Every gun is an individual. Some shoot specials better than magnums, some the other way around. The jump to the forcing cone CAN be blamed for a lack of accuracy in a magnum, but it ain't always so!
Any gun, when fired, will gum up eventually. There will be a fouling ring from shooting specials in a magnum cylinder, just as there will be a fouling ring from shooting magnums. Shooting magnums after specials may be harder, but the same amount of cleaning will put the gun right, no matter what was shot that day. And while I have heard of chamber erosion, I've never seen any, except with blackpowder guns that weren't quickly/properly cleaned. At the low pressures involved, I'd guess it would take many many thousands of rounds to cause erosion in a cylinder.
Papajohn