Tap doesnt touch the stock afaik.
Yeah, thought about different pellets. Only use JSBscause they do well in 95% of the guns, incl this one but never tested other pellets.
Its also easiest to remember if you use same pellet in all these
Maybe 4.51 is too small a headsize aswell? I started with this headsize cause my Diana 31 Pro prefers it. Actually never looked back. But most seem to use 4.52 or 4.53