6.8 sub loads new AR build

[Bigshooter]

I'm thinking about a dedicated AR upper strictly for sub loads. I originally had my sights set on a 300/221 but I started thinking about using the 6.8 instead, mainly due to the fact that the cases are easy to find and I already the dies and such for it.
Was thinking a 1:8 twist-10" barrel with, 150-160 gr. bullets.

Has anyone tried this?
Pros-cons?

Thanks in advance

[Alleycat]

Are you looking to hunt with it or just hit steel?

[LouBoyd]

The most important cons are:
1. The 6.8 SPC has considerably more case capacity than the 300-221, yet it would be shooting lighter bullets. It will be difficult to get good subsonic velocity consistency without filling the cases to reduce the capacity.
2. The case is too long to properly seat heavy low drag .277 bullets and still fit in an AR-15 magazine. There's not much point in an AR-15 single shot.
3. The lighter subsonic bullets won't deliver as much downrange energy as the 300-221. Even the 300-221 is marginal for hunting deer. The only way to get decent downrange energy from any subsonic is to use both heavy and low drag bullets. That's why the 300 Whisper, 338 Whisper, 458 Socom AR-10), and 500/510 Whispers make sense. If you're only shooting at short range a pistol cartridge wouild be easier, perhaps 40 S&W 45 ACP, or even 50 Beowulf.

All around I see little if any pros for the 6.8 SPC as a dedicated subsonic compared to a 300-221. You can easily make 300-221 brass from 17 rem, 221 or 223 cases. Already prepared 300 Whisper cases are pretty easy to find too. Have you looked on Gunbroker.com?

[Bigshooter]

Thanks for the replies, it looks like the 300/221 is back to the top of my list again.

[snipecatcher]

You'll love the 30/221. There is a huge variety of 30 caliber bullets, and 223 brass is easy to find, and easy to form, although it is a bit time consuming. It is currently my favorite caliber to reload and shoot.

[320pf]

LouBoyd pretty much sums it up. A subsonic 6.8 would be pretty weak. The kinetic energy of a bullet is given by the following equation:

KE=1/2mv^2;

Since the bullet is limited to subsonic velocities (~1080fps) the only way to increase energy is to increase the bullet weight. Thus you want to shoot the heaviest bullet that you can. The big attractions of the 300-221 Fireball (a.k.a. 300 Whisper) are:

1.) runs well in an AR15 platform and uses a standard Ar bolt/carrier and standard mags (including beta-c mags). You can not do this with a 338 Specter 458 Socom etc... )

http://www.quarterbore.net/forums/photopost/showphoto.php?photo=426)


2.) 240gr 30 cal bullets are readily available.

3.) The long heavy bullets seat deep into the case and limit the case to about 8-10 grains of powder. The load density is 90% or greater of the available case volume so the powder position is pretty constant. This give pretty constant subsonic velocities and pretty good accuracy.

4.) Long heavy bullets tend to be sub-critically stabilized, thus the readily start tumbling upon impact. A 240 gr Sierra bullet does not deform much if at all but tumbles soon after impact. Imagine a object ~1-5/8 in x 0.308 in tumbling through the target at about 900-1000 fps and think super broad head.

320pf