Macros Are Evil

[NaN]

Hmm.
I'd say they use 2 Pi / 360.0 instead of Pi / 180.f
And yeah, they use 3 times double to float: once in btScalar(2.0) then in btScalar(360.0) and in PI define itself
and that forces to use float division on those 3 floats with already conversion errors.
My guess is that the result is different from just float( 3.1415926535897932384626433832795029 / 180.0) or even /180.f
But yeah, btScalar can be a double if ussing double precision with Bullet, then it probably is okay. But for that one would have to include bullet sources in project, since all distribs have it off probably (just single float).

Nope, precision is not the problem.