Found this in a post on the Internet somewhere in the past:
I disagree with the negative comments.
A collimator is an excellent tool for checking the 'scope has not moved or failed between outings, after a knock, or after an unexplained miss.
After you have zeroed the rifle on paper at your preferred range, make a note of the position on the collimator grid. Then whenever you take the rifle out, do a quick check with the collimator.
If the 'scope is still showing the same position on the collimator grid then it is almost certain that the optical zero has not shifted.
But if possible you should still take a test shot, because other things can still shift the zero e.g. moderator on/off/loose, looser or tighter stock bolts, change of ammunition batch, temperature effect on powder etc. etc.
If however the collimator shows a different reading then either the scope has shifted, or the collimator has moved, or both. Most likely you have not inserted the collimator into the barrel correctly, or a bit of dirt has shifted the pin slightly. Check and retry. If its still out, you must test on a target.
The collimator is also a useful tool for many other reasons. Here are just a few:
Adjust scope to get first shot somewhere on paper. Easier than boresighting for small calibres, or when you can't squint up the bore, e.g. semiautos, air rifles etc. You may find boresight isn't quite the same as centre of grid, but once you know the offset its usually repeatable between rifles.
Find out limits of adjustment of turrets i.e. wind turrets until reticle stops moving against collimator grid and note position.
Test repeatability of adjustments by "boxing" the grid and checking return to zero.
Test for smoothness of adjustment of turrets, detect stickiness, or when tapping the turret after adjustment causes shifts, or when turret has to be first backed off, then wound back to be repeatable.
Test for moving zero when altering zoom or parallax.
Test for moving zero when temperature changes e.g. indoors versus outdoors.
Find zoom setting when mildots are true against grid.
Check calibration of turret scales is true.
Centralise scope in adjustable mounts. Find limits of turret adjustment then set to halfway point. Then adjust mounts to bring zero back to the right position on the grid.
Set up vertical mount adjustment to maximise turret elevation adjustment for long range shooting. Same as above, but set vertical turret so it is nearly at lowest point at close range zero.
Check return to zero when using scopes in take-off mounts, and/or switch barrel rifles.
Swap scopes around several rifles and minimise time/ammo to re-zero by noting collimator grid settings for each rifle.
AFAIK most collimators seem to be the same design, and look as if they come from the same factory, varying only in price, not quality. £40 will buy one. Be aware some large diameter scopes in high mounts don't line up very well with the collimator and can make it difficult to see the grid, but I've always managed to use it.
I've also tried laser boresighters that fit in the breech and in the muzzle, and neither seemed of any practical use to me, whereas the optical collimator gives a repeatable quantitative measurement IME.
