[LouBoyd]

That much vertical strining is most likely to be from velocity variations, I'd suggest you shoot over a chronograph and record the velocity of each shot and it's point of impact at e target.

Velocity variations can come from several sources, but in all cases a slow bullet (of equal weight) has received less energy as it passes the muzzle.
Reasons bullet can have less energy are:
1. less powder charge - weigh each charge precisely until you find the problem.
2 poor ignition. If time it takes for the powder to burn varies with time the bulet can
recieve a variable amount of energy. Slow powders and low peak pressures usuallly make that worse.
3. engraving force. High engraving force will raise chamber pressure and may actually give higher velocity. It can go either way, but variable engraving force comes from varying seating depth. Engraving foces are very sensitive to seating depth when the bullet is near (toching +/-) the lands. Slow powders can make the time to engrave bullets more variable.
4. blow by. if the bullet is seated away from the lands a significant amount of propellant may go around the bullet due to clearances in the neck and throat and out the bore before the bullet moves enough to seal. That can vary from shot to shot depending on neck tension, neck thickness, and of course seating depth variations. Powder burn rate variations can make shot to shot blow by variabilty worse.

5. some bullets my sit on the lands without providing a good seal. It's similar to 3 but for a different reason As a quick test drop one of the bullets (not cartridges) into the bore from the chamber then blow air into the chamber gently. You can use a piece of rubber tubing which fits the chamber or remove the barrel from the rifle. If the chamber isn't sealed by the bullet just sitting into the lands the throat is mismatched to the bullet your using. This means the bullet has to actually start engraving before the bore seals and more propellent is likley to blow by.
6. variable bore friction. If the barrel is copper or powder fouling the shot to shot friction can vary and produce vertical stringing. For short barrels I'd expect that to be less likely than 1-5.

All of the above are more critical to subsonics than to typical supersonic cartridges.
Bulet drop is always D=1/2 G* t^2 where D is drop in feet, G is the acceleration of gravity
which is 32 feet/(second^2) and t is the time of flight in seconds.

If a chronograph does not show velocity varation possible problems inclued something loose or bullet instability. Pointing error could be anything loose. Action, barrel, scope, scope base, reticle, etc. I bought a 223 AR-15 upper on Gunbroker once which produced 4" stringing at 100 yards!. Turned out the barrel nut was barely finger tight. It couln't have simply come loose with the gas tube in place. It shot fine after the barrel nut was properly torqued.

If your actual muzzle velocities are over 1050 fps the problem could also be related to bullet stability though that usually produces random point of impact variations. I'd suggest keeping your loads at 1000 to 1025 FPS at least until you find the cause of the stringing problem. Nothing magic happens at exactly the speed of sound. The transonic region where bulelts have c reduced stability is 200 ( 1050 to 1250) to 300 (1000 to 1300) fps wide.

Trying to diagnose subsonic loads without a chronograph is just asking for frustration.