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Buying several boxes of the same ammo is a great option for ensuring supply and probably cheaper in the long run, certainly less trouble.
All the best and please keep us informed on progress.
GPM.

Greetings again,
Some notes on loading for the .303.
From my experience quite small changes in loads can result in large differences in point of impact with lower velocity loads shooting...

"Provided the ammo used is constant". You nailed the biggest problem right there. Buying a box here and a box there will probably give you trouble. Handloading can get over most of the problems but...

Greetings again,
The ratio from the sight radius to the range to target is 1:218 (0.420:91.4) so to drop the POI say 450mm on target requires raising the foresight by a little over 2 mm (450/218)....

That is an old one with that rear sight. It would have originally been regulated for 215 grain projectiles at a little over 2,000 fps so it is not surprising that it shoots high. A higher front sight...

Greetings shift14,
Lee Enfield rifles are notorious for shooting different loads to vastly different poi. If the rifle is a No1 rather than an earlier Long Tom it would have been regulated for 174...