BT-5
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Arcade Battle BR: 1.0 → 1.3
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Realistic Battle BR: 1.0 → 1.3
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Reload Speed: 2.9 s → 4.0 s (also affects RBT-5)

Rationale
The BT-5 is competing for the title of best reserve tank (British reserves might slightly better overall). The 45 mm 20-K gun however, is easily the best gun at 1.0 due to the combination of good penetration, good post-pen damage, and fire rate. Something has to give.
The in-game BT-5 seems to be visually based off an early-mid production BT-5, which means there is a way to historically nerf it to bring it more in line with other vehicles.
Most BT-5’s used the 45 mm 20-K Model 1932, which did not have a proper semi-automatic breech, or at least not in practice with all common ammunition types. It wasn’t until the Model 1934 gun that they seemed to have mostly resolved this issue, but these were only mounted on the late production BT-5’s. This is a similar case to the M13/40 (I), which has lower RoF than the M13/40 (II) & (III) due to the lack of a semi-automatic breech. It would therefore make sense to bring the BT-5’s fire rate in-line with M13/40 (I)’s, at 4 seconds aced.
Despite the reload nerf and BR increase, the BT-5 would remain fairly competitive since it is still very good mobile and has plenty adequate penetration and damage. USSR only has two 1.3 TT vehicles at the moment, so the BT-5 is already routinely uptiered to 1.3 anyways, and the RBT-5 is also already 1.3 despite the only difference being two very hard to aim rockets. This nerf will not actually be too impactful apart from giving the less competitive1.0 vehicles a bit of breathing room.
The BT-2 would be a more appropriate vehicle for 1.0. It’s even faster than the BT-7M, but has a much more appropriate gun for 1.0.
BT-7
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Arcade Battle BR: 1.3 → 1.7
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Realistic Battle BR: 1.3 → 1.7
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Reload Speed: 2.9 s → 3.1 s

Rationale
By nerfing the BT-5’s rate of fire, the BT-7 now has a more notable advantage over its predecessor, which makes a difference in BR more logical. The BT-7 variant modeled in-game should have Model 1934 gun with a semi-automatic breech afaik, but the reload is still a bit excessive at the moment, allowing it to out-trade many 37 mm-armed vehicles. While it should still have a decent reload given the size of ammunition and its placement, it should not be able to outtrade so many tanks with smaller guns.
The BT-7 should have no real issue at 1.7. The gun is still more than capable enough of handling most tanks, and the mobility is still impressive.
BT-7M
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Rank: Rank I → Rank II
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Arcade Battle BR: 2.0 → 2.3
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Realistic Battle BR: 2.0 → 2.3
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Reload Speed: 2.9 s → 3.1 s
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Add BR-240P (APCR)
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Add O-240 (HE)
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Add Scouting

Rationale
The improved top speed of the BT-7M makes this a pretty big step up from the previous BT’s since it can fully take advantage of its insane p/w. In its current state, it’s massive missed potential that it has all that mobility, but can’t be used as a scout vehicle. Moving it up to rank II would allow for it to fill the conventional scout niche, considering the T-50 is acting as more of a pseudo-medium.
As the latest BT variant, that did see service even fairly late into the war, it should have access to some of the better rounds other rank II vehicles get.