I spent some time checking the Algore-web trying to find concrete info on XM193, and if it really is a hotter load than M193. I found one reference that said that XM193 is really M193 that the military rejected. I'm interested because this is what I keep in my 16", 1/9 twist M&P15T, and I'd like to know if this is the best FMJ round given my barrel length and twist. I also read that with the 55 gr. M/XM193 rounds, the twist rate really isn't that important, it's only when heavier bullets are used that the twist rate becomes important. I did find some good info discussing fragmentation of M193, and that velocities need to be above 2700 fps for fragmentation to realiably occur. From what I read, XM193 does about 3075 fps. from a 16" barrel, and keeps above 2700 fps out to about 150-200 yards. So my questions are: -Is X/M193 (55 gr.) the proper bullet weight ammo for my rifle (vrs. heavier 62 & 77 gr. rounds) given the twist rate and barrel length of my rifle? -What is the real difference between XM193 and M193 ammo? -Does barrel twist rate effect velocity to an appreciable degree? I hunted for answers to these questions before I posted this, but my google-fu was weak today!