Subsonic .308?


October 5, 2011, 08:02 PM
I'm thinking about ordering a custom .308 barrel for my T/C Encore set up to shoot heavy projectiles at subsonic velocities. Probably order a can in time, too. What would be the best rate of twist for this? 1:8? Fast as 1:7? I figure at subsonic velocities, I shouldn't need anything longer than 18 inches.

Yeah, I know I could go .300 Whisper/Blackout, but I shoot a lot of .308, and have the brass, bullets, and dies already. No expert by any means; just looking for an economical way to get into subsonic .30 caliber.

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October 5, 2011, 11:34 PM
Do you really mean subsonic - 900-1000 fps? Proper twist is a function of speed and bullet length, several twist calculators on the web. Most probable problem with heavy bullets in 308 is the short neck. 170 gr is ok, but a 250 gr would be a problem. H4895 might be a good powder to use for slow loads, don't know what the fps would be.

October 5, 2011, 11:45 PM
Yes, interested in topping out around 1,000 fps. I know the neck is relatively short, but I'd have thought you could set a long bullet deeper since you'd not be filling the case. Something to think about ...

October 5, 2011, 11:54 PM
Most .300BLK barrels seem to be 1:8. Since you are basically going to be duplicating the ballistics of that cartridge by downloading .308, that's the twist rate I'd go with.

October 6, 2011, 12:07 AM
Same thoughts as hirundo82.

You will need to consider what powders you can use in that fairly large case to get your leisurely subsonic velocity without having issues of powder position, failure to ignite, and that purported risk of detonation with some powders. I'm wondering if a full case of Trail Boss would work in some loads. A possible alternative is inert filler materials on top of your 10-15gr powder charge but that sounds like a lot of hassle to me.

October 6, 2011, 12:25 AM
Midway has them on the shelf !

October 6, 2011, 06:43 AM
And at that price, it won't take long to pay for the price of a custom barrel by reloading components I already have!

I've been reading a number of places about using Trail Boss for subsonic rifle loads; I've been using it for some time for soft .357 and .44 loads in my revolvers. I should try some in my current .308 rifles, but they may not be fast enough twist to stabilize. That's why I was thinking of a dedicated barrel, threaded, for my Encore.

October 6, 2011, 08:55 AM
Hodgdon's has load data for subsonic .308. Covers 168gr rounds using Titegroup or Clays.

451 Detonics
October 6, 2011, 09:09 AM
One in 8 should be fine if you are looking at bullets in the 200 grain range.

Trail Boss is one of the best powders I have found for sub sonic .308. It is bulky enough to provide good case density which will result in a much lower standard deviation. The lower the SD the more likely you will see good accuracy out of the ammo. I am using Trail Boss in my suppressed Ruger 77 in .308. It works great and has proven to be very accurate.

October 6, 2011, 10:06 AM
Detonics, that looks like a nice rig. Do you have a tight-twist barrel on your 77? And what's the barrel length? Your can looks like it attaches just beyond the forend.

October 6, 2011, 11:21 AM
I shoot a lot of sub 308.
One rifle has a 1/11 twist and does great with trailboss and 150 flat noses.
The other has a 1/10 twist and likes the 180 round nose better.

Both loads and rifles will shoot about 1.5 inches at 100. A little better may be possible, but that has worked for everything I have needed it to. Consistency is much more difficult than with supersonic loads.

If I were going to have a dedicated .30 sub gun, it would definitely have a 16 inch barrel and fast twist. 1/7 or 1/8. For subs only, id go with the 1/7 and use the 240 smk (think the 1/8 can handle it too depending on conditions...)

I shoot a lot of 308 supersonic as well, so I have been in a similar boat. No reason sub 308 will not work for you, but you have to understand that the case fill will not be right to give you very very good consistency. Even with trail boss it is difficult to get anything to shoot lights out. Just start at probably 12 grains and work your way down, preferably with a chronograph, till you get velocity and accuracy you want. 14 grains will just about fill the case, but you will probably end up needing 10.x grains. The powder company has some decent info on their site about the powder.

Be mindful of seating depth as that can have a big effect on the load. For me, I had the best luck with federal match primers. Some people say to widen flash hole and use mag primers (probably helps with less fluffy powders), but using trail boss i have never had an issue.

What twist are you running in your rifle? The short stubby bullets are easier to stabilize. You can always load a regular pointy bullet backwards in a pinch and it should do fine. the 180 rn is about 1.1 inches I think, and stabilized from my 10 and 11.25 twists. Be sure to test them for stability before a suppressor or anything ends up on the muzzle. Shoot paper or better yet paper behind cardboard at various ranges, especially up close. If you get clean round holes you are good to go. Keyholes/ oval is problematic lol.

Definitely neck size when you can. The brass will last a long time. I would like to swap a 300 xxx subsonic barrel on to my savage (mcgowen usually stocks them) but there are often ejection issues due to the super short case and rem 700 or savage ejectors. If you are running a T/C then those issues would not be there...

Maybe try the subs and once you get a can see how you like it? If the 308 subs don't live up to expectations then I would get a 300 whisper or blackout barrel on the TC. The tiny case gives MUCH better consistency. You have to get some additional tooling, but for a subsonic platform it is tough to beat it.

451 Detonics
October 6, 2011, 01:54 PM
Detonics, that looks like a nice rig. Do you have a tight-twist barrel on your 77? And what's the barrel length? Your can looks like it attaches just beyond the forend.

If I remember correctly it is a 1 in 9 twist (the new 77's are 1 in 10). Here is a pic with the can off...The shroud serves to protect the threads and makes it look like a plain heavy barrel.

October 6, 2011, 07:27 PM
What velocity are you all shooting sub-sonic? Blkout web says they are >2000 fps, which is NOT subsonic. It's more like slow and suppressed. Sort of interested for my 30-30, maybe 220 gr. CB

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