Re: M21 Scope recommendation
Same rifle here. I choose a USO 1.8-10x37mm Tpal for mine, and Sadlak ABN mount. I don't have the scope yet from USO, so I can't comment yet on that. However the reasons for the choice were this; it is streamline for mounting it low, I was able to get every feature I wanted in a scope from USO, and mainly I wanted the very best US made scope for my US rifle.
My particular M21 I figured would be doing more shooting inside 600 yards. Conservatively speaking the M21 is not the ultimate 1000yd rifle. 1000yds is pushing it. I thought of the 17x USO, but was reminded of them M40s in the Marine corps I drooled over with just their 10x Unertl optics. That and the wide field of view with the low 1.8 power made me think the Scope I have on order might just be perfect for the limits of this rifle while optimising it for close target engagements.