Just got to looking at my grandfather's 870 wingmaster, it needs a good cleaning and going over but for some damned reason the mag tube is gummy.

So pulling the cap off there is not a retention cap but a flat plug.  
Any ideas oh wise sages how to get the damned thing off to clean it, or should I just take it to a gunsmith and while there have a new barrel put on?

The factory retaining plug was a cup-shaped split bushing with a big hole in the center.
You use a screwdriver in the center hole to gently work the cup out of the mag tube.

A flat plug is not something I've ever seen in an 870.

Take it to a gunsmith to have whatever it is extracted and buy a new steel factory cup retainer.

Remington standard barrels are total drop in, no fitting needed.  Just buy the correct barrel and install it.
Make sure to buy a compatible barrel.  
The Wingmaster and Police have a spring loaded plunger in the face of the barrel support ring.
The Express models don't have the plunger, but some Express barrels may have the hole for a plunger.

You need the correct barrel for the version gun you have or the magazine cap will unscrew itself.

Clean it buy a new retainer plug good to go.  Look up parts list on Numrich Guns parts. Thats all you need.  Also clean the whole gun , barrel , trigger group , bolt and magazine tube.  Just look at the
parts list and any normal halfway mechanical person should be able to do this.  If not take it to a gunsmith or a gun buddy that knows how to take it apart.  Godd luck

 

Just going to clean the old gal up really good and keep her as Gramps had her.
Bought a new shotgun for HD.

Trying to find info on recrimping 3.5 shells that I ended up picking up by mistake.

Re Crimping 3.5” shells? I have to ask why they’re open? It’s a simple crimp bushing on the press.

Not 100% sure about 12 ga, but my 20 ga has the flat plug.  It's held in by tension and as already mentioned, just use a screwdriver to lever it up a little at a time until it comes all the way out.  Remember to watch out for the spring as it's under tension.  Then when I put it back in I just turn the screwdriver around and use the back of the handle to push it back down.

Guessing he plans to take 3.5" shells & "cut them down" to 2 3/4" or 3" size and "recrimp" them.

If that’s the case, it’s not a good idea on many levels, the least of which is the economics. OP, just buy that shells made for your gun. I don’t see any good coming out of trying to modify 3.5” shells.

Yeah, whole lot easier, not to mention safer to trade the 3.5s to a waterfowler for 2.75 or 3 inch. I'd trade 1.5 to 1 short for long factory shells to someone in my area looking to swap, OP should be able to find someone willing to do similar.