You're either damaging multiple enemies at the same time without hurting any individual enough to kill them, or you're looking at the score streak rather than the "Enemy Hit" points. There is no other explanation for getting 250 points without a kill.
I sit there and shoot somebody with a 50 caliber machine gun and it doesn't kill them like it says up to 250 points and they're still not dead and then they shoot me twice and I die how do I fix this
You fix this by not sitting there with an MMG. By being stationary you're making yourself an easy target, this is especially true if you're laying prone because in that stance half of your cross-section is covered by your head which takes up to 2x damage.
Meanwhile the enemy is free to duck and weave from their standing position and throw your aim off. The caliber of your gun doesn't matter if you cannot land your shots.
If you want to stick to playing Support my suggestion would be to switch to an LMG suited for the typical engagement distance of the map you're playing.
Short to medium range (e.g. Underground, Marita, Rotterdam): KE7 for a fast-firing gun with a decent magazine size, or Lewis Gun if you want a larger magazine.
Medium to long range (e.g. Twisted Steel, Hamada, Panzerstorm): Bren Gun (3x scope with full right-side specializations) for the best all-rounder, or the Type 11 (3x scope with RLLR specializations) if the magazine blocking your screen bothers you.
Finally,
I've watched people sniping me through mountains and buildings like how are they doing this
I can assure you they aren't. It's the death camera giving this false impression because it takes some time for it to turn around and face the opponent, and by that time they've already moved behind cover. The death animation on your character can also cause the camera position to shift from where you actually were the moment you got shot (classic example: rolling down the side of the mountain on Fjell).
On top of this there is the unavoidable network latency which can also contribute to the discrepancies.
