Hi Tom,
Welcome to the forum. I built my own .30 shooter. I've had a couple of the Interarms guns, neither was very reliable or accurate. If I wanted one off the shelf, I would go with a DWM Alphabet Commercial (AKA "1920 Commercial"). They are available from several on-line dealers as well as Gun Broker. Most will have the standard 3 7/8" barrel, but occasionally a 6" barreled one will show up. Vlim is right, these are also usually cheaper than the Interarms guns.
Or, get an Alphabet Commercial and have a 6" after market barrel installed. We have a couple of members who could do it for you at a reasonable price.
Here's an example at Simpson Ltd. Prices will run from $600 to $1000+, often cheaper at local shops and gunshows.
http://www.simpsonltd.com/product_in...b5fe525a23bd73
Ron