This is the average time I managed with using only one weapon :

• Magnum ~1:25
• Shotgun ~2:16
• Handgun ~1:43

Before Ver 1.1.5 the shotgun and magnum were pretty tight in terms of time, they did give Bosol more hp and the magnum always has a 100% chance to deal max damage + probably deals more dps than the shotgun anyway.

I then did a final test where I tried switching weapons, and ended with a time of 1:41.

Overall, magnum rocks !