By John Bibby
For those of you who are adept at reloading, this is not a difficult task. It does however work a bit differently than working up a supersonic load. With a subsonic load, you start above your desired velocity and work towards slower. This “backward progression” is done for a couple of reasons.
With supersonic loads, we are attempting to keep our load below maximum pressure and look for a load that provides good accuracy with a stable velocity. The brass is normally filled at 70-105% of the capacity, so there is little concern with unreliable ignition due to position of the cartridge. Even in lower velocity rounds like .30-30 Winchester, there is no concern with cycling the action or the bullet not leaving the barrel (with a properly built cartridge). The concern is to not rupture the brass or primer by over pressurizing the round.
In contrast, subsonic rounds are often using less than 50% fill volume of the case, and some powders do not behave well in that condition. Also, the goal of a subsonic round is to balance the need to cycle an action (if in a semiauto) without excessive gas volume or velocity. Either can create a significant sonic event. This defeats much of the purpose of developing a subsonic round. For this reason, most people start their load testing with a combination that is known to be a supersonic round, but well below standard ranges. The ladder works down from there in small steps. With each lower powder amount, the projectile must be confirmed to have exited the barrel, the target must be checked for flight stability and the brass must be checked for pressure signs.
If the powder is finicky with lower powder volumes, a squib may occur due to partial ignition of the powder. This might leave a bullet in the barrel. The powder may burn inconsistently and impart enough energy to exit the barrel, but not stabilize the bullet in flight. The same load might also have delayed ignition and flash all the powder at once, creating distinct overpressure and a supersonic (read as explosive) round. For these reasons, developing subsonic loads is painstaking work. Even when using a proven load, a different barrel length or twist rate on your firearm may yield different results. Be aware, loads that work great in a bolt gun may not cycle a semiauto. It is less common but loads that cycle a semiauto may go supersonic in a bolt gun.
For my development, I am working with a 200 gr Maker Bullet .308 projectile designed specifically for subsonic hunting and self-defense work. The bullet is designed to expand from .308” to +.800” at +900 fps. The manufacturer suggests a maximum velocity of 1120 fps to properly utilize the expansion potential. I assume this means the folded back copper petals risk coming off at impact speeds exceeding 1100 fps. Considering my target subsonic velocity is 1050 fps, that will not be an issue with my usage.