It is I....subsonic man. Kiwi here is a quick run down on subsonic .308 ammo.
Bullets....It is best to use a 1:10 or 1:8 twist barrel on your weapon. I have a 1:12 twist barrel on my 700 P and I get marginal performance. A good manufactured subsonic round at 100 yards or meters should yield a group of .50 to .75 MOA with the fast twist barrels. With my slow twist I get anywhere from 1" to 3" groups at 100 yards or meters.
I was taught to use jacketed flat top heavy bullets. The one I use the most is the Hornady 170 grain jacketed flat top bullet. Lapua and Swiss P market a subsonic round using pointed bullets, they may work good in fast twist rifles. I was taught to use long cylindirical bullets like the Hornady. Now, with the fast twist rifling you can use the heavier bullets, from 170 grains to I have seen some over 200 grains.
If you have a chronograph, you want to strive for a muzzle velocity of between 1000 fps to 1070 fps depending on the weight of the bullet. It has been suggested that the primer flash hole be enlarged and a magnum rifle primer used to ensure that the powder is ignited properly. DO NOT USE ANY FILLER IN THE CARTRIDGE WITH THE SMALL POWDER CHARGE.
DO NOT USE ANY TYPE OF RIFLE POWDER FOR YOUR PROPELLANT. IF YOU DO YOU CAN DEVELOP A SECONDARY EXPLOSION WHICH WILL RUIN YOUR DAY AND MAYBE YOUR LIFE. USE ONLY THE FASTEST PISTOL POWDERS SUCH AS RED DOT, BULLSEYE, OR UNIQUE.
As for cases, some people recommend reaming the primer flash hole to 9/64" or 3.5 mm. Mark these cases and use them for subsonic loads only. I had purchased some subsonic ammo from one outfit and they used the standard primer flash hole with out enlarging it.
Here are a couple of sample loads you can try.
Hornady 170 grain flattop jacketed bullet
Hornady Match Brass
Federal Large Magnum Primers
8 grains Bullseye (I really like this powder for subs)
I press the bullet on a 2.015" .308 cartridge and press the bullet just to the bands. Do not crimp.
Sierra 150 grain flattop jacketed bullet
Hornady or other brand of Match grade brass
Same primers as above load
7 grains Bullseye
I press the bullet to the bands
Make sure your bore is clean before testing. Have someone listen when you fire (without the suppressor). You want to hear a boom, not a crack.
Becareful with decreasing powder grains, gradually is the norm. There are other thoughts on subsonics but this is how I was taught by my instructor. Now, one last word, do not expect the trajectory to be like a high powered round. The power band does decrease quite rapidly and the bullet drops like a rock at 300 meters. Most info I have seen mentions 100 to 200 yards. When you use subsonics there will be considerable holdover on your scope. Feel free to contact me on this if you need to.
I hope this information was useful. The main rules are to never use rifle powder, only fast pistol powder. Lapua makes a pointed type bullet specifically for sub sonic load. It is lead with a thin gilding over it. I believe it comes in 200 grains.
I think when the bullet gets out past maybe 200 yards depending on the twist of the rifling the bullet can tumble. Here is some data that I came up with and it is comparable to some of the manufactured rounds.
100 yards 947 fps 16.99" bullet drop
200 yards 873 fps 72.56" bullet drop
300 yards 811 fps 173.63" bullet drop
As you can see, the bullet really poops out at 300 yards. When I first started messing around with this stuff I tried using a Sierra MatchKing bullet and it did actually tumble at 100 yards when it hit the target. It did it right before it hit, it left a keyhole. When I switched to a flattop jacketed bullet as instructed I got a nice clean hole at 100 yards at three hundred yards I saw the makings of a keyhole. Another reason for the flat top bullet is they expand easier. And expansion is what you want. If you use a bullet with a pointed tip it will not expand, except for the bullet made by Lapua. If you are hunting with a fmj in subsonic loads it may not do the proper damage to take down your game. With my 1:12" barrel I think I am doing pretty good to get 1" to 3" groups. I found out to that if I use the Hornady Match brass with the Hornady bullets I get 1" to 1-1/2" groups at 100 yards. If I use LC brass it comes out to 3" groups. When I get a 1:10 barrel I should be styling at about .75 MOA. Hope I did not stray too far off course.
I am attempting to load for a friend of mines .308 internally suppressed bolt gun. Its 18.5" Bbl 1:11.25 twist as far my measurements reveal. I know very little about the suppressed world. He left me with the ammo that was sold with the gun. At this point, I have not broke one down to weigh the bullet. The bullet is a round nose with a deep concave with lead showing. My guess is its in the 200 grain range. Where do start with this combo? I do not have a chronograph.