1:10 twist VS 1:8 twist | AR-15 7.62x39 Barrel

[lawndoctor1]

I currently have a 1:10 twist barrel on my AR-15 7.62x39. I can buy a new barrel with 1:8 twist that is 11.6 ounces lighter. What are the differences in these barrels?

[Unclenick]

One is lighter and has a faster twist.

Not sure what the fast twist is for. Generally it is needed for bullets that are either longer or same size but lighter due to being made of less dense material. Long bullets eat up powder space fast in that cartridge unless you have enough throat to load them too long for a magazine and will single-load them from a SLED.

Does the lierature say what it was intended for originally?

[Jim Watson]

8 twist is so you can treat it like a BO/Whisper and shoot heavy bullets subsonic. It should do ok with standard bullets.

[lawndoctor1]

I am looking to reduce weight in my rifle. I have 2 options in regards to the barrel. I can buy the brand new barrel that weighs 11.6 oz less, or I can have my original barrel fluted. The person that would do the fluting tells me that after he finishes that my org barrel would weigh 12-14 oz lighter.

The fluting job will cost $140 plus me shipping the barrel to him.
If I buy the brand new barrel it would cost $179 plus the shipping cost to me.

Any thoughts,etc??????

[Unclenick]

It gets back to the bullets. It doesn't sound like you intend to add the weight of a suppressor, so the subsonics are probably of no interest. The problem with shooting the more standard 123's with the fast twist are exaggerated wobble in flight (eccentric spin due to small mass asymmetries in the bullet) and the potential for core stripping (when the bullet jacket loses its grip on the lead core due to too rapid angular (rotational) acceleration, and that leads to groups opening up. This can happen to .30 cal in a 10" twist gun at velocities of around 3100 fps, and with cheap bullets whose cores are already loosened by a deep collet-type factory crimp, it may be as low as 2700 fps. This is one reason cheap ball ammo groups are sometimes 3 or 4 inches at 100 yards, when the Hornady FMJ's you carefully handle and load yourself can group 1". It's tempting to warn you 8" twist could do that at 20% lower velocities, but the 7.62×39 has about 20% lower peak pressure and therefor lower peak angular acceleration, so you still may not see it at all. I'm just bring up the theoretical possibility.

[lawndoctor1]

So...should i just have my barrel fluted as opposed to just buying the new barrel?

[Unclenick]

I don't know what kind of ammo you want to shoot or what sort a accuracy you expect. I don't know how many rounds you have through the current barrel. So I don't really have enough information to make a solid suggestion.

[lawndoctor1]

All I do is target shoot with this rifle. I like to plink a few hundred rds per month.

[Gunplummer]

It sounds as if you are just trying to save a couple bucks. That rarely works out well with guns unless you have the ability and means to do the work.

[Texas45]

Originally Posted by lawndoctor1

All I do is target shoot with this rifle. I like to plink a few hundred rds per month.

If this is all you use it for why does the weight reduction matter.
Your not toting the thing around daily
You haul to and from the shooting station and car.
I have a 762x39 AR also used pretty much as you say.
If I was to spend on another barrel Id opt for a 556 upper and shoot both cals.
Otherwise spend the 200 on ammo and go shoot.

[Unclenick]

If you are target shooting and you are using the usual 123 grain bullets, they are already "overstabilized" by a 10" twist, and 8" is going in the wrong direction. In general, Sierra says a good rule of thumb is a gyroscopic stability factor of 1.3 to 3.0 for "hunting accuracy" and 1.4 to 1.7 are close to ideal for best target accuracy. I have a file for a 123 grain Lapua flat base bullet and for 2350 fps from a 10" twist, the stability factor is estimated to be 3.23 in ICAO standard conditions. For an 8" twist it is 5.04. Ideally, you would want something more like a 14" twist for that short bullet for target shooting. For the longer shape 125 grain Sierra MatchKing, something more like a 12" twist would be best.

The faster spin is mostly going to exaggerate the effect of any small symmetrical bullet flaws. I don't think you are likely to see bullet stripping at these velocities.

[lawndoctor1]

Thank you Uncle Nick!