Hello there. I have from uk Neil 4.48 4.51 4.52 and 4.53. Like you I started with the 4.48 as my pro target seemed very pellet fussy. I did my best to accurately digitally measure some JSB head sizes into groups. The 4.48's seemed to group best at the time. So I sized some 4.53's and it just about touched all the heads. This seemed to tighten groups and I thought I had cracked it.
However after a while I tried some 4.53's just weighted and started to get consistent 8mm groups at 45 yards. The conclusion. In my barrels JSB likes a nice leaded barrel to perform best.
I was not used to this as the cz barrel likes to be perfectly clean with mozzies which I find simpler to maintain.
I have been testing sized pellets in my tuned hw98. It likes 4.52's or 4.53's. But in this case sizing makes a huge difference and that is at 25 yards. The 4.53 die is not even touching the head, just resizing the skirt.
In my s200 shooting mozzies, I neither have to size or weigh pellets apart from to weed out the odd freak