Thanks again, Ken...that part is "hidden" from view when looking at a fully assembled 444. The 444 case fits nicely in the 3030 tube, but I haven't assembled it to check feeding. At least the tubes aren't all that expensive to buy.
Any other "kinks" you can think of doing this swap about?
bteague...the 444 case fits tight in the 3030 and might work but a 444 bolt has a small cutout on the bottom quadrant just like the 45-70 and 450. My 3030 bolt seems to work OK so far without the barrel and mag tube attached, it might also need the rim around the boltface opened up 0.010", the 356 rim measures 0.501" and the 444 measures 0.508" actual measurements, but I will buy 444 bolt just in case as I don't want to mess up the swap-in parts.
I keep having daymares about other caliber barrels using the 356/358 case...243, 260, 7mm-08 or 6mm-7mm Waters or maybe a 25 cal. This might end up like my SA Savage with a barrel for every caliber from 22 to 358. WAY too much time on my hands in winter. ??? ;D
There are pictures of a 450 bolt, 45-70 bolt and modified 3030 bolt on the 444 link now showing the differences. ALL the different bolts have different part numbers so there must be some kind of difference...Marlin uses the same part numbers for parts that are interchangeable for the different calibers and so does Gun Parts Corp.
I think by the time it's all said and done it will end up costing close to half what used 444's are going for right now. It's a fun project but for all intents and purposes it isn't all that utilitarian...it will take about an hour to swap things around and resight and HAVING another rifle to grab and go makes much more sense. But having the bragging rights to a "swap barrel" Marlin is something very few have. Anyone for a 17 or 22-243 Marlin, anyone?? Hahahahaha
I know...WHY ??? ??? :

Hahahahahah(totally sick laugh)
LUCK