I've never had the chance to try a Donnyfl, but I've tried every Huggett there is. Assuming that the statement made by the OP holds true and they're very similar in performance, I still have never found a silencer that actually outperforms a Weihrauch?! I have one as a sort of benchmark to test others against, and nothing locally made (we have some very good silencer makers here in SA) or from Huggett has ever beaten the old HW either subjectively or in dB measurements. Now they're neither the fattest/biggest volume or the most advanced design out there, so I have no idea what black magic went into them. Of course I'm not talking big bore here, but that said the huggett magna did not outperform the HW on my 30fpe fx crown or my 35fpe red wolf. Given conventional wisdom of volume etc, it really should have shown some advantage.
I generally use other silencers for the reasons others have mentioned - supporting local craftsmen, local service, perceived better build quality and often looks on a certain gun (I hate a tapered silencer that ends up skinnier than the shroud it's attached to, and the HW always ends up looking like that) while providing 'enough' sound suppression. Just find it incredibly strange how far removed airgun silencers and logic/science seem to be sometimes. Makes buying very tough at times! Can only imagine how irritating it must be to design these things and find performance not quite what you hoped and not having any idea why!
