I had to email them for the barrel. In the email, just let them know what barrel you want, they'll price it out for you, tell you where to pay, ect. and they'll send it right out. It may take them a few days to respond, especially with the holiday.
Now to answer your other questions...
1) Pretty much nothing....I've modded mine to hold a Faux mag, but it's purely cosmetic.
2) The reason the cylinder wont slide all the way forward when the mag is empty is because the cylinder head is hitting the BB follower, and since it doesn't move out of the mag well, the cylinder wont slide all the way forward. So, pretty much it's the cylinder head hitting something that isn't meant to leave the mag - like a BB does. It is an indicator that the mag is empty, but that's not exactly what it's intended to do. Also, don't force it forward when it does that, because you can break the BB follower off and then your mag is pretty much screwed.
Now to answer your other questions...
1) Pretty much nothing....I've modded mine to hold a Faux mag, but it's purely cosmetic.
2) The reason the cylinder wont slide all the way forward when the mag is empty is because the cylinder head is hitting the BB follower, and since it doesn't move out of the mag well, the cylinder wont slide all the way forward. So, pretty much it's the cylinder head hitting something that isn't meant to leave the mag - like a BB does. It is an indicator that the mag is empty, but that's not exactly what it's intended to do. Also, don't force it forward when it does that, because you can break the BB follower off and then your mag is pretty much screwed.