Large error in Zeroing No 1 Mk III* at 25 yards - your thoughts, please?

[22SqnRAE]

Yes Mike, I do reload. Have found little difference at 100m between reloads and factory.

I am not seeing a correlation between performance (holes through paper) and basic ballistics.

[CINDERS]

What I would do if you have a good mate that trusts you with a known 303 rifle MkIII that shoots well and you always FLS your cases (All the way not just short so it just does the neck) poke some rounds through his rifle with yourself shooting it and see what gives.
If that rifle groups with your reloads then obviously its not the reloads, go to the muzzle look for wear/damage, find someone with a bore scope and look at the lede/throat it may be like a train tunnel if the projectile enters the bore off axis it won't straighten up in there but will always be crooked if all's well with that side start looking at the bedding and correct contacts for the rifle.

I base this on my own experience when I purchased my sniper T when I first got it the grouping was @20MOA at 100yds took the scope off to eliminate that and it was still sh*te so not the scope at that stage, everything else looked good tore the rifle down (stripped Peter) result of the investigation was absolutely pulped draws and a front collar that was too long allowing the receiver to rock in the stock which probably caused the issue.
Sometimes things are easy to spot others are less obvious the only other thing I can really think of also if the barrel has been ringed we just do not know with these weapons and their past lives so it is all a bit of detective work and getting assistance from the knowledge base here.

Then it just may be one of those mongrel rifles that never shoot worth a hoop as was attested by my mates brand new Rem 700 22/250 went back to Remington 3 times and still would not group worth a split pea so they swapped it out for him as he was a professional roo shooter it impinged greatly on his enterprise.

Hope you get to the bottom of it 22sqn then can start slinging lead and having a bit of fun.