I use mostly original Lehigh NFPJ conversions in all of mine but i also have a Bestill tungsten/cabide bushing plug made especially for my Super DISC. Its custom fit to that rifle but i imagine it would fit my 54-120 also as long as i use a Fed209. A Win209 is pretty tight in it already and Bestills plug was made to be a tiny bit longer.
I would love to have a Extreme based 45cal brush gun setup to shoot mostly heavy conicals. 1-24 or faster twist.