Best Webley Rifle made?

[Geezer]

Is it because the piston travels the wrong way?

That and a long transfer port that forces the air to do two right-angle turns before it gets to the pellet.

In a pistol, those aren't such big issues, and they allow an appealing compactness compared to the more conventional layout. In a rifle, the performance limitations if the design are more apparent.

And I've never been sure about the quick-change barrel, from an accuracy point of view.

Don't get me wrong, they are fascinating, charming, important rifles. But as a shooting proposition, the LJ BSA and similar knock them for six on both power and accuracy. Which is probably a factor in why Webley replaced the MkII with the BSA (via Diana 45) derived MkIII.