AR Rifling help

[Guest clsutton21]

Alright, as most everyone knows, Hero Gear has launched a line of AR's called the Hero 15 and I am having one built, specifically a DMR. It is going to be in 6.8 spc with a 20" barrel. I know that the 6.8spc is generally a 16" round, but I like the looks of the extra length.

Now onto my question: I read somewhere that you generally want a 1:10 twist with a 16" barrel which gives you approx 1.66(repeating) rotations of the bullet. Since I am going with a 20" barrel, should I go with a 1:12 twist rate because that would also give 1.66(repeating) rotations?

I'm not really sure if that is the proper way to think about it, but I thought with a longer barrel, I could have a slower rotation since I have more room to rotate and thus, have more velocity than I would with a faster rotation. Or, since this is going to be a dedicated long range gun...should I go with a 1:11 or some other twist rate so I have more spin, thus making it more accurate? They haven't cut the barrel yet, but I wanted to be informed when I was asked what I wanted.

Edit: Also is the SPC II chambering the norm nowadays? I noticed some ammunition requires it.

Edited August 4, 2009 by clsutton21

[Superman]

I am new to this, but as far as I know the twist rate is determined by bullet length. Longer bullets perform better with a faster twist. Heavier bullets are longer than lighter bullets. My suggestion would be to talk to Joe or someone down there at Hero Gear. I am sure they could explain to you what twist you need and why.

Edited August 3, 2009 by Superman

[Guest clsutton21]

I am new to this, but as far as I know the twist rate is determined by bullet length. Heavier bullets are longer than lighter bullets. My suggestion would be to talk to Joe or someone down there at Hero Gear. I am sure they could explain to you what twist you need and why.

I talked to Jamie(sp) the guy that builds them and he told me it was basically personal preference on what I want. The default is 1:10, but 6.8 is not usually in a long barrel because it accomplishes all it will in 16" so I didn't know if I needed to think outside the box to reduce pressure while increasing velocity and still keeping accuracy. He's gonna talk to the guy that cuts the actually rifling and see what he says and call me back, but I wanted to be somewhat educated in case I was given options.

[Guest Mugster]

The rifling has nothing to do with pressure. Thats a function of the bullet weight and the powder charge.

To shoot a longer bullet, you need a faster twist all things being equal. Barrel length has nothing to do with it. You either have enough twist to stabilize a given length bullet and that pretty much the end of it, even if you turn it into an SBR.

Go with the tighter twist, 1 in 10. The only reason not to do this is too shoot fairly fragile varmint bullets at warp factor 5. Alot of revolutions can cause them to actually fly apart in mid air if the jacket is thin enough. There are no loadings in 6.8 which would cause this to happen unless you handload, as far as i know.

[Guest clsutton21]

The rifling has nothing to do with pressure. Thats a function of the bullet weight and the powder charge.

To shoot a longer bullet, you need a faster twist all things being equal. Barrel length has nothing to do with it. You either have enough twist to stabilize a given length bullet and that pretty much the end of it, even if you turn it into an SBR.

Go with the tighter twist, 1 in 10. The only reason not to do this is too shoot fairly fragile varmint bullets at warp factor 5. Alot of revolutions can cause them to actually fly apart in mid air if the jacket is thin enough. There are no loadings in 6.8 which would cause this to happen unless you handload, as far as i know.

Thanks for the advice. This clears it up quite nicely for me. I still don't understand why some ammo says you need a 1:11 twist rate for pressure reasons, or is that only for when you don't have a SPC II chamber(which I don't know if I have)?

[Superman]

Well I have looked into the 6.8 before because I plan on getting one. And it looks like everybody in the 6.8 world is saying 1:11-1:12 is the best way to go know with an SPCII chamber.

[Superman]

Here is some info I found

a. Barrel Length:

For most practical shooting, a 16" Mid-length chrome lined barrel will be ideal if you plan on shooting 80-115 Gr. bullets. If you plan on shooting the heavier (130 Gr.) bullets, the 18" or 20" barrel is a good idea since you will be able to get an extra 60-125fps more than a 16" barrel.

b. Barrel Twist:

1. The ideal twist for the 6.8SPC is the 1:11"-1:11.5" twist. It will allow you to shoot the 80-135 Gr. bullets.

2. The 1:12" twist is ideal for the 80-115 Gr. bullets.

3. The 1:10" twist barrels will allow you to shoot the 80-135 Gr. bullets, but the barrels with the slower twist slightly reduce the pressure.

c. Chamber:

The best chambers for the 6.8SPC are the SPCII Chamber and the DMR Chamber. The SPCII Chamber has a longer throat which allows you to shoot the hotter loads without worying too much about pressure signs. The DMR/DMR-C Chamber was designed for precision while still capable of shooting the hotter loads. For anyone that has a SAAMI Chamber, the one thing you could do is have someone lengthen the throat to the Improved Chamber specs.

SAAMI Chamber: Tight Chamber. (Similar to the the .223 Chamber.).

DMR/DMR-C Chamber: Longer throat than the SAAMI Chamber. (Similar to the Wylde Chamber.).

SPCII/Improved SAAMI/SAAMI II: Longer throat than the DMR Chamber. (Similar to the 5.56 Chamber.).

[Guest clsutton21]

Here is some info I found

a. Barrel Length:

For most practical shooting, a 16" Mid-length chrome lined barrel will be ideal if you plan on shooting 80-115 Gr. bullets. If you plan on shooting the heavier (130 Gr.) bullets, the 18" or 20" barrel is a good idea since you will be able to get an extra 60-125fps more than a 16" barrel.

b. Barrel Twist:

1. The ideal twist for the 6.8SPC is the 1:11"-1:11.5" twist. It will allow you to shoot the 80-135 Gr. bullets.

2. The 1:12" twist is ideal for the 80-115 Gr. bullets.

3. The 1:10" twist barrels will allow you to shoot the 80-135 Gr. bullets, but the barrels with the slower twist slightly reduce the pressure.

c. Chamber:

The best chambers for the 6.8SPC are the SPCII Chamber and the DMR Chamber. The SPCII Chamber has a longer throat which allows you to shoot the hotter loads without worying too much about pressure signs. The DMR/DMR-C Chamber was designed for precision while still capable of shooting the hotter loads. For anyone that has a SAAMI Chamber, the one thing you could do is have someone lengthen the throat to the Improved Chamber specs.

SAAMI Chamber: Tight Chamber. (Similar to the the .223 Chamber.).

DMR/DMR-C Chamber: Longer throat than the SAAMI Chamber. (Similar to the Wylde Chamber.).

SPCII/Improved SAAMI/SAAMI II: Longer throat than the DMR Chamber. (Similar to the 5.56 Chamber.).

So if I'm ordering a DMR built, it will most likely come with the DMR chamber?

[Superman]

That's the way i understand it. Apparently they have changed the way the make the 6.8 ammo and if you use this "new" ammo in an older gun or in a saami chamber it is equivalent to firing 5.56 in a .223 chamber.

[Guest clsutton21]

That's the way i understand it. Apparently they have changed the way the make the 6.8 ammo and if you use this "new" ammo in an older gun or in a saami chamber it is equivalent to firing 5.56 in a .223 chamber.

Yeah, they're pushing to have 6.8 for the older chambering and 6.8x43mm separate for the new chambering so people won't be confused.