The five shot tube has really been holding me back in 3 gun matches (not that I'm all that great with the rifle or pistol). I'd like to get a Saiga, but until I can afford to buy one, maybe a longer barrel and magazine might be in order. Could I install a 590 barrel and magazine on my 500A? Is there another combination that would afford me higher capacity that wouldn't cost an arm and a leg?

590 barrels are threaded (i thinkwell either way they attach different)

you could get a 18.5" 500 barrel that will screw right in they are like $85 direct from mossy i've seen them as low as 75 elseware

my 500 and 590

I'm pretty sure Brownells sells a barrel/shot tube combo for the Mossy 500 with a longer shot tube.

oh yeah i've seen those around too

Yes you can.  The recievers on the 500 and 590 are the same.  
You can get a 590 magazine tube and barrel assembly and put it on a 500.  The 20" barrel with an 8 round magazine tube would let you get a total of 9 rounds.  If you add a Vang extension you could get 10 rounds int he shotgun.
You can also get the Brownell's 500 8 round barrel and mag tube assembly.
The magazine tube assemblies screw into the receiver, they are not brazed on.  It isn't a hard process.


No they are not.
Some of the 500 barrels have the same mag tube arrangement as the 590.  

I believe he already has the 18.5" barrel on the 500, which is why he has the 5 round mag tube.