I’m curious how the Mosin–Nagant bolt-action and the Dragunov semi-automatic rifles are regarded as sniper rifles?
In Soviet Union, rifle snipes you.
A few points:
Common use of “Sniper Rifle” for various rifles used by snipers is a misnomer. Any rifle that a sniper uses is a sniper rifle because he/she is the sniper, not the gun. Just because you use a “sniper rifle” doesn’t increase your skill, abilities or accuracy one bit. If you jerk the trigger, a bullet from your three rounds in a 1/2" "sniper rifle may miss the target completely.
Most kills by snipers aren’t at unusually long range. Just as good hunters don’t take a shot they’re not confident will take down their prey in one shot, a good sniper won’t take a chance on a shot that might miss. A cannonball that misses is just as physically deadly as a BB that misses. I’m far more impressed with Carlos Hathcock’s 700 yard kill that took him three days and nights to travel 1200 yards in enemy land to take, than his long shot. Jump to 8:54 on this video to hear the beginning of his story here https://www.youtube.com/watch?v=UCXOLmcGer0 and the rest of the story here: https://www.youtube.com/watch?v=Shp0vaaAgQ0
Since a bullet fired from a rifle typically travels at more than twice the speed of sound, it can take seconds for the sound report from a rifle to be heard at long range. Mythbusters tested dodging a bullet and proved that even if you moved when you saw the muzzle flash (which travels at the speed of light), it would be near impossible dodge a bullet, which would hit you in at least half the time before you heard the sound from the rifle or anything else (an ejected case, a sneeze, a fart, etc).
A pox on whoever designed the Remington 700. May their children and their children’s children be forever cursed. There has been a continuing occasional problem with unexpected firing, and resultant lawsuits. This is googleable.
I personally experienced this back in about 1968. I was unloading a Remington 700 30.06 by cycling the bolt (stupid, I know) when it fired. The bullet went through a wall, a clothesline pole, a fence and another wall before it lodged in the headboard of a bed in a house most of a block away. The people in that house were not home at the time, fortunately.
Exactly where was your other hand’s fingers? 
A good sniper, one like Carlos Hathcock would be allowed to use any weapon he wanted because he was good at the job. What he preformed best with no matter the engineers ‘proof’ of accuracy. What superior would try to force a sniper of Hathcock’s proven ability to use a weapon he did not like or trust?
LT says, “Here Sargent, take this fancy rifle and go to the roof and be ‘High’ cover.” does not make the Sargent a sniper, just a regular GI sitting on a roof. No better a shot than anyone else in the platoon.
Most soldiers follow orders but they also know how to work around stupid orders when are stuck with a stupid superior.
They try to put them, trained elite soldiers, with the smart and proven superiors.
My experiences, yours may have varied.
It’s less about restricting the type of rifle a sniper uses, than the sniper having fine tuned his rifle to be accurate. Hathcock primarily used a Winchester Model 70 chambered in 30.06. They probably didn’t handload their own cartridges, but certainly would choose select batches of cartridges (which can vary in accuracy by production lot) for their work. In the YouTube video I posted above, Hathcock states that the general he shot after stalking him for 4 days, 3 nights was 700 yards away because 700 yards was one of the distances he sighted his rife to.
His legendary long shot was done with a 50 cal. M2 machine gun in single shot mode with a scope attached. Again, it’s not the gun that makes a sniper/marksman, but the sniper/marksman’s extraordinary skills that enables the gun!
The shooter’s skill is critically important, but also important is the rifle. Together, excellent skill with excellent equipment (including rifle) will tend to produce excellent results.
Even the best marksmen cannot shoot MOA shots with a sub standard firearm.
Remington rifles are exceptional shooters right out of the box in many cases and a lot of that exceptional accuracy is possible because of a “Crisp” trigger.
NEXT
Mr. Duality posts;
Don’t put the blame on Mike Walker for the defective Walker trigger! Mike tried to get his trigger corrected (In 1947) but the Bean Counters and investors of Remington are the ones to blame for that trigger.
Read Jack Belk’s book, “Unsafe By Design”. And watch this video
For game purposes, of course, play balance matters more than verisimilitude. Slow fire rate vs automatic weapons with fast fire rate and lower damage. They make bolt actions do the most damage because “bolt action” is an easy excuse for why it has 1/32 the fire rate of some SMG.
Bolts are more tolerant of ammo quality, temperature, etc. and are less hard on the ammo. The first is important if you have to make do with found ammo or of a questionable provenance. Also, military units that are in subzero conditions (e.g. certain units in Canada, Norway, or Denmark/Greenland) still issue only bolt actions.
Also when manufacturing ammo, generally a heavier crimp is used for ammo intended to be used in semiautos/autos. I’m not sure what militaries are doing in that respect.
Interesting. Why is that? I’m not doubting, I’m curious.
If, given this is true, it may also explain why bolt actions make for better accuracy. If bolts are ‘less hard on the ammo’, then it’s reasonable to assume that ammo coming from a bolt rifle is ‘less stressed’ (my language here) and thus are allowed to fly more truly.
As I recall reading, military cartridges have a very heavy crimp making the bullet harder to pull (to reuse the case). Also, IIRC, there we some issues with the early .233 cartridges for the M-16 where the crimp wasn’t tight enough and bullet would creep slightly out of the case, causing the cartridges to jam because they wouldn’t load properly from the magazine. I think the modified the loading mechanism to accommodate the extra length caused by the bullet creep.
I personally haven’t shot either one, so this is based on what I have read about each weapon.
The Mosin-Nagant has a reputation for being rugged and accurate. The famous Finnish sniper Simo Hayha (over 500 kills) used a Finnish variant of the Mosin-Nagant during the Winter War between the Soviet Union and Finland (1939-1940). He preferred using iron sights instead of a scope since a scope forces you to lift your head higher, and he preferred to keep more hidden (he still ended up getting shot in the face).
The Mosin-Nagant was first used as a sniper rifle in the 1930s and is still used as a sniper rifle even today. That says a lot, I think.
Fun fact - In early WWI, the Russians needed more rifles than they could produce, and actually contracted with both Westinghouse and Remington for a while. Many people these days are surprised to learn that there were quite a few Russian rifles made by the U.S. 
The Dragunov wasn’t really intended to be a sniper rifle. It is actually what is more properly termed a “designated marksman” rifle. The difference between a sniper and a designated marksman is that a sniper is more highly trained and operates fairly independently (snipers often work in pairs, but aren’t part of a squad). A designated marksman, on the other hand, is part of a regular infantry squad. The purpose of a designated marksman is to fire quickly but effectively at ranges up to about 600 to 800 meters. A designated marksman gets more training than a regular infantry grunt, but nowhere near as much as a dedicated sniper.
The things that make the Dragunov a good marksman rifle also work against it as a sniper rifle, to some degree. The Dragunov is light so that it can be quickly aimed at different targets, but being light, it suffers from greater recoil and is therefore a bit harder to keep on target. It is semi-auto, intentionally, so that it has a higher rate of fire and can more quickly shoot at multiple targets, but as noted upthread, being semi-auto does make it less accurate than a bolt-action. It is also designed to fire armor piercing rounds, and for that they had to shorten the barrel twist, which also makes it slightly less accurate compared to a true sniper rifle.
The Dragunov is a favorite for video games and for gun collectors just because it looks really cool, but as far as a true sniper rifle goes, it’s a bit mediocre. But in its designed role as a designated marksman rifle, it’s a pretty darn good weapon.
The quality of the trigger is an often overlooked variable when it comes to a rifle’s accuracy. A good trigger will produce more consistent shots, all else being equal. Many authors, including Col. Jeff Cooper, have written extensively on this subject.
Though there are exceptions, semiautos tend to have heavy or rough triggers, while bolt action guns have smooth/crisp triggers.
Side note: my semiauto FAL, which I dearly love, has a heavy/rough trigger. But it was designed to be that way. Why? Because one of the original specs for the FAL was that is must not fire (with round in chamber) when dropped from a helicopter hovering 30 feet off the ground. A smooth/crisp/accurate trigger would have failed the test. They had to design the trigger mechanism to be “stiff” and heavy-duty to pass the test.
And the sound of the actual firing and the mini sonic boom wouldn’t?
It the action, or lack therof.
For which part? Every time you cycle around into the chamber and back out, it may inch out slightly, and done enough will cause failure to load. Crimp makes this less likely but not impossible.
Also a fired case gets stretched by firing to the dimensions of the chamber. When reloading for bolt actions you can choose to just resize the neck to proper dimensions, stressing the brass less. Semis need the entire brass crushed into shape. However this is probably moot for 99.9% of military situations; they’re using new brass.
The Romanian PSL rifle is popular because it kinda/sorta looks like the SVD/Dragunov that video games popularize but is much cheaper. Unscrupulous or ignorant sellers may sell them as a Dragunov.
That doesn’t give away the position. The sound will reach the (missed) target from a direction perpendicular to the bullet path.
Depending on range, the sound of firing will only give away the general direction.
Actually there’s a sort of range gap in the infantry- out to 300 meters, your average iron/ACOG/red-dot sighted infantry rifles do just fine.
Outside of about 600 meters, actual snipers are typically used, with the bolt action rifles, and in the typical sniper/spotter teams.
Between 300-600 meters, that’s where the designated marksmen come into play- far enough out that they need that scope to actually hit what they’re shooting at (bullet drop is significant), but not so far that it necessarily needs a “real” sniper.
https://www.army.mil/article/206463/army_to_field_squad_designated_marksman_rifle_in_september
And FWIW, the Mosin-Nagant was the standard Tsarist/Soviet rifle starting in the late 19th/early 20th century. It’s a contemporary of the Mauser 98K, M1903 Springfield and the SMLE.
Just like all of those, some were converted into sniper rifles- AFAIK, there wasn’t a lot of really special testing- they just converted some to have scope mounts and turned-down bolt handles.
The Dragunov was designed as a designated marksman rifle. Its “sniper rifle” designation is more of a consequence of it always having a scope, being pretty common, and probably being used in a way that would be considered sniping by a lot of people who aren’t aware of the difference.
Having shot one, all I can say is that it’s heavy and kicks like a fucking mule. As in, that same day, I fired a M1 Garand, a M14, an AK, a BAR, a MG-3, and a M1919A4, and the Dragunov was by FAR the most punishing one.
As will any associated sounds of the sniper. The point being that to worry about the bolt action noise to the exclusion of all other sounds in regards to bolt vs auto seems implausible to me. I’ve fired a lot of guns and hunted quite a bit. When guns go off anywhere near you, you can tell, generally, where the shooter is.