You can certainly load up jacketed bullets, proper size would be .001" over groove. Here comes the big crapper on the 44-40, the pressure needed to drive the bullets to a velocity where they will expand is higher than you should be going with the .44-40 so all you will do is limit yourself to a .430" hole and more expensive bullets. You are well ahead to use a middlin' hardness lead 10-11 and take the bit of expansion you will get from that rather than jack the pressure up enough to get the speed you need to expand the jacketed .44 cal bullets.