1. barrel, not "barrow"
2. barrel length those have importance beyond hopup, namely the maximum usable air volume from the piston
What you're talking about applies mostly to crappy spring "shotguns" that only fire 1 BB; TM (and clones) and most gas shotguns fire 3 or more BBs per shot, making that the biggest difference between a shotgun and other guns.
Would it be possible to upgrade a spring, single BB-firing shotgun to perform like a bolt action rifle? Probably. But seeing how you're starting with a lower end shotgun to begin with (exception perhaps to the G&P/Maruzen ones) it would require a lot of parts and work. You'd maybe be able to match some of the lower end bolt action, but high end sniper rifles are hard to match even for other (lower quality) bolt actions.