To answer your question, pure lead will not be good if you are going up to 1200 fps, unless you are using a gas check. In any case you would want to add a little bit of tin to help the mould fill out easily. However, anything more than 2% or so is a waste of expensive tin. Tin is not there to make the bullets harder; antimony is the ingredient that makes for harder bullets. Wheelweights have antimony, at least the older style clip on ones.
So, try a bit of tin, use gas checks and a good lube and see how they work. Good luck.
If you are using a bullet without a gas check, it is another game, and you will probably need a harder bullet and pay more attention to sizing diameter and lube.