308 subsonic

[JF]

I'll jump right in here and share my success with .308 subsonics:

I begin with a starting load of Accurate 5744 powder. I then work backwards until I reach the subsonic speed I desire. I will chronograph each load for a particular gun and write it down in my log book. In my 308 Encore barrel (23 inch, 1:8 twist) I hit a good subsonic velocity at about 12gr of powder using a 220gr Sierra HPBT match bullet. I have been using magnum primers and a good crimp on the bullet but this is not always necessary or desired. My accuracy in this barrel with 220gr subsonics has easily been MOA at 100 yards. The velocity spread also has been low.

I find that each barrel will be a little different in resistance so one load recipe will not always work in a different barrel. My bolt gun (18 inch, 1:10 twist) was way off of a good subsonic speed with the same load that worked great in my Encore. I believe that this critical subsonic window is why there are not very many over the counter 308 subsonic loads. What may work great in one gun may be too slow in another etc....

I don't like to guess a subsonic load because that is where you may end up with a stuck bullet in your bore. I always work backwards from the published starting load a little at a time until I reach a good subsonic speed.

Don't use just any powder for loading subsonics in rifle size cases. There is dangers associated with using low volumes of slow powders. Accurate 5744 is one powder that I keep coming back to because it just works. It is not effected by low charges in a case. In fact, the person I talked to over at Accurate powder tells me that it was designed to not be position sensitive with low charges in relatively big cases. It also has a higher nitroglycerin content for easy ignition.

Another powder to try is IMR SR4759. They sell it in the metal IMR cans but you only get a 1/2 pound since it is so bulkey. It is in the same class as Accurate 5744 but is a more bulkey powder.

Fast pistol powders will work, but there is a danger of a double charge of powder causing more pressure than is safe. I stay away from them for this reason. With 5744 and 4759, you can visually inspect the charged cases before seating a bullet and easily see if there is a possible double charge. A double charge of these (when measuring for subsonic loads) will not be over the maximum load listed in the manuals.

If all you have is a 1:10 twist barrel, you will have problems with accuracy if you go heavier than a 180-200gr bullet. I would suggest that 180gr bullets be max for the slower 1:10 twist barrels. If you use a bullet that is too heavy for your twist, you may have a hard time hitting the target at all. They will flop around in the air like a poorly thrown football and you can actually hear them wobbling through the air down range.