Probably the easiest thing to do is buy the gun with a 16" barrel, then send in the SBR paperwork. When it comes back, you can order the short barrel and it can be sent directly to you.
If there are any gas system differences, they should be included with the short barrel... as I understand it, each barrel comes with its own gas block installed.
If you order it built from the factory as an SBR (I have never done this, so don't know for sure), I believe your Class III dealer will have to order it, then when it gets to the dealer, you will have to send off the paperwork and then have it sit at the dealer till the paperwork comes back.
One period of waiting with no rifle at all beats two as far as I'm concerned, so I'd take the former approach. Also keep in mind that if you do it this way you will have to have the reciever sent off to have your information engraved, but the turn around on that should be a week, or two tops.
If there are any gas system differences, they should be included with the short barrel... as I understand it, each barrel comes with its own gas block installed.
If you order it built from the factory as an SBR (I have never done this, so don't know for sure), I believe your Class III dealer will have to order it, then when it gets to the dealer, you will have to send off the paperwork and then have it sit at the dealer till the paperwork comes back.
One period of waiting with no rifle at all beats two as far as I'm concerned, so I'd take the former approach. Also keep in mind that if you do it this way you will have to have the reciever sent off to have your information engraved, but the turn around on that should be a week, or two tops.