Two major factors are the weight of the bullet itself and the amount and type of powder used, which will change the speed of the bullet. There are other factors like shape of the bullet, length of the case, the crimp of the case to the bullet, etc.

That said, when my groups open up, I tend to look back to the fundamentals before I get too suspicious of the ammo. There’s more to shooting well than just aiming. There’s also stance, grip, breath control, trigger control, and follow-through. Shooting pistols accurately has got to be one of the hardest shooting sports.

For more on the fundamentals, take a look through the U.S. Army Marksmanship Unit Pistol Marksmanship Guide. Use the links on the right side of the page to read the chapters of the manual.