Loading subsonic question. How slow is too slow.

[Nic56]

What bullet are you using and the OAL? I do load mine short, I measure the bullets ogive and find where it measures .264 in diameter and set that mark on the side rib of pmags. Thats a start point then play with it for accuracy once I have my velocity between 1000 and 1050 fps. I'm using 208 amaxs with A1680.

I have been mostly experimenting with the Leatherhead 240 grain cast bullets, which are Hi-tek coated. I seat these out to 2.100, mainly because it just barely hides the bands on the bullets. I realize they aren't match grade bullets, but I think I can do better.

I have 8lbs of A1680, but have been experiment with IMR 4227, which is starting to run low. I may play with 4227 a bit more, but may be forced to switch back over to 1680.

I will play with the seating depth. I need to chrono the loads with my suppressor attached. I would like to stay around the 1000-1020 speed. Once I confirm that, I will work on depth if accuracy does not improve.

I plan to load up my next batch with the cases I most recently annealed. I think I did a pretty good job annealing this time.

On a side note, I have bought some of the Wilson 7.62X40 magazines. They are simply the Lancer magazines with the ribs shaved much lower. Big bullets can be loaded to full magazine length without any pinching. They are nice to use.

[Klem]

The sonic boom starts 90fps below the speed of sound and increases until its maximum decibel level 150fps above the sound barrier.

You want to be about 100fps below what the sound barrier is in your location. Below that you will not get any quieter and apart from tiny price savings from loading less powder there are no advantages and only disadvantages.

Below 5,000lbs at the gas port (Quickload) your AR will stop cycling reliably.
This is below 7.5gns of powder in a pistol length gas tube and about 11gns in a carbine length gas system (using 220gn projectiles).

Below about 9gns of powder and the case volume drops below 80% which risks two dangerous situations; Secondary Explosion Effect (SEE) and Flash Over.

As the velocity drops you will see less impact on flesh.

Subsonics are already 2/3 the available peak pressure of the calibre so there will be no barrel-life saving.

The aim of the game is to get your projectile to about 100fps below the speed of sound at the muzzle. Anything less is less efficient.