When they are getting ready to assassinate the German general they are using a M-1 .30 caliber carbine. This weapon should have had a 15 round box magazine loaded into the weapon in front of the trigger guard. There's no magazine in the rifle. Only the single bullet he placed into the chamber. This weapon was NOT a sniper weapon and would not have been accurate at all for this "one shot, one kill" type of mission.
When the guys are at the opera, Tom is clearly wearing an adjustable, pre-tied bow tie when seen from behind.