Mouse acceleration in games is a bit complicated thing to deal with.
First of all, the game itself does not implement any mouse accel unless you ask it to do so explicitly, either by passing “-useforcedmparms” on the command line without passing “-noforcemaccel” and “-noforcemspd” at the same time or by using customaccel configured in-game. To be on the safe side always pass “-useforcedmparms -noforcemaccel -noforcemspd” as commandline to the source-based games you play. Failing to do so might result in game auto-enabling Windows-provided mouse acceleration even if you have it turned off (i.e. if you have no checkbox in “Enhance pointer precision” in mouse input control panel dialog).
Next possible source of the acceleration is having m_customaccel cvar set to anything other that 0. Some people (me included) use it to gain more precision for extra-slow mouse movement while having fast and non-accelerated mouse response when moving the mouse in a non-turtle-like fashion. It could be achived by configuring the game to use extremely aggressive acceleration curve coupled with the sensitivity cap but it is a bit advanced topic on how to properly do it and it’s safer to not use it unless you’re absolutely sure you could configure it correctly.
Then in case you have non-standard mouse driver installed (i.e. not the “HID-compatible mouse input device” by Microsoft) you could get some amount of acceleration both from your mouse driver of from your mouse itself if it is capable of providing acceleration in hardware and is instructed to do so by mouse driver. There’s no general way to handle this kind of acceleration - it is mouse-driver specific task you would have to learn how to deal with by trial and effort. Alternatively you could simply disable vendor-specific mouse driver by installing default MS driver for your mouse instead.
As far as I had tested BM does not alter the mouse acceleration settings in any way with its default config and I don’t get the behavior you describe with my Logitech Performance MX mouse, both when I use MS driver or when I use Logitech-supplied driver configured to perform a “game detection” and automatically turn maccel off.
