I think it was anctually Tony saying .25 wasn't any better than .22
Personally I'm not sure, but the problem with .25 may be that the larger volume of air required to keep the bore pressurised to an extent that the pellet continues to accelerate the pellet might be a limiting factor at 12 FP, and this may counteract the greater surface area the air has to act over (providing more accelerative force). This clearly isn't a problem with PCPs that generally have a lot more air volume at 12 FP. ANother problem is pellets getting too heavy for springers - but I've always had fairly good results with 20 grain H&Ns and 19 grain Rhino's.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.