Measure length from muzzle to breech. Write down this number.
Remove barrel from gun.
Resize two cases.
Drop in barrel or cylinder. They should "plunk" right in and fall right out when you turn barrel or cylinder upside down.
Expand both cases and ensure there is enough flare/bell at the case mouth that a bullet can be lightly pressed into the case and will stay in place when you turn the case(s) upside down.
Drop bullet in barrel and push very lightly into lede/leade/rifling.
Measure distance from muzzle to bullet tip.
Subtract this number from the muzzle-to-breech number. This is your Max COL for THAT bullet in THAT barrel.
For a revolver, you want the distance out that a bullet can be seated and not extend beyond the cylinder. You can do this by removing the case mouth flare just enough so the case chambers, then placing the round with the pressed in bullet into the cylinder and seeing if it chambers. If not, gradually seat bullet until the round does chamber. Be sure it chambers in all chambers (is there a different name for the chambers of a cylinder?). This will be max COL for that bullet in that cylinder.
Now, your goal is to establish a COL that fits the magazine, feeds, and chambers in your gun.
Since there is no powder or primer in the two "rounds," we can safely play.
Seat the bullet to the cartridges max SAAMI COL (9x19: 1.169: MAX and 1.000" Min; .40S&W: 1.135" MAX and 1.085" Min; .45 Auto: Match (SWC)--1.255" MAX and 1.140" Min, Std (FMJ, etc.)--1.275" MAX and 1.190" Min).
Try the max COL and see if the two INERT rounds will fit in the magazine and feed and chamber. Watch for bullet set-back.
Rounds will probably NOT pass all three requirements.
Seat bullets gradually deeper until both inert rounds function properly.
This is the MAX effective COL for that bullet in that gun.
Now, load up 5-10 rounds at this COL. Then load up several other sets of 5-10 rounds at progressively shorter COLs, not to go below the minimum COL in manual being used and certainly NOT below the SAAMI minimum COL.
Take these to the range and fire them from a rest for accuracy. It is possible that some of the very long COL rounds and some of the very short COL rounds will not fully function for all 5-10 rounds. Unless you see some problem with the round itself, consider that COL to fail. You should easily find the mid-range COLs all feed and chamber easily and, if you shot carefully, you can even get some idea if a certain range of COL is more accurate.
This will establish the proper COL range for that bullet in that gun.
Now, that COL will "probably" also work for many other guns in that caliber, but it will probably NOT be optimum in the other guns.
Your shooting and loading will establish what COL works best for your and your needs.
In my experience with my guns, 9x19 is by far the most fussy cartridge in terms of COL--too long or too short will produces a jamamatic and the "sweet spot" for accuracy in very fine and EVERY different bullet will do best at its own COL.
.45 Autos seem the least finicky and do very well with a wide range of COL and will, at least all most all mine will, feed an empty case without any trouble.