Some info on loading SS rounds.
The practice of down loading rifle calibers to subsonic velocity levels must be approached in a special way.
THE FUNDAMENTALS ARE:
At subsonic velocities the chamber pressure is extremely low in most rifle calibers.
This means the combustion efficiency is not good.
It means that case obturation is bad.
Therefore faster more efficient burning powders are required.
This leads to low loading densities, which might cause ignition problems.
Because the requirement is to have subsonic velocities while maintaining the best possible combustion efficiency, the velocity should be kept as close as possible to the sonic threshold for the specific conditions, preferably between 1025 Fps to 1100 Fps.
Projectiles with sharp pointed ogive’s do not make good subsonic projectiles because
External and Terminal ballistics at subsonic velocities does not exploit the design features of high BC jacketed SP or HP-BT bullets. Therefore, it is actually a waste /moot point shooting with these designs at subsonic velocities.
Subsonic loads using standard components will not cycle high power, semi auto rifles.
Warning: — Since fast burning powders are used, it results in very low loading densities; Therefore the possibility of double charging is very high!! —— Please take extra precautions to prevent this from occurring.
WHAT TO DO?
Use only powders with good ignition characteristics.
Use any well-known Cast Bullet Publication, such as Lyman®, Speer® etc as a guide with the above equivalent powders. Begin at the recommended minimum/Start loads and incrementally load lower towards the subsonic range, of between 1025 and 1100 Fps.
In most cases extremely fast burning Handgun/Shotgun powders are the better options. However, there are exceptions involving slower burning powders with excellent ignition characteristics such as our Accurate 5744®
To ensure the best highest possible loading density, select the bulkiest powder with the lowest bulk density (weight/Volume).
Extruded flake, or -tubular or -multi perforated powders, are usually the recommended types, however there are some exceptions utilizing special spherical/Ball® powders.
Neck-sizing the cases to ensure proper obturation
Adding Fillers to keep the powder on the primer side of the case (Dacron® or Pufflon® – see below).
The loading is done inversely to the standard practice with normal loads and is best executed by loading from a higher Velocity usually in the 1300-1400 fps range towards the lower level.
Confirm statistically, with proper number of rounds per iteration (min 10 rounds) by measuring the velocity with a chronograph and verify standard deviations and extreme spreads to make absolutely sure that no supersonic “flyers” occur.
Make sure that projectile has left the barrel when a blackened/sooted case is extracted.
Also be aware when extremely low velocities are recorded < 900 Fps. This is indicative of extremely low pressures which, is an early warning sign that bad ignition "squib" loads = stuck bullet maybe eminent.
Use projectiles with the shortest ogive possible, preferably Round nose projectiles. This will ensure better stability due to a more centered COM (center of mass)
Use the heavier projectiles for a particular calibre. The higher inertia ensures better ignition and combustion.
The USE of FILLERS:
Whenever inconsistent velocities are detected, it is recommended that a proper filler material be used to keep the powder at the primer end of the case.
The following filler materials are recommended:
PUFFLON®, which is a dedicated speciality filler product. www.pufflon.com
DACRON®. This product comes in various forms, and the “fluffy” version must be used. This must be lightly stuffed and not tightly compressed; the purpose is to merely keep the powder at the bottom of the case against the primer.
Powders recommended for subsonic loading from our own product lines:
Accurate SOLO 1000®, Single base Extruded Flake
Accurate Nitro100® New Formulation. Double base Spherical/Ball®
Accurate no2®, Double base Spherical/Ball®.
*Accurate A-5744®, Double base extruded tubular.
*Caution: This powder will leave some un-burnt powder granules behind, which can cause irritation when “flush-back” occurs into chambers and mechanisms of guns.