i have a friend interested in shooting 20ga for shotgun matches. he had heard somewhere that you could use a mag extension for a 12ga on a 20 and it would work fine. Any of you heard anything like that??
stinger out

Nope, won't work.  

Wilson sells them for the 870 20ga and Benelli for the Nova and Super 90 20ga.

the wilson site says that the extension is for either 12 or 20. so is it safe to assume that a longer extension for 12ga will work for 20?

I suspect Wilson's actually mean their extensions are AVAILABLE to fit either.

Call 'em and ask.

The problem is: Remington lists totally different magazine parts for the 12 and 20 gage.
As I recall, the 20 gage mag tube AND IT'S THREADS are smaller than the 12 gage parts.  This means, that the 12 gage sized and threaded extension won't screw onto the smaller tube.

In addition, the larger 12 gage tube would leave a "step" where it attach's to the 20 gage tube.  This means that the shells would catch on the reduced diameter "step" and fail to feed.

In addition to Wilson's, Choate makes a 20 gage-specific extension kit.  Their kit comes complete with the extension, spring, clamp assembly, and a better follower.

There is a gentleman who offers good discounts on Choate products, which is cheaper than you can usually find, and much cheaper than Wilson's kit which doesn't have the clamp assembly.
The clamp should be used.  
The tubes and threads on the gun are thin, and a good bump will either bend the extension, or actually knock it off the tube.  
The extension greatly reinforces the assembly.

To contact him, email him here:

magazine extensions
Eldon King
[email protected]

Search complete. Found some at tanksrifleshop.com
7 or 8 round extensions for 1100. $43 plus shipping.