Yes Bismuth is lighter than lead , most shot bars drop light any way so it will be even lighter with Biz . You could try using the next up size shot bar but I suspect it will still dop light with Biz I always weight all my Nontox and most of my lead hunting loads .

If you have niceshot I have a tested 16ga 2 9/16" 7/8oz low pressure load if anyone is interested . I think I have posted it here before .