After shooting it, I'm actually glad mine is in .44-40. I didn't like having to source supplies for producing the ammo, but shooting it just felt "different" than shooting .45 Colt--in a good way. My Ideal 42498 mold casts fairly large, so these actually get a little sizing when run through my .429 lubrisizer die. Accuracy with this bullet over a card wad and 40 grains FFF Olde Eynsford in semi-balloon head cases was excellent, and I found that hitting 6 inch steel plates from a standing position at 100 yards was quite easily managed--very few misses with this one. And 35 gr in modern solid head cases, even without a wad was also an accurate load.
I brought along my Miroku '73 in .45 Colt that day as well, but all I had loaded for it were PRS Big LubeTM bullets over 35 gr FFF Olde "E". This load proved the rule that these bullets just aren't designed for long range accuracy. Misses at 100 yards far outnumbered hits. I had also loaded up some PRS Mav. Dutchman over 35 gr FFF Olde "E" for the '66, and these also proved to have trouble hitting the steel at 100 yards. Closer in, I had no complaints, but I did determine that Olde "E" does a good job of avoiding foul-out, even with the smaller lube grooves of the Ideal 42498. I fired over 50 rounds with the older bullets that day, and the last was as accurate as the first. I'll be saving these big lube bullets for when/if I can no longer find Olde "E".
Since I didn't have any long range loads on hand for the .45 Colt, my developed preference the the .44-40 '66 was probably somewhat subjective. But that was a happy accident for me, as it kept me from being irritated that I'd bought the '66 before Miroku came out with the .45 Colt version.