drilling into sprockets

[greencactus3]

we have these HEAVY sprockets we would like to drill holes into to lose weight. they are the black ones. im assuming hardened steel. we've tried carbide bits, cobalt bits, and regular bits and each one burns up with one hole or less. we have been applying oil generously but they still die on us. what are we doing wrong? we drilled lots of holes in the same sprocket with one bit with no problems before. and we barely had to use oil then.. we arent sure which bit we used back then but we know it was a 1/2 in bit. same as what we are using now. why can we not drill a 1/2 in hole into our sprockets without killing them? or we know we can take off the hubs. can a bandsaw cut them off? (we are not allowed to use a lathe... this is for OCCRA. which lathes are not allowed.)
well, any other way we can drop weight from these sprockets? (we are not allowed to use smaller sprockets/chains.)