Exactly as Quiet said. The Title II AOW classification doesn't have anything to do with action type. The distinction there between a Title II AOW and a Title I "Other Firearm" hinges on keeping the overall length above 26". If it is less than 26" OAL, then it is considered a "concealable" weapon, and therefore falls into the smoothbore handgun definition of AOWs.
Now, this next point ABSOLUTELY HINGES on the firearm having been made as an "Other Firearm" -- without a stock. If it had a stock from the factory, and you swap on a barrel less than 18", that's a Title II Short Barreled Shotgun (NOT an AOW).
But, if it somehow came from the factory as an "Other Firearm" you can put any barrel you want on it so long as the total package in firing condition is 26" long or more.
...
Having said all of that. What we're not discussing yet is the question of Title II "Destructive Devices." One of the big reasons, it seems, that manufacturers generally don't sell semi-auto PGOs is that they're already sort of pushing the limits of what the ATF will allow as a firearm over .50 cal that is exempted from DD classification. Most 12 ga. shotguns are, by default. But there's no guarantee ANY shotgun would be. So far, the ATF has allowed the exemption to stand for pump-action PGOs. If a manufacturer started making semi-auto PGOs, they may quickly discover that the ATF declares their product to be a Title II DD, and they, at the very least, have to recall all of them and have them destroyed. (Or, like the SPAS-12s, etc., the owners are required to register them.)
I believe these discussions have already taken place between the manufacturers and the BATFE, back when they started putting pump-action PGOs on the market.