I'm new a reloading and I made a noob mistake that cost me probably about 600 rounds of 9mm. Here goes.
My first loads were 115gr plated bullets. I got it setup and fired 150 problem free rounds so I proceeded to finish the rest. When I ran out of plated bullets I switched to 125 gr lead bullets. I only changed the powder charge but did not realize that I would have to adjust the OAL of the round! DUMB NOOB!
I found out it was a problem when I dropped a round in the chamber to see how well it sits and guess what, it is too damn long!
Now that I'm loading .45 ACP I have several different test loads with different OAL to see which ones work. I guess you learn from you mistakes.