The BEST ( Are no longer around ) only two left and the late comer to the game was Tim Hill doing parts and pieces to this day & Cothrin machine making power house "Dump" valves. The Likes of WAR / JSAR who were cutting edge in the innovation of Valves, barrels upgrades, breechs, SSG's etc are no more.
Seems @ 10 years of marauders the Want & Desire of upgrading them has came and went with so much NEW GUN & Manufacturers in recent years.
Many of us still have and work with these rifles, but for those just coming aboard the M-rod train ..... It is a hobo's jump aboard to find any of the really desirable parts and pieces as it left the station a few years ago.
I still do performance work on .177 & .22's with regulation conversion, light hammers ... barrel & trigger work. The .25's not so much.
Scott S