All metal or is there some plastic? If plastic, then disregard what I'm about to tell you. If it is one of the all-metal versions, get a can of cheap non-chorinated brake cleaner spray and a can of any good spray gun lube (I'm partical to Breakfree CLP, but any good lube will work). You'll also need a source of compressed air (you do own a compressor right? If not, canned air works fine).
Go outside and spray the brake cleaner liberally into the mag. Push down the follower while spraying to get it inside good. Let it sit for a few minutes upside down to drain. Then, hit it with the compressed air to get it well dried. Again, push down the follower while blowing to get everything dry. If it is still gummy, then repeat.
Now, give it a LITTLE shot of the lube. It doesn't take a whole lot. Hit it with a blast of compressed air to force the lube down into the magazine and blow off any excess. Your mag is now clean and relubed.