I don't have any expeience with it, however assuming it is close to the width of the bore, and the barrel is long enough that the angle of light that can exit is limited (also dependent on caliber/barrel width), it shouldn't matter as long as you can see it at range.
For very short barrels, or large calibers you might need one that is to closer tolerances, as it will need to aim more precisely since the barrel won't restrict the light.
Laser diodes come in several wave lengths. Many in cheap products use one closer to infared that for the same amount of power is not that bright to the human eye. The better ones are closer to the orange spectrum. Those are now only a little more, so I would aim for those. (The human eye is even more sensative to green light at the same power, so green lasers can be seen the furthest for a given power, but those still cost a bit more.)
I imagine the cheapest ones will use the cheapest diodes which are probably not very visible in well lit conditions at range, and the cheap lasers have optics that will often spread too rapidly, which in a barrel short enough to permit, would make sighting in with it more difficult.
I have been meaning to rig a laser diode on some wires to drop into some barrels myself.
Keep in mind where the laser points and where the bullet will impact are two different things. The laser will always aim at where the barrel is pointing. However the bullet trajectory will change the actual vertical impact locations. Wind and other factors will of course change the lateralimpact point.
So if you just sight in on the dot you will just have your point blank range sighted in.
So using the ballistic coeffecient and velocity of the bullet you will need to plot the trajectory, and actualy sight the firearm in above or below the laser the proper number of inches for the trajectory you are seeking.
It does work well though, I have sighted in a couple firearms just using really bright light in total darkness, and using the very cinter of the projected light as the center point of reference. One shot to verify it is correct is all that is necessary, and they have been correct so far.
