People (including you, Optibotimus) always say that Masterpiece Megatron is too tall to display with other Masterpiece Decepticons. While I will agree that a newer MP Megatron with better engineering and more in scale with the current Masterpiece scale would be nice, in the meantime I always thought he looks just fine with Starscream and Soundwave.
Speaking of scale, you imply the reason the gun was oversized was as a way of "getting away with a gun mode." No, not really. Takara could care less about US child toy safety laws. (Well, at least for toys they design primarily for their own market, such as Masterpieces. They might care a bit more when it comes to regular figures.)
Besides which, size has nothing to do with it. The orange cap is what is needed to make it compliant. The reason his gun mode was oversized was because they designed him with his robot mode's height in mind, so that he would stand approximately the same height as the original Masterpiece Optimus.
(That gives me hope that if we do get a new MP Megatron, maybe his alt mode will be closer to a 1:1 scale!)
Also, the barrel and stock extensions totally
are third party products. Not sure why you seemed confused on that issue.
Really should write my own review of this figure one of these days... Minor problem is, I'm not quite sure what I did with my mace, sword, and gun. But the short version... I completely agree he is not a perfect figure... but I still don't get the vehement hate many people have for him. I still love him!