there's no real reason to ever use a bolt action rifle in tarkov except for style points. Why would I use an m700 when I could use a hunter or an rfb which is easier to put a scope on and semi auto. Why use a mosin when I can get an svd. Yes there is the balance by price, but after a scope and a silencers (which you'd probably want for sniping) the cost becomes neglible.
To offset this I see 2 ways of balance.
the soft way would be to make the highest quality rounds have a very high malfunction chance (as these rounds are most likely loaded very hot) while also making bolt actions have a much smaller chance to malfunction and accuracy degradation with durability. So you could use these incredible rounds in your full auto fal with a drum mag, but you'd likely not be able to get through a full mag without a jam.
The hard way would be to just do what they did the the kedr-b, a gun that can accept both normal 9x18pm ammo and 9x18pmm (which isn't actually any better), this would also let them make ammo that would make the bolt actions more viable without making the larger caliber select fire guns unstoppable.
Now you might say that making the mosin more powerful would just lead to the mosin meta of old once again, you could make this less prevalent by just buffing the ammo in ways other then just damage/pen. I think these should be buffed yes, but not to the point where you can reliable one tap thorax. Give this ammo buffs to accuracy, velocity, frag chance, make it have a high chance to make enemy's bleed/fracture.
That's just my two cents.