Joel, its a neat idea, but at the end of the day, you'll wish you had stayed with conventional (i.e., have the gun ON you) methods.
Here's the main reason why: Even a light shoulder bag is going to get annoying after a while; and you will be shifting it around to stay comfortable. What are the odds that the flag goes up precisely when you have just moved the bag to your other shoulder and now cannot reach the gun, etc., etc.....
That said... I have the small FatBoy, and it goes with me everywhere; as a mini BOB, so to speak. But I don't tote it around like a man-purse.
The MOST important thing I learned as a young grunt was this- You're only as good as what you've got. That means what you have strapped to your ass... NOT in the rucksack you just put down, NOT back at the rally point, NOT stowed behind the co-pilot's seat in the Huey... etc.
Use your imagination. There are many ways that you could be relieved of your gat while carrying it in a shoulder bag, murse, whatever its called.
Let me guess... carrying is cramping your style (and maybe something else ) with the tight jeans thing?????
The FatBoy, even the small one like I have, is too big for a bicycle; unless you've got a pannier rack to strap it to.
Cycling jerseys have perfect spots to stash a J frame in a pocket holster; those big pockets over your kidney, intended to hold snack bars, etc.. Keep your wallet, etc in an extra tool bag under or behind the seat, and you're golden.
Of course, all of that assumes you're talking about a bicycle. If you mean a motorcycle, just strap a horse scabbard on that bad boy and carry a shotgun or lever rifle...