For a 1 in 48 twist, which is most likely what you have, you should look at conicals up to around 400 grains (for a 50 cal) and a max length of around .750". This is not a hard rule but gives the approximate optimum for that twist. As is often seen, every gun has it's likes and dislikes and yours will have a personality all it's own. With that said, get some bullets and go to the range. You'll get to know your gun better and get better with your gun, like any relationship it only gets better the more quality time you put into it. And don't forget to be safe and have fun.

BTW, I believe the K in your serial number indicates the barrel was part of a kit. Did you build the rifle yourself or did you purchace it from someone?