Re: douglas
I have known, worked with and shot with Tim and the other guys at Douglas for many years now and I can attest to the quality of their work and products. Unfortunately, however, MWM is correct re: Douglas not making "finished" or drop-in AR-10 barrels. They will sell you a Premium Air-gauged SS barrel blank, but it would be up to you to provide it to your AR-smith to perform the work necessary to get it running on your AR-10.
Personally, and with absolutely ZERO offense intended toward Douglas and its fine people, I would go a different route for an AR-10 barrel. There are tons of companies that offer high-quality barrels ready to run in the AR-10/LR-308 platform and likely for less money than you will ultimately end up paying for a Douglas plus the work involved to install the receiver extension, chamber, drill for the gas port, etc., etc., etc.
If you'd like some alternatives, I can provide you with a list of different mfgs/retailers that you ought to consider.