TL;DR: deflection due to coriolis force depends strongly on the angular velocity of the station. In slow rotating stations (eg. very large ones) coriolis effects on short range engagements will be negligible, but can be significant at longer ranges (eg. 100s of metres). In very fast rotating stations (eg. very small ones) the coriolis effects will be so big that you really shouldn't be shooting projectiles at all.
High velocity rounds will partially offset coriolis effects, the faster the better. A combination of training and specialist equipment is probably the way to go.
Neither the mass of the projectile nor the radius of the station affect the strength of the coriolis force, but they do affect the strength of the centrifugal force. I won't consider this here, but you should care about it.
Edit: sudden obvious realisation... the equivalent of "the high ground" in a rotating habitat is the ground spinwards of you. If you're shooting spinwards, coriolis forces will reduce your range as your bullets appear to be dragged downwards. The people you're shooting at on the other hand, will find their range increased, and may be able to "shoot round corners" (or at least, shoot "up" under an obscuring ceiling). Defensible areas in a rotating habitat will therefore be more strongly fortified to the spinward side. People trained to fight in a rotating habitat will preferentially advance in the antispin direction. Untrained people may not realise this, and problems will ensue (including blowiong themselves up with grenades because they couldn't throw as far as they thought).
Edit 2: the coriolis force doesn't just affect projectiles, but any moving object. This includes your arms, legs and head. Even if you do have lasers which will always shoot in a perfectly straight line, your ability to aim or track a target will be limited by your familiarity with and acclimatisation to the environment and your training. Even apparently simple operations like reloading a weapon could become challenging in a high-RPM environment, and using a laser will not prevent this. Have a read of Artificial gravity station station phsiological effects and design criteria, a NASA report from the early 70s.
Disclaimer: I Am Not A Mathmologist nor a Physician, so things like mechanics and vectors are a bit of a foreign language to me. Naturally, this answer requires both. E&OE.
I'm assuming you're in a rotating habitat with artificial gravity provided by centrifugal force. I'll assume the specific example of an O'Neill cylinder, so I'll use terms like "endcap", but this can be translated to any other rotating habitat, I believe.
I'm not going to consider centrifugal force here, as your question is explicitly about the coriolis force, but in a rotating habitat with substantial vertical distance changes (eg. walking up steps on the endcaps) the effects of centrifugal force will also do odd things to projectile trajectories. Don't forget this! I'm also not recomputing the effects of the coriolis force once deflection occurs. A projectile deflected down a little by coriolis effects will then be deflected antispinwards a little, and so on. Trajectories can form very strange shapes, though at high projectile speeds and low station velocities things don't get too crazy, so I'll just talk about the initial forces affecting the projectile. I think my approximation here is OK, though it won't hold for things like thrown grenades in smaller stations!
The strength of the coriolis force is defined as $F^\prime = 2m\Omega \times v^\prime$, where $F^\prime$ is the resultant force vector, $m$ is the mass of the moving object, $\Omega$ is the rotation vector of the station, and $v^\prime$ is its velocity vector relative to the rotation vector of the station.
Lets make a coordinate system with Z pointing radially inwards at the axis from the rim (ie. "up" in the artificial gravity field). $\omega$ is the angular velocity, and $v$ is the projectile velocity. If I shoot the projectile along the X axis, in the direction of rotation:
$$F^\prime = 2m\begin{bmatrix}0\\\omega\\0\end{bmatrix}\times\begin{bmatrix}v\\0\\0\end{bmatrix} = 2m\begin{bmatrix}0\\0\\-v\omega\end{bmatrix}$$
you get all the force in the -z direction, or your "massive bullet drop". How how massive is massive? Well, the downward acceleration experienced by the projectile will be $a = -2v\omega$, and the drop $s$ over time $t$ will therefore be $s = \frac{1}{2}at^2 = -v\omega t^2$. Doubling your velocity doubles the strength of the coriolis force, but quarters the time you experience it for so the drop will end up being half as much.
Lets start with the 50km radius station. To get a 1g apparent gravity, you need a spin rate of about 0.134rpm (or 0.014 radians per second). Lets shoot a projectile at 400m/s (a conventional 9mm pistol round might go this sort of speed). I'm gonna be super, super lazy and neglect bullet drag... lets assume some idiot has been firing projectiles in a space station, and punched a bunch of holes and let the air out. The acceleration due to the coriolis force experienced by the projectile will be...
$$a = -2v\omega = -2 * 400 m/s * 0.014 rad/s = -11.2 m/s^2$$
...about the equivalent of 1g, approximately. At a 400m range, which seems a little far to be shooting a 9mm bullet, you'll get an additional drop of about 5.6m. Shoot antispinwards, and you'll get a drop reduction of 5.6m. Shooting a round twice as fast will get you half the drop. A NATO 5.56mm supersonic rifle round travelling at 880m/s will get a drop of ~2.5m firing spinwards, etc. At a more cosy 20m range, your pistol round's deflection will be more like a couple of centimetres... enough to spoil your score in a shooting range, but not really enough to make you miss all by itself.
Take home message: for close engagements, no special equipment or training is needed, probably. For longer engagements, eg. the sort of ranges that typical modern-day military forces expect to fight at, problems will arise.
Now lets look at the other end of the spectrum, with a 20m radius centrifuge spinning at a dizzying 6.6rpm, or about 0.7 radians per second. The strength of the coriolis force is therefore 50 times greater. The distances involved are vastly smaller, of course... you're shooting 40m, tops. In that time, an object in the station will have rotated 0.07 radians, or 2.8m around the circumference (which is the maximum displacement if you're shooting someone diametrically across the centrifuge from you). It'll certainly feel like your bullets are bending in that situation!
Take home message: don't go into a centrifuge with a gun that fires bullets. Something bad will happen, to someone. Just thrown in some grenades first.
Edit: prompted by Harper's comment, lets have a look at 2001's Space Station V. It has a 150m radius and rotates at a little under 1rpm to give lunar-equivalent gravity. Its fast rotation rate leads to strong coriolis forces (about 3.7x stronger that the massive O'Neill for the 400m/s bullet) but under a 3m ceiling your longest line of sight is only going to be no more than 60m. This gives a maximum z-axis deflection of ±0.92m. This means that if you lie on the floor and look antispinwards and can see their toes, you can shoot them in the abdomen. This is also very approximately the spread of a long-barrelled shotgun with a full choke at the same range, so in this environment a shotgun might well be a sensible choice if you weren't expecting to face armoured opponents. It has a the nice knock-on benefit that unlike supersonic rifle rounds, you're much less likely to shoot a hole in the world and let all the air out.
You've thought of lots of neat ideas, but here's my take:
Use smart sighting devices. For closer-range engagements, for example, combine a laser sight with an inertial measurement unit and a laser rangefinder. The sight can detect the rotational parameters of the current environment and the direction in which you're pointing the gun, and then steer the laser to point where the bullets will actually land.
On a planet, or a really big habitat at close range, it'll work just like a regular laser sight. As the strength of the coriolis force increases, the laser will point in slightly (or extremely) different directions to the gun. The relationship between the two will be confusing for the untrained, who will wave the gun around frantically in the wrong direction in the heat of the moment and then get shot. They'll need lots of practise in varing environments to learn to use the sight properly.
Use guns which shoot high velocity rounds to mitigate the effects of the coriolis force. These are generally preferable in most circumstances, unless you wanted to be stealthy in which case supersonic weaponry is undesirable.
Use guns which shoot a lot of bullets in a short time, perhaps tracers, if you're feeling brave. You can walk your fire onto the target, given small aiming errors.
Use grenades, flashbangs and gas in confined spaces at high RPMs.
Use guided projectiles at long range.
Consider using shotguns against unarmoured targets in smaller stations, for both ease of hitting and environmental safety.
Deliberate use of low-velocity projectiles to shoot around corners and under obscuring ceilings in fast-spinning stations might be a good tactic, given some practise.
