Reply To: Varying pellet speed and accuracy.
Varying the pellet speeds can affect accuracy but I doubt the difference would be that noticeable at such close range. I would normally expect to notice a difference in accuracy caused by over or under-stabilization in the form of spiraling further down range.
If I was struggling to group even at short distances like that, I would start by cleaning the barrel. Then I would try a bunch of different pellets as not all guns group well with all pellets.
As somebody already suggested, it could be clipping too, so check the baffles on the moderator (or try it without it).
Also, with it being a 177, the wind could be playing a part in widening groups, even at that range. Have you tried it at an indoor range?
If the gun has never grouped well with any pellet, it could be an issue with the barrel. Have you checked for damage?