well I don't know the exact requirement for it, so here are my matches I hit today with the MR6,

Pistol Marksman 19-10.jpg
Map: Redwood
score: 19-10
witnesses: None
1/5 maybe?

bertus pistol marksman 2. 18-8.jpg
Map: Exodus
score: 18-8
Wittnesses: None
2/5 maybe?

bertus pistol marksman 3. 18-6.jpg
Map: Exodus
score: 18-6
witnesses: None
3/5 maybe

bertus pistol marksman 4 16-6.jpg
map: Havov
score 16-5
Witnesses: None
4/5 maybe

bertus pistol marksman 5, 20-5.jpg
Map: Red Wood
score 20-5
Witnesses None
5/5 or 1/5