To folks unfamiliar with firearms, and who just don't know any better, it is believable that a semi-automatic rifle that looks like a military weapon would function in the same way.
This.
A lot of people think that the AR-15 is the best, most powerful weapon available since our military uses them. They're surprised to find out it's a beefed up .22.
But propaganda needn't be hampered by inconvenient facts in a world where people don't research things for themselves... not that we're immune to that either..