My mosin is good for 3MOA.
With the original ladder sight.
Or, with the mojo rear sight I have on it.
Those are sturdy sights.
The only time it starts wandering its zero is when it is hot from 20+ rounds fired rapidly (about 1 shot every 5 seconds) and a heat mirage develops along the barrel. If I let it cool for 10-15 minutes, it settles right back down to its original cold bore zero.
However... every time I take it out of the safe for a range session, the zero is intact. No sight adjustment necessary. The front and rear sights keep their orientation to the bore despite heating/cooling cycles.
The thing that everyone needs to remember here when bashing this product is that these rifles are 70+ years old and are surplus rifles for around $100 at that. So why is everyone so worried about them getting heated up and loosing accuracy? If you want a good rifle buy one, if you want something cool to plink around with and cheap to buy and shoot that is fairly accurate then get a mosin.
My theory is that because this metal strap involved is so malleable, it will also stretch under the tension of acting as a strap (if you can bend it, then you can stretch it) and this, coupled with heat and abrupt recoil, will shift the scope.
It has nothing to do with being a 70 year old peasant-conscript's rifle, as noted by Cowboyup. I love the mosin. There isn't a better .30 caliber rifle out there for less than $100. Or even $200.
Read the critique more attentively, Cowboyup. The mosin is fine. There are good ways to scope a mosin, even.
However: This is an aftermarket product that has a distinct weakness, IMO.