Straight off the desks of the New Canterlot Republic: Office of Weapons and Logistics. Here is a much annotated design draft for an earth pony compatible sniper rifle battle saddle, along with the submission brief.
The pony body is from ~KP-ShadowSquirrel
's excellent pony models. The rifle is somethign I found for free ages back on the net. Made in 3DS max.
See it in action here here!
This is tangentially related to Kkat's excellent fan-fic Fallout: Equestria, which has neither the NCR, or the OWL, or even such a battle saddle...
I totally blame
for putting this idea into my head!