out there for not stupid pricing. So it will be interesting to see how long would work for the 12.5" barrel. Or the 10.5" barrel. While I DO want to start ordering parts, I need to make sure I line things up properly first. I should put a list together of where I'll get what and the cost. That way I know what the total spend is going to be and can space it out better. Probably over a few paychecks.

 

[Golddiggie]

Earlier today I sent a message to Columbia River Arms asking about what length handguard to use for the 10.5" and 12.5" 308 Winchester barrels. I got a reply a little while ago with the info. Looks like an 11.5" handguard will just start covering the threads at the end of the barrel. So it would work for the muzzle device I plan to get.

Also found out that they sent a 12.5" thin profile AR10 barrel (308 Winchester) out for testing. This is the comment made for this:
"We just sent a 12.5” 308 1x8.5 in light carbine profile out for testing with standard and subsonic loads and the company testing was very pleased with the accuracy and performance."

So, I'm now thinking that the 12.5" barrel will be what I go with. I have asked for some additional information around what they used for rounds in the testing. The twist rate of the barrel sent for testing is the exact one I've been looking at. Mostly because I saw how it had the highest accuracy potential of the twist rates in the projectile weight (or range of weights) I expect to use.

The handguards I'm finding are running about $250 for the 11.5" version. I'm trying to use only a few vendors for the parts to build this firearm to make things easier and possibly save on some shipping charges. I need to see if I can get a DiamondHead rail in that length for the AR10 platform from a reseller (their site isn't open for placing orders or even looking at info right now).