Speedbird, the two use different systems to hold the mag cap tight. If you put an Express barrel onto a Wingmaster the mag cap will come loose. The Express barrel doesn't have a ball detent (although scott points out that the older models did), it is the detent that holds the Wingmaster mag cap tight. You can have a detent installed by a gunsmith (simple and cheap) or you can buy a sling plate that has a ball detent on it...those are made by Wilson Combat, GG&G and some others. Dlask also has one that works a little differently but very well.
Cheers