I don't know what the answer or reasons are Dan - I hear you - but feel it's got to be something to do with the Swiss powder. I've never used it, but would like to test it sometime in my .40 and .45 along with the big .69.
Just why your .50 shoots cleaner than and with much more powder than the .58 is a puzzle for sure. Perhaps the breech is part of the answer - maybe not. My Pedersoli Kodiak has cupped patent breeches in it's .58 tubes. It seems to foul not at all with GOEX - from 82gr. 2F GOEX to 120gr. 2f GOEX, the highest I've shot due to regulation problems starting with that load. There is seemingly no 'felt' difference in bore fouling, over that 40gr. charge increase. I use the rifles' 3/8" hickory rod for loading. It does best with 110gr. 2F, where the barrels regulate perfectly shooting exactly parralel at all ranges - neither converging nor diverging and shoot exactly the same elevation as well. It is my most accurate 'trail walk' rifle. Imagine- a DR! The 50th load goes down exactly the same as the first.
I have, over the years, used about 4 other makes of powders and all of them have shown me virtually the same deal - for me, 2F fouls less than 3F - I had always chocked it up to faster, hotter burning thus higher early peak pressures of 3F, which would logically tax the patch/ball seal intergrity more greatly, due to higher pressures exerted on the ball/bore junction, ie; patch seal. Seems this would be especially prevelent in the smaller bores, with their higher working pressures to start with, the trend indicated in Lyman's BP loading book. This goes to the powder weight vs. ball weight that Dan was hinting at. Incredibly, look at how much powder it took to develope even modest pressure in the .58 and .75!!
Cleaner shooting and patch integrity is why Tracy shot only 2f in her .36 Seneca - and used to beat us (Taylor and I) both- o, we didn't shoot at 25 yards- hardly every except for card and string cuts- small stuff - yeah- we could see then. The cleaner burning trait also showed in the larger bores, which is why we then used 2F exclusively in just about everything.
When I got the .45(re-barreled .50) - my first ML since the .69 in .'86, I tried 3F GOEX in it at the range as the local store didn't have 2f and I was out of powder. I found it would print nice 1/2" groups with LHV lube and 75gr. 3F GOEX. I had no fouong problems using a .445" ball and .022" denim patch. It also printed the same groups using 85gr. 2F, later on in testing, with what seemed identical fouling - virtually none. I also found, with a water based lube, I could reduce those powder charges to 50gr. 3F and 60gr. 2F and maintain the prior accuracy, although muzzle velocity was very much lower than before - dropping to the piddly 1,600fps range.
Incidently, where the LHV (slippery) lube demanded relatively huge powder charges & had a very narrow accuracy range of charges it would allow, the water based lube (WWWF/Oil) gave virtually identical accuracy as the slippery lubes(1/2"-3/4" @ 50yds) - accuracy over a very WIDE charge range.
The oil was just not so easy to get long with as it had very narrow range of working well - but - knowing what charge is needed for an oiled patch for hunting, is necessary. The only way to find out what powder granulation and how much of that granulation is needed, is to shoot both and find out what the gun wants.
It is a foolish 'guess' to work up a load for trail walks, sighting, of course for that load, then merely switching to a 'hutning-type charge with an oiled patch for hunting, without first seeing what the gun wants to use with that oiled patch- with the granulation. It could be, it wants X grains of 3F for oil and Y grains of 2f for water lubed patches - only after testing all the varaiables, can an 'accurate' decision be made.