How to make an object accelerate and *then* rotate at a constant speed? Say I am designing a mini-gun. One of the key things it does is spin up, and then continue to spin at a constant rate. I'm having trouble making it look consistent when this transition happens from the 'acceleration' motion to the 'constant speed' motion.

For the sake of round numbers, say I am using a Bezier interpolation from frames 0 - 50, as I want to control the slope of the acceleration. Then, I transition to a linear interpolation from frames 50-100. The problem is that the rotation speed coming out of the bezier curve is awkward, because there is still that easing.

Basically I want it to rev up, then continue at a constant rate (though I'd like control over how long it takes to rev up). 

I saw this answer but I don't understand drivers well enough to know if it solves my problem or not.
The tangent to the curve reaching frame 50 must be exactly in line with the linear progression after frame 50. Also the 1-50 curve must also stay to the left of the linear ascension otherwise it will look like it accelerates before getting to the linear part.