I've been through the import process to get the Barrett .416 brass and it's not overly difficult but will take 3-4 weeks to process. You can do it all online and it's free so no money up front BUT it will have to be collected by a registered firearms dealer that you nominate on the B709 forms. The dealer may charge you a fee for this service as they have to go to customs to collect the shipment. If you purchase from the U.S then check if they require an export permit cause that's another $250USD on their side and not many online shops over there are prepared to go through the process. The way I was going to do the brass was to bring in 2000 cases and split the cost with my dealer but after getting all the approvals to import I managed to convince NIOA to import it for me directly from Barrett which worked out a fraction cheaper.
I'd look at alternatives such as the mag used on the Ruger Gun Scout .308 or possibly see if you can get hold of an "Accurate" mag as used by Colt on the 2012CLR. I have 2 of the Accurate mags sitting over at Magnum Sports in Chatswood being sold on consignment, possibly if your over that way have a word to the boys and see if they will fit your Remington. The Rugar and Accurate mags are very similar, in fact the Ruger mag will fit into the Colt but has feed issues every other round so not recommended. If you're not local to Sydney I'll call the shop Monday and see if they still have the mags and can post them to you, if they work then $ changes hands and if not then post em back and we both learn something