MP Megatron isn't as oversized to a real gun as some people are thinking.
A comparison photo was posted up when the toy came out, and it was as close in size to a real handgun as Gen1 Megatron.

I can't seem to find the photo now, but these were some photos I could find with a google search...
Gen1 Megatron next to a real Walther P-38, which looks about 30% bigger than the Gen1 toy.
Gen1 Megatron with Masterpiece Megatron, which only looks to be about 60% bigger than the Gen1 toy.
As such, the real gun would be close to midway between the two Transformers guns.
The wiki page for the real gun says it is 216mm long.
The Gen1 Megatron is 172mm long, and MP Megatron is 280mm long.
That makes it 4cm between Gen1 and the real gun, and 6cm between MP and the real gun.
Not that much difference... and close enough for both Transformers toys to be classed as "replica" to fool someone behind a counter in not questioning if it is real.
(a replica gun is just as dangerous as an unloaded gun - are you going to challenge the offender to prove it is a loaded or lethal weapon?)


BTW, in the second link, four of those Transformers toys are considered replica weapons, as they only have to be close enough to fool people at a glance to count as "replica".