>>64266545
To install a barrel on a Mauser 1898 action you need to clamp the entire action into a vise grip and turn the barrel with a wrench that's like 3 feet long. If you want the rifle to actually work and not do things like lock up after firing or explode in your face, you also need to frequently stop and double/triple check headspace with a set of go/no-go gauges.
An experienced gunsmith working in a shop with all the right equipment, with everything more or less dropping into place with no adjustments required might accomplish this in about 30min but realistically we are looking at an hour or so.
A jabroni fucking around on a rooftop probably can't do it at all. If he somehow dragged a vise grip and barrel wrench up there it would probably take about the same timeframe assuming he's skipping the headspace checks.
The idea of this guy just spinning the barrel in by hand or with basic tools is not realistic with that design.