So I have noticed something interesting that didn't make sense to me. I load both 460 mag and 45 colt with H110 and IMR 4227 and have noticed that only my 460 loads with H110 have any kind of significant muzzle and cylinder gap flash and boy is it significant! My 460 loads are 300gr LCSWCGC with 42.5gr H110 or 38gr IMR 4227 in an 8 3/8" barrel. The IMR load doesn't flash at all which I find interesting because its not much slower than H110 but I think its a single base powder which may have a difference. My 45 colt loads are a 300gr LCSWCGC with 22gr of either H110 or IMR 4227 in a 5.5" barrel and neither of them have any noticeable flash.

What causes H110 to flash in one gun but not the other considering that powder charges are roughly similar for each caliber?

IMR means Improved Military Rifle powder.    It is flash suppressed.    WW296/H110 is not.

Dont know about the 45 Colt loads.  For a sporting gun, flash doesnt really matter unless it bothers you.

I don't have an answer to your question just a observation of my own. Years ago I shot a lot of hot .44 mag loads using H110 & a 240 gr bullet. I do not remember the charge weight but it was max from a Speer manual. I had shortened a 7.5 Super Blackhawk to 5.5 inches and I had a impressive muzzle & cylinder gap flash. The gap flash was at least afoot in diameter when firing in low light. I attracted a lot of attention when ever I went shooting. Also I had a significant amount of unburned power in the barrel. Not any helpful info for you....you just made me take a trip down memory lane.

Not yet. Both are in great mechanical shape so I figure no more than .006

It doesn't really bother me but the interesting thing is the flash occurred in the longer barreled gun not the shorter one and the only combo resulting in significant unburnt powder was IMR 4227 in the 5.5" balckhawk. I ended up with a bunch of powder grains that look like hollow tubes  left over.

In my experience....revolvers and H-110/w-296.... is always a lot of muzzle flash



....and velocity