Factual error: As Walker and Atwood are falling from the building in 1929 New York City, the reflection of Vancouver's Harbor Centre is visible in windows. Furthermore, the Harbor Centre was completed in 1977, so this is a double goof. (00:22:45)
Factual error: We're told they carbon dated the gold to determine its age. Problem is, the gold hasn't aged by the normal processs - as it's been brought forward in time it will appear to be extremely new. As such carbon dating wouldn't work. Also, carbon dating only works on things that were alive and absorbed carbon-14; gold, having never been alive, could not be dated this way.
Factual error: In 1929 New York, the license plates on the cars are wrong. In 1929 they used an A1-234 pattern and these are 123-456 patterns. 1921 used the 123-456 pattern, but they were white on black plates, not the black on yellow we see.
Answer: I'm pretty sure it was mp5 with muzzle brake.