.45 Colt is noted for having a wide variety of chamber dimensions. When shot in a gun with a large chamber, the cases can expand down near the case head, beyond where many carbide sizing dies touch the brass.
That should be OK if the brass is going back into the same gun after it is reloaded. But, it can cause problems if it goes instead to another gun with significantly tighter chambers.
So, if you are shooting your loads in something with tight chambers (perhaps a Ruger) and you give some reloads to a friend who is shooting them in a gun with a much looser chamber (perhaps one of the Colts or a lever gun), HE may not find any problem shooting them. But, when you get your brass back from him and reload it, you could find that you can't shoot it in YOUR gun, while he can still use it in his.
Some dies are better than others at sizing the bulges in the bases of .45 cases. There was an article in a recent edition of Handloader Magazine that talked about this. One of the dies that I remember was supposed to do a good job fixing the .45 Colt bulge problem was the Lee carbide die. One of the reasons that the bulge is hard to get sized is that many die manufacturers have started recessing their carbide rings so that they cannot contact the press and be cracked. And, dies that are "optimized" for progressive reloading presses also have more taper in the carbide section's mouth tho aid in rapid feeding of cases in the progressive machines.