CoP, Jupiter Plant Outskirts, administrative building of Jupiter Plant. Stash consists of OTs-14 "Groza" ("Thunderstorm"), two boxes of 9x39 mm ammo and two VOG-25 grenades.
I didn't use Thunderstorm much. It's a lovely assault rifle, compact, lightweight and with integrated grenade launcher. It also can mount a silencer and a scope. The problem is... it uses 9x39 mm subsonic ammo. Sure, it shares ammo with AS Val and VSS Vintorez, but those guns are equally rare. Duty uses them frequently though.
Also, it's a leftie model. Then again, so are most vanilla S.T.A.L.K.E.R. guns.