I've built 3 repeater Vartargs now on R700 actions... an ARC Archimedes, a Remington 700 223, and a Defiance Deviant Tactical.
Magazines were the sticking point, and the best mags I've found to use are the Ruger 223 AICS plastic mags, #90562. Their feed lips are differently shaped and longer than the old genuine AI 223 mags, the MDT plastic and metal 223 mags, and the Accurate 223 metal mags; none of those would feed reliably with Vartarg for me, but the Ruger magazine works very well with Vartarg. Load them to the back of the magazine and go. The Archimedes will actually CRF with the stubby little 20VT out of the Ruger mags.
One thing to note, if you feed the bolt very, very slowly or stop the bolt right after the round comes free of the feed lips the little Vartarg can fall down vertically into the gap in the magazine in front of the other rounds in the mag, but run the bolt normally and it never happens. At some point I want to try 3D printing a new follower and a spacer to fill the front of the magazine to totally eliminate that possibility.
I think I paid about $25/ea when I picked up a bunch of the Ruger mags from Brownells. What's funny is while they feed 20VT great they don't feed 223 or 223AI worth a damn in either of my 223AI bolt rifles, lol
For all 3 of those actions the Ruger mags needed to be shortened a bit at the very front of the magazine as they seem to be made tall in that area and hit the action preventing the magazine from inserting high enough and latching in, but if you pull the follower out it takes about 5 minutes with a file to remove 0.050" or so off the front 1/2" of the mag and then they no longer interfere with the action. The feed lips are also pretty thick and the outside of them may hit the side of the action depending on the mag cut in the action; if that happens a round needle file can help make a tiny bit of clearance on the outside of the feed lips for the mags to sit high enough to feed properly.
