this is is correct. When assembled guns were test fired for accuracy, the best were set aside for installing scopes. The Soviets, the Germans and the BritishFrom what I was told the Soviets did not use armors to make their sniper rifles. They started in the factories with barrels which appeared to be top quailty. Then they made the rifles and twicked them to accuracy. Izzy and Tula were the ones making them with the large majority being made by Izzy.Commonwealth Nations did this. Remington was the oddball in selecting certain barrels and then assembling them onto the receivers. It still had to meet certain specifications of accuracy to be accepted as a sniper rifle.
Lyudmila Pavlichenko mentions some of the things she did to improve her M91/30. You can read about it in her autobiography, Lady Death.
There is also a translation by Maj. James Gebhardt on Sovietsniper rifles that covers what was done to improve the M91/30. Suggest you get that too.
I would look at what the Finns did to accurize a MN but rememeber, it it shoots poorly to begin with, don't expect exceptional results. Start with a good shooter.