Originally Posted by
yocan
There is a lot of speculation that if there are any machine or other marks in the metal of your gun (especially the chamber, and particularly the end of the chamber) that metal casing will hit it and cause it to exacerbate and become worse, while brass will smooth it out, hence why I've been told and repeat to only use brass the first 500 rounds. Especially with guns that aren't as precicesly machined. IE the S&W M&P which occasionally has machine marks inside.
Basically when breaking in a gun, use brass, whatever break in for that gun is. Brass does do less wear and tear, but with the price of ammo, unless your gun is worth a fortune, I"d have no issue using steel. I won't use it in my carry gun though unless I run out of ammo to shoot.