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Old May 6, 2011, 02:49 PM   #1
Yellowfin
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6.8 150-160 grain subsonic load?

Does anyone here load a subsonic 6.8 SPC in 150-160 grain bullets? I'm wrestling big time between 6.8 and .300 AAC. I really would like a 6.8 better for hunting deer and hogs, but my wife REALLY likes shooting subsonic and suppressed when the chance arises. I'd rather not need to have 2 different uppers just to have the latter for maybe 20% of the time, but also keeping her happy is...well...you know, a BIG priority too--I want to have as much fun for her as possible so she enjoys shooting in as many ways as she can, and a recent trip to MD for a suppressed shooting event showed me that she likes that the best.

From what I gather, it looks like the biggest bullets I can get for the .277 size are 150 and 160 grain. The goal is to have them just barely subsonic in a 16" barrel and have it cycle my AR's action 100% with a suppressor on (helps add gas pressure). Can this be done, and does anyone here do it or at least have done it?

(In case anyone wonders, we live 15 minutes north of PA w/ plenty of friends and family there, plus plenty of family in other free states as well.)
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Old May 7, 2011, 10:09 AM   #2
HiBC
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The bullet length/bore dia determine the twist rate necessary to stabilize the bullet.As you are trying to achieve sufficient rpm,lower velocity would compound the problem.Not saying it could not be done,but I think you will require a much faster twist barrel than common for the 6.8.
I have no experience with suppressors.Getting it to cycle could be a challenge.
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Old May 7, 2011, 10:27 AM   #3
Loader9
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Yeah, it can be done but you'll find shooting the 130's out of that 6.8 are an easier thing. You'll do best by using SR4759 and starting at 10.5 grs under a 130 and go up. Using a chronograph and reliable cycling of the action will tell you where the sweet spot is for your particular rifle. Consider a max velocity at 1050'ps at the muzzle. This is the velocity the commercial loaders use as max for subsonic ammo.
If you've never used SR4759, it's extremely bulky. 19.0 grs in a 3006 case is about 80% loading density. That's what I load for the Boy Scouts locally and it's a hoot to watch them shoot the o3a3's with no recoil. SR4759 will not meter thru any measure so it's a weigh ever charge kinda powder. Burns exceptionally clean. If you've never loaded subsonic ammo, make sure to keep the brass separate from normal loads and only reload it a max of 5 times or expect issues. The firing pin action and low pressures will eventually shorten the brass at the datum line giving you an excessive headspace condition.
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Old May 7, 2011, 10:46 AM   #4
Brian Pfleuger
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You might try Trail Boss also. Instructions for load development here:

http://www.imrpowder.com/PDF/Trail-Boss-data.pdf

Trail Boss will help keep noise down also since 100% powder burn occurs in the first inch or two of bullet travel. On the other hand, that may also cause low enough pressure to not work the action. I'm not familiar with the AR internals.
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Old May 11, 2011, 10:20 AM   #5
Yellowfin
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Fantastic. Now to pick out a lower and a stock she'll like.
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