Absolutely, when guns come from the factory many of them have excess tool marks and burrs, polishing reduces the amount of friction between two mating surfaces, it also helps to polish the breech face which reduces the friction when the back of the cartridge comes into contact with the breech face. Same for polishing the feed ramp and inside of the chamber. The hammer and trigger pivot points can also be polished along with the trigger bar where it rubs against the frame and the channel in the frame where it rides. The holes in the hammer or trigger pivot points can be polished by rolling the sandpaper into a small tube inserting it into the hole and rotating the hammer or trigger a couple of times along with the pivot pins. You will also have to cut the sandpaper into small strips or pieces depending on what you are polishing. The key word is "polish" as the goal is not to remove metal, this can be done by starting to use 400 grit wet or dry sandpaper then 600 grit to 1500 grit to 2000 grit. Flat surfaces can be done by folding the sandpaper over a steel machinists ruler or flat needle file. It is best to do this by hand and not use any type of power tool such as a Dremel, which will definitely remove metal. Some guns do not need any polishing at all, typically the more expensive models, one of the reasons that they cost more is because they go this extra step. Polishing the bearing surfaces also smooths out the action giving a better trigger pull while not reducing the spring tension or factory settings. In a sense you are "breaking in the gun" without having to put a couple of thousand rounds through it. You can tell which areas that should be polished by looking at all the shiny spots where there is metal to metal contact. I wouldn't recommend polishing any of the critical points such as the sear surfaces, hammer to sear contact points etc. this is done with special "jigs" that do not change critical angles where it comes into contact with the hammer or trigger. All firearms are different so it is important to study and fully understand the "mechanics" of any particular gun before attempting to do any of this, also know your own limitations. I've just given you a general idea of what this involves. Revolvers also benefit from this process.