How to get SD as low as possible when reloading for .308 subsonic

[Mr.Knowit all]

To the few of you that actually shoot and reload for precision subsonic loads I am seeking your advice if you have any ideas how to get the SD as low as possible when loading for subsonic (.308 in particular).

I shoot a 200gr. Speer with a flat base at 994fps with 11.5 gr. of Trail Boss. My SD is about 14 fps which is a little too high for me. I double weigh everything but with an SD that height really opens up my groups when shooting at those speeds. My supersonic loads are always in the single digits but I just can't get my subs down even when I change the projectiles. Maybe that is just the nature of the beast when reloading for subs??

If anyone knows first hand please shower me with any knowledge you may have. I have been trying to fix this issue for some time now.

Thank you for your time.

Eternal

 

[DaveTooley]

Try pistol primers.

 

[Nightraider]

Try pistol primers.

At Subsonic distances 14sd is perfectly fine. I hate saying this, but a good crimp will typically lower SD/ES in sub cases, but it also effects accuracy in most cases too.

 

[kindabitey]

Getting low SD in a 308 sub load is going to be difficult. Do all of the same brass prep as if it were a bench gun. Uniforming flash holes has helped my 300BLK loads.

 

[gary0529]

As has been previously posted, Why bother? I shoot a LOT of 300 Whisper (Blackout) sub loads. Cannot see any difference in POI at usual distances I shoot-i.e. under 125 yards and typically less.
I would not think you are shooting bench rest with subs- I use them to cull deer and 1 MOA is more than I need or can hold even with a rest in the field.
Don't get lost in the weeds trying to make a BR round where there is no need.
If your applications are different and you require this level of precision please let me know what you are using it for - just for the sake of my curiosity .

Gary

 

[garandman]

I'd guess neck tension is prolly a pretty significant factor, more so than in supersonic (2700 fps+ ) rounds.... ????

 

[mgunderson]

Questions- are you sorting bullets and how? How extreme have you taken the seating depth?

Statement- not sure trailboss and the low fill ratio of the case is going to give low Sd. Been playing this game for a few years now for fun and keep coming back to wanting to try another powder.

 

[golong]

My initial reaction is to echo that of others - why are you trying to do this?

Yes neck tension matters, but with subsonic you are fighting powder that is just kind of floating around in the case. Throw the bolt and the powder may be slumped right behind the bullet. Settle into a bag and it may level out. Point being, getting a consistent burn is almost impossible when filling a small amount of the case capacity. If you can find a powder that fills the case and creates just enough pressure to get the rock out of the barrel then you may be able to get low SD's; To the best of my knowledge no such powder exists for a .308 case.