6013 is a very good all-purpose, mild steel rod. It is used a lot on thinner metals because it doesn't penetrate very much. 7018 might be a better choice because you're going to need penetration into each side of your pieces. When I used to be a welder when it had to be strong we used 7018, when it just had to be in one piece, 6013. 6013 is a somewhat easier to weld with though. Since you'll probably have to start and stop a lot to keep your temp down, be sure to grind out any slag at the start and stop point. I could be all wrong as I welded pressure vessels, not rifle bolts.
A TIG or MIG would certainly be the best way to go.