Your last paragraph pretty much sums it up. Grease does work well as a lube because it stays where it is needed and is long lasting. However, many greases get tacky and sticky over time which can actually glue the mating surfaces together. Worse off, the grease will catch and hold all of the contaminates from the environment and from the firing process and make them build up. Oil based lubes suspend and float the contaminates away from the mating surfaces.
I do lube my AR-15 and a few other things with grease. I use superlube grease and only apply it to the high friction areas that are further away from chamber and bolt face. 90% of the time though, I am using corrosionX as the general lube and surface wipe down oil.
I do lube my AR-15 and a few other things with grease. I use superlube grease and only apply it to the high friction areas that are further away from chamber and bolt face. 90% of the time though, I am using corrosionX as the general lube and surface wipe down oil.