Reposting Luger solution.
To beat Luger challenge:
Aim near the top of the second box from the top.
OOOOXO
OOMOXO*
OOXOXO
OXXOXO
If successful, the green box should roll off onto the next step.
OOOOOO
OOOOOO
OMXXXO
OXXXXO
And then it should roll off and stop at the bottom.
OOOOOO
OOOOOO
OOXXXO
MXXXXO
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I should be able to help with all Gun Game challenges on expert.
Notes:
I. I've unlocked all Bigger Clips, Recoil Reduction, and other Gun Upgrades, so what I do can only approximate what you may have. (Low to no bullet spread has its problems..)
II. As far as I can tell, the version on Armor Games has 5 less boxes in Speed Challenge than versions on other sites.