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Centerfire Rifles - Semiautomatic or Gas Operated Centerfire rifles, carbines and other gas operated rifles.

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  #1  
Old 09-05-2012, 10:33 PM
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Default Best barrel length and twist for 7.62x39?

Howdy all.
So I started thinking about what would be "best" for my 7.62x39 AR project.
Specifically what barrel length and twist perform the best with the 123 gn surplus ammo that is si abundant these days? I know I'm WAY over thinking this, but its just what I do
I'm sure that there are quantifiers for "best", but I don't know what they are. truthfully I know little about barrel length and twist applications, hence the question.
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  #2  
Old 09-05-2012, 10:56 PM
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A good bolt would be essential to start with.

1:10 or 1:11 twist would work.
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  #3  
Old 09-05-2012, 10:58 PM
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Honestly with that ammo and using junk ammo your wasting your time..

Get a 308 win ar10 if your serious about grouping. Way better ballistics.
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  #4  
Old 09-05-2012, 11:03 PM
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Quote:
Originally Posted by nick View Post
A good bolt would be essential to start with.

1:10 or 1:11 twist would work.
I have an LMT on the way. Hopefully AR precision will have more of theirs in stock in a month or so, at which point I will get one of them as well.

Im guessing I probably wont get much in the way of improved MV going for a barrel longer than 16" though?
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Old 09-05-2012, 11:04 PM
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Get a 308 win...
Not interested, thanks.
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  #6  
Old 09-05-2012, 11:29 PM
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Get an AK and just shoot the crap out of it. U will forget about all this silly tech speak
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Old 09-05-2012, 11:31 PM
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Quote:
Originally Posted by C.W.M.V. View Post
I have an LMT on the way. Hopefully AR precision will have more of theirs in stock in a month or so, at which point I will get one of them as well.

Im guessing I probably wont get much in the way of improved MV going for a barrel longer than 16" though?
I was able to hit at 600 yards with my saiga 20", don't know the twist.

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  #8  
Old 09-05-2012, 11:36 PM
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Quote:
Originally Posted by MrPlink View Post
Get an AK and just shoot the crap out of it. U will forget about all this silly tech speak
I have several. Thank you.

Not looking to build a competition rifle here, just need to know if there is an ideal barrel/twist combo for this round.
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  #9  
Old 09-05-2012, 11:46 PM
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Quote:
Originally Posted by C.W.M.V. View Post
I have an LMT on the way. Hopefully AR precision will have more of theirs in stock in a month or so, at which point I will get one of them as well.

Im guessing I probably wont get much in the way of improved MV going for a barrel longer than 16" though?
Not likely. Besides, considering that you'll be shooting cheap ammo, it'll be inconsistent anyway.
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"Thou shalt not interfere with the Second Amendment rights of "law-abiding" citizens who want AK-47s only to protect hearth and home." - Paul Helmke finally gets it :)
Quote:
Originally Posted by SJgunguy24 View Post
Some people are so open minded, their brains have fallen out.


WTB: Saiga .223 bolt; HK G3 bolt; Chinese AK pistol grips; milled AK cut receiver pieces and stubs.
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  #10  
Old 09-06-2012, 5:08 AM
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Get a model one 20" M4 upper with a free float tube, with Yugo surplus they shoot two MOA, with handloads 1.5 MOA maybe a bit better.

I have shot mine out to 500 meters (silhouettes) and wih trigger time you can figure out wind drift fairly easy.

I also shoot a custom bolt gun 10 twist 26" barrel and its a 1/2 MOA gun.
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  #11  
Old 09-06-2012, 10:15 PM
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Quote:
Originally Posted by C.W.M.V. View Post
Howdy all.
So I started thinking about what would be "best" for my 7.62x39 AR project.
Specifically what barrel length and twist perform the best with the 123 gn surplus ammo that is si abundant these days? I know I'm WAY over thinking this, but its just what I do
I'm sure that there are quantifiers for "best", but I don't know what they are. truthfully I know little about barrel length and twist applications, hence the question.
If you really wanted to do it properly, you'd be using a twist most folks would consider outrageous if the load is the short (roughly 1.1") 123 grain lead core, copper or bi-metal jacketted loads typical of the caliber.

Using the Greenhill equation yields these interesting results: 1:15.8" (1 full twist in 15.8" inches of tube) though a 1:15" twist would be adequate. If you are wondering why that is, the bullets are stumpy, stubby and thus don't need a lot of spin to stabalize.

The inputs I used are for a quality load like the Lapua or Golden Tiger 123 grain FMJ, pretty close to "match grade" factory ammo in this caliber, which would achieve about 2650 FPS from a 16.3" tube, uses a .311" bullet, has a 10.9 specific gravity for the lead core, copper jacketted bullet, with a bullet length of 1.1" (approx, I haven't measured to verify). With a 20" tube, you add a relatively small 135-140 FPS or so, small enough most folks would likely do without.

To get ideal accuracy you need to 1) Handload your own ammo, buy several load books to copare. Milsurp is very inconsistent shot to shot, accuracy was not the goal 2) Buy a quality barrel, though you'll find out pretty soon your options for twist are severely limited for a .311" bore blank - primarily 1:9.5" and 1:10" until you get into expensive custom twists. 3) Use a tight chamber spec and reamer, CIP preferably.

R
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  #12  
Old 09-06-2012, 10:34 PM
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If you want it to really shoot, get a match grade .308" dia. blank (10, 12 or 14 twist it doesn't seem to really matter) chamber it with a JGS 7.62X39 LAPUA reamer, and set headspace to SAAMI minimum. Load .308 dia. 125,135, 150, or 155 Sierra MK's. It will also shoot surplus ball with no pressure problems, but like others have said, most of that stuff is garbage anyway. Leave the .310 bores for the AK boys.
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  #13  
Old 09-06-2012, 10:44 PM
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I am just curious my AK can shoot scoped maybe a 3-4 " 5 round group at 100 yards
what will an AR upper shoot?
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  #14  
Old 09-06-2012, 11:09 PM
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Quote:
Originally Posted by Mikelarry View Post
I am just curious my AK can shoot scoped maybe a 3-4 " 5 round group at 100 yards
what will an AR upper shoot?

Read 6mmintl's post above. Those figures are with good handloads. With Yugo ball in the AR the 2MOA doesnt last very long. Move out past 200 meters and it goes to hell real fast. At 500 meters I 'd say an honest 15 MOA or more with the flyers and wild uncalled misses. The other gun will drill them very consistently, but the wind plays havoc with the bullets due to the lower velocity.
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