By simply knowing these tanks I have 50% of your authority. By playing the game and this BR I have 80% of your authority. By playing M-51 I have 90% of your authority. That’s more than enough to undermine your argument, as it’s weak to begin with.
The gun is very good for a 6.0. Regardless of our personal experiences the fact of the matter is that it’s a gun that is not lacking by any means on AMX-30, which is at 8.0.
The M-51 as a platform, while not particularly amazing and much worse than AMX-30, is more than enough to utilize the gun for sniping. It’s even better for hulldown in some regards than AMX-30, because the turret is smaller, but of course it’s still much worse.
The gun is so powerful, that M-51 being a bad platform at 8.0 doesn’t justify it being at 6.0. The tank as a whole is much closer in performance to AMX-30, than a god damn Sherman 76.
Since the gun is too strong and unfair for the tanks at 6.0, it definitely can deal with 8.0 opponents, the platform doesn’t make it impossible to effectively utilize the gun for sniping and there are historical premises to put it next to T-54s, it should be put at 8.0. To emphasize that AMX-30 is a better tank, BR decompression should be done with AMX-30 being put at 8.3 for example.
I would even argue that for example T-54 1951 is a worse tank for sniping than M-51. M-51 has better optics, twice the gun depression, normal turret rotation (doesn’t matter much, but still) and arguably better HEAT shells with slightly less shell velocity and TNT equivalent, but more pen. Of course the main advantage of the T-54 though are powerful APHE shells, which easily one shot all early western MBTs, but when encountering a well-armored enemy, M-51 would be better.
Yes, top speed of 38kph is decently fast and it reaches it decently quickly even with a very bad crew and gear shifts. It’s also pretty agile, it’s not hard to turn this tank around 180 degrees. It’s definitely much more mobile than something like a Tiger 2.
What?
Spoiler
From the ones you listed I would consider only Shturm-S to be fast, when compared to tank shells, but even it is just 550m/s. ATGMs aren’t a bigger threat than shells for unarmored tanks. A long 88mm 1000m/s APHE shell is more dangerous.