OK, I'm more conscious now.
I suggested the SBS route because you had mentioned wanted something concealed, etc. Like the other said, for practical use you can get a Mossy or Remmy for 200-300. Take it to the range and shoot the crap out of it. Burn in muscle memory (of course, you want to do that with whatever you buy)
Now it seems as if you need to submit paperwork to buy shotguns?
I'm sorry, I wasn't really conscious when I posted earlier, and didn't elaborate very well. The paperwork/etc that I was rambling about earlier is only in regards the Short Barreled Shotguns (SBS) Or the Serbu that you posted earlier, which is technically an AOW. These are NFA items, and require compliance with the National Firearms Act of 1934. Basically, anything extraordinary - machine guns, short barreled rifles (<16") short barreled Shotguns (<18"), silencers, etc. - requires filling out a Form 4, getting certification from the Chief Law Enforcement Officer (CLEO) in your jurisdiction, getting fingerprinted, sending it and $200 to the Alphabet Police, waiting a couple of months, then finally getting approval. The Serbu above is technically an AOW - any other weapon. Under NFAclassification, this means pretty much anything else not covered (pen guns, cane guns, etc.) as well as shotguns which have a pistol grip only, and never have had a stock attached to it. That's why the transfer fee on that would be $5, as opposed to $200. (It still costs you $200, and the same rigamarole, in order to make an AOW/SBS.)
But yeah, like they all said, get something, and use it, learn it. BA/UU/R. If down the road you still want a shorty, for easier concealment etc., you can always pay the $200 and chop it down.
ETA: Also, before you go too far down that road, you may just want to check and make sure NFA items are legal in your state - you didn't say where you are.