How to Put a Scope on The Mosin (Quick Guide) - Escape From Tarkov
Items Needed: Kochetov Mount + PU 3.5 riflescope
To put all other scopes including Thermals refer to Part 2 here • How to Put All Scopes and Thermals on Mosi...
Make sure your Mosin is Bolt Action Sniper Rifle Not Infantry Carbine
Take note, there's an item called Kochetov Mount already with the scope.