Playing Day of Defeat Source I see a lot of players using smoke grenades to try and sneak past machine guns. While this has slight success, there is a better way.
An MG in a good bunker can be difficult to throw a grenade into for a kill. However, it seems many players dont realize how a near-miss grenade really shakes up the MG. The Source engine violently shakes and blurs the gunner's view. So for a second or two, the MG is unable to effectively see and aim. This provides a great oppertunity to get a clear shot on the MG.
Just time your attack to take place the moment after the nade goes off and you will greatly increase your chance for success. Good luck!