QUOTE: "...I have 2 questions that the knowledge base here can help with....
1) Why would powder build up of the revolver be different than a semi? Is it the fact that the chamber is detached from the barrel and in a semi that build up occurs inside the barrel?
2) In the store, the sales person also matched the GP side by side with a MKIII ( also a 5.5" barrel). All things equal, should the revolver have greater accuracy due to the fact that the MK's barrel has the chamber included in that 5.5" where as the revolver is all barrel ( beyond forcing cone)?..."
Long-time, competitive pistol shooter (primarily Bullseye) Gil Hebard wrote an article for the 1961 issue of Gun Digest titled "Selecting A .22 Target Gun", the information of which still holds truths relevant today. Mr. Hebard opined:
"What is the best gun for competitive .22 target shooting? I am asked this question often. It is a difficult question to answer, and one that usually starts a hot argument among serious handgunners. I have my personal opinion, and so has everyone else. Put this question to a dozen reputable gun dealers or a dozen top shooters and you'll get a dozen different answers! Confusing? Definitely so! Add the copious and inexpert advice most beginners receive in the matter of gun selection (and this was before the internet!) and it's surprising indeed if they end up with a gun that is well suited to them.
"There are today (keep in mind this article was written over a half century ago) over 173 different models or variations (domestic and foreign) of .22 auto pistols, single-shots and revolvers available to American handgunners, the requirements for a target gun worth its salt in hot competition rule out perhaps 85% of these, so the problem of wise selection is somewhat simplified at the start. In the remaining 15% are to be found the real tools of the targetmen, and these I'll discuss.
"What are those guns in this 15% bracket which meet the requirements of the competitive .22 shooter? Here they are: Ruger Mark I, Hi-Standard Trophy, H-S Citation, H-S Tournament, Smith & Wesson Model 41, S & W Model 46, Colt Match Target, Browning Medalist, the custom automatics (modified factory models) as built by Clark, Shockey' Dinan, Chow, Giles, etc., and the foreign Hammerli-Walther and Beretta Olympic.
"Note that no revolvers are listed. This is not because there are no good revolvers. It is simply because revolvers are not quite as accurate; have a longer hammer fall (hence slower lock time); require cocking for each shot (handicapping timed and rapid-fire); (and) have a tendency toward leading which impairs accuracy...Practically all our best shooters have tried .22 revolvers at one time or another in their continual search for perfection, and all have returned to the semi-auto if for no other reason than they can score better with it..."
Though this article doesn't fully address or answer your questions (its primary focus was on Bullseye competition, the only real competitve shooting endeavor at that time) and whether you agree with Mr. Hebard or not, I found it interesting and at least partly applicable for today's shooters.