Its not at all impossible to make 1,000 yard shots with a Mosin Nagant, but most , indeed, need 'tuneing' to get decent groups past 300 yards. Most M-39's, in 'As issued' can be a good start.
We use a spotting scope, an old school desk, a folding chair and a 55 gallon oil drum and pace off 1,000 0r so yards in 100 yard incremets and have at it. drums are 2 feet across and 4 feet tall , or there about.
We do this in winter, and we use a snowmachine on the frozen river to go back and forth.
At that distance an oil drum in visible and the hits make a deep resounding sound. Takes some prectise and a sheet of paper and pencil can help keep track of your settings.
Some guys I have seen can keep the groups of 5 all on the drum, I get about 1/2..........