I have tons of 9mm lead bullets since that was all I was reloading. When I started reloading 38 special cartridges, I wondered if I could use the 9mm bullets that I already had or if I should go out and buy 38 bullets. I found several blogs discussing the merits mostly saying that 9mm bullets would not be as accurate coming out of my S&W Model 10 as 38 bullets would be. But what is "not as accurate"? To answer this question, I loaded up some 38 special cartridges with both 125gn RN 9mm bullets sizing in at .356 and 125gn RN 38 bullets at .358. Off to the range I went. I shot three groups of six each (18 per group, 36 in total) at six different targets from a distance of seven yards. Back at home I took each of the six targets and calculated the Average Group Radius (find the group center, measure the distance from center of group to center of each shot, then average the distances). Then I averaged the three 9mm (.356) groups and the three 38 special (.358) groups and found the difference. And presto! I now know how much more accurate the 38 bullets (.358) are over the 9mm bullets (.356) - less than a quarter of an inch (<0.25"). That should be less than an inch at 15 yards which is plenty accurate for IDPA. Not anywhere accurate enough if I was trying to shoot 100 yards, but I'm not. From what I read in the other blogs is that gas escaping around the bullet because the bullet does not fill the barrel completely is what causes the smaller .356 bullet to wobble a bit as it comes out making it less accurate. But gas escaping around the bullet also means that some back pressure is lost and the bullet is flying slower, right? IDPA has some strict power floors that must be met, so the next question is, how much slower is the 9mm .356 bullet traveling vs. the 38 .358 bullet??? That's the next thing to find out - back to the range. Disclaimer: I'm not a mathematician or a gun smith.