DC Brush Motor Current

A 25 A motor at what voltage? If it's a low voltage motor (12 - 24 Volts, for example), the number of windings will be low and the inductance will be low. If it's a high voltage motor (90 - 180 Volts, for example), the inductance will be a lot higher, and a current pulse will produce a significant voltage spike unless the voltages are clamped or otherwise suppressed.

You also don't say how the "shunt" is connected. Is it in parallel or in series with the motor?

You can also characterize the motor's impedance before it is connected mechanically to a load (the armature acts as an inertial load itself) by observing a pulse applied to it with an oscilloscope, which is really the easiest way. Keep in mind that brushes can be in contact with one or two commutator bars each, meaning that a DC brush motor armature has two distinct characteristic impedances.

The shunt is in series with the motor, looking for +/-0.125V to feed an A/D. It's a 28V motor. I plan to do lab tests to get a real number, but that's some months away. I'm just trying to get a handle on what to expect as I write some code for the BIT function in the control FPGA.

The motor terminals will be clamped. The motor will be connected to a significant load at all times.

I read online today that the starting / stall current is just V/R, R being the winding resistance - is this true?

Stall current is just V/R, but V seen by the armature windings may be less than that applied to the terminals, and the difference is brush drop. At 28 volts with a PWM amplifier, brush drop is insignificant.

Motor impedance is a little esoteric. Motors generate a back emf, so the current through them is a function of winding DC resistance, brush drop, back emf, and inductance.

When the armature of the motor is carrying near it's rated current (partly rated for temperature and partly for magnetic saturation) the magnetic field in the armature is established and changes nearly sinusoidally as the armature rotates. The apparent inductance of the motor when a magnetic field is established is due to the change in winding angle (magnetic field direction) from one pair of poles (commutator bars) to the next.

Yeah, I had to take motors twice in college. That's why I try to stick to digital design.