Originally Posted by Broz
Hornaday makes a tool with a special threaded case you put in the chamber. Then a push rod extends the bullet down until it touches. Then lock it down and remove from chamber and measure it up. Works great.
I always just pull the bolt, drop a pill down into the throat, and tap lightly with a cleaning rod. I then go to muzzle end and put a register mark on the cleaning rod using a tip I cut flat. Tap out the pill, reinstall the bolt (without dry firing) and put another register mark on the cleaning rod. Measure and you have your length to jamb with that pill.
The hornady tool is useful on a ratchet gun or auto where you can't get at the bore from both ends.