Windage issue with lower velocity loads Lee Enfield

[Gunfreak25]

Can someone give a theory why a lower velocity bullet could have wildy different impacts on target compared to full power loads?

Below is two targets from my US property Savage Enfield. The left is Greek ball, normal for this ammo it's good stuff but not match grade. The right target is my reduced cast load which shoots well, but hits 7 inches right. I'm guestimating 1500fps.

My theory, slower bullet is in the bore longer and more affected by harmonics and barrel whip. I see shimming in my future....

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[country gent]

It may simply be the harmonics point the 2 loads are exiting the barrel at. The greek load may be exiting while the harmonics node is at 12:00 and the cast load is exiting with the node at 3:00. The slower load will be affected more by recoil and follow thru than the faster loads.

[Larry Gibson]

Not so much harmonics, the greatest difference is from how the rifle recoils with each load before the bullet exits.

[303Guy]

It would be interesting to to compare the bore line to the sights. You might find that cast shoot closer to the bore line than the jacketed.

[Gunfreak25]

It's a tight stock, all matching Savage parts. Armorers manual says 4 to 5lbs of down pressure at the muzzle. Mine is only half that meaning the barrel is flopping around with vibrations. The fix is shims under the rear of the receiver which cants the barrel down more.

Looks like I have some shim experiments. If I can dampen the barrel harmonics enough to correct the windage issue with this load, it will be interesting to see how it affects full power loads.

Larry I suspect recoil movement from shot to shot may also be a big one here.

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[GooseGestapo]

Enfields have light "floppy" barrels. Hence, the need for some bedding pressure to dampen the vibration.
I find that my .303 does something somewhat similar to what yours does.
I have a milled vernier sight on mine and have markings that allow me to quickly go from jacketed bullets (up 2.5 turns from bottom) to cast (up 5turns).
The horizontal shift is due to barrel torque differences due to mass of the bullet. Changing to a similar weight cast bullet will reduce the amount of horizontal shift.
My rifle likes a heavier larger bullet (.312" jacketed), particularly the Hornady 174gr .312" BTHP. A Lee .309-200RNGC cast of w/w sized .312" and running close to 2,000fps shoots close to the jacketed bullet, windage wise.
It also likes the Lee .312-155PTGC.

With my scoped rifles, I usually find a magnification where the lower post taper matches the bullet impact of the slower cast bullet, and put a label on the scope so I can remember how to set and hold for different loads.

Added: My rifle is a '43 Fazerkerley that was fto in '50's. It's got a like new, tight 5-groove barrel, and I bought a Fazerkerley marked vernier rear sight for it. It shoots vastly better than yours appears to. My match loads of 174gr Hornady match bthp run at/under 2" at 100yds. My cast run about 3".
Somebody previously refinished the stock with a thick gloss finish. I plan to redo it sometime.
I also plan to shoot it in some vintage military rifle matches. I took it to CMP Talladega last year to get 200yd zero. A fellow shooting down from me with a Swiss straight pull got his feelings hurt! My ole Brit was kicking his supposed "tack driver". He came over to see my home made front sight adjuster I modified from a SKS front sight press. He couldn't believe my 200yd target.
It shoots "almost" as good as my 1943 Remington 03A3 with barrel that gauges 0.0 te, and 0.5 me. It's shot several 1.2" 5-shot groups at 100yds (Hornady 155gr A-max, 48.0gr H4895).