My experience with Mosins suggests that they weren't a slouch when used as a basic battle rifle.
But battle rifle accuracy and sniper rifle accuracy are both different and sometimes diametrically opposed.
Battle rifle Massed Musketry fire and maneuver doctrine depends upon natural dispersion to blanket a targeted area with randomly impacting fire. That randomness is what makes fleeing such fire into such a terrifying crap shoot. It's what you use when you can't scare up a genuine Sniper.
But Sniper Riflery doctrine cannot tolerate such dispersion.
My experiments with a modernized version of corking brought dispersion well below average in my three Mosins.
As for the video, I am not at all surprised. Those sight mounts which replace the rear sight blade are horrifically unstable and flimsy.
I grasped an obscure Internet post about taking off the entire sight, base and all, and replacing it with a pair of high aspect air rifle rings clamped onto the barrel's sight mount grooves. That produced a rock solid scope mount.
Then, the customary cheapo Chinese Scout scopes had abysmal clarity. I replaced mine with a high quality 'Trophy' handgun scope. It still lacked rifle quality magnification, but conversely provided a reliable sight adjustment capacity, so the trade was a fair one.
Don't let anyone tell you that a Mosin can't be adequately and cheaply modified to respectable accuracy.
The only honest joker in the deck is bore wear luck of the draw. You could cure that by cutting off some of the barrel, but noise and flash become intrusive. A heavier duty bloop tube could help with that.
Most Mosin ills are consequent to a near-century's worth of softwood stock shrinkage. I found that corking and trigger housing shims were the most logical and effective remedy for that, including rim lock issues.
Handloads that worked for me used the Hornady '303 caliber' 150gr Interlock, PPU brass, IMR-4064, and mildly increased 308 load recipes (start low, etc...), I forget how much 4064. My SIL has been whacking between a quarter to a half dozen deer a year with that load in one of my former three Mosins. I gave him two hundred rounds of my handload along with the rifle.
My remaining Mosin has the above mount, scope, and an Archangel stock.
The ten round Archangel magazine stays in place (just like the Enfield), and I can load better with two strippers than by changing out mags when one considers that one has to load those magazines to begin with. That's why I stuck with the Scout Scope mounting system; some compromises make sense from a practical point of view.
I consider it my perimeter defense, and scare 'em off with the bang, rifle. If I wanna bag 'em; they ain't gettin' away. But sheesh, does Mamushka pack a kick.
But:
This guy is the
real Mosin expert.
Greg