Ken Waters did the wildcat larger bore chambering on a .303 a long time ago - in .338, .358 and .375, I believe.
: Everyone interested in Wildcats should buy the big book on Wildcats", available form Wolf Publishing called "Wildat Cartridges "Combo Edition".
: Dave Kiff is the ultimate for 'odd' chambering reamers. His prices are perhaps the best as well.
: I believe chamberings on the #3 should be kept to rounds you won't be running over about 50,000psi or under even with the straight sided "improved" ones. This might eliminate normal loadings in the .375 JDJ. It developes greater velocities than the .375 Winchester, a 50,000PSI round. Many of these JDJ wildcats were run up around 50,000CUP, which is closer to 55,000psi and 60,000psi depending on the ctg. imho.
; If built on a #4 or #5 in good condition, pressure isn't a problem as everything form .223's to .308 Winchester work good on them. The .308 (and it's offspring) are 60,000psi+ ctg's. although normally listed as around 55,000 CUP.
; If I were going to do this, and I might someday with my 'spare' #3, I'd go with the .444 Marlin case as the base and probably in a .358 or .375 chambering. For chamberings in .44cal or larger, I'd use a .45/70, .350 Rem mag or .45/90's 2.4"straight case. These are easily usable from 8mm on up to 45 cal.
; A SMLE in .45/90 is no slouch - it will drive a 500gr. bullet at 2,000fps in a #3 and 2,150fps in a #4, if throated properly.