Cheap method - uses a fired cartridge case and a bullet that you can use again.
1. Take your once-fire case and neck size it.
2. With a fine-blade hacksaw, cut three or four slits in the neck down as far as the shoulder.
3. Insert the bullet - it should be held fairly tightly by the neck, in spite of the slits, which act as a spring-loaded 'retainer'. Insert it just far enough to be held without falling out.
4. Use a felt-tipped marker to mark the bullet all round.
5. Insert the complete item into the breech and close the bolt.
6. As the bullet touches the rifling, that will push the bullet into the case.
7. Carefully open the action, and drop the case with the bullet in it gently into your waiting hand.
8. You now have a dummy cartridge that exactly fits into YOUR breech and 'just' touches the rifling. Exactly WHERE it touches the rifling will be indicated by rifling marks in the felt-tip marking, if you see what I mean.
9. Using the calipers we told you to buy a couple of months ago, carefully measure the OVERALL LENGTH of the COMPLETE CARTRIDGE. Let us say that it is 2.75"
10. Whatever that measurement is, begin reloading a series of cartridge, using the same style of bullets, at 0.010" shorter than 2.75". You are now making a finished cartridge that is 0.010" SHORT of the lands of the rifling, for a start, and see how you get on. Some rifles like a bigger 'jump', but remember that being actually in contact with the the rtifling at the moment of ignition meansa that the pressure will peak virtually instantly - having even the slightest 'jump' will reduce that effect by a small amount, possibly resulting in an increase in accuracy. You can increase the amount of shortness by 0.005" at a time until you get the resaults you want. My Krico 650SS likes a 'ten 'thou jump' with Lapua Scenars, but shooting VLD Bergers I have to make allowances as the ogive is so long that the bullet would have nigh-on disappeared up thr barrel by the time it touched the rifling.
Remember, too, that this will be different of every type of bullet, as has been noted here many times. The longer the ogive, the more the bullet needs to be set out of the case in order to touch the lands - the opposite is therefore true - old-style, short ogive bullets will need careful finagling.
Expensive method. Buy a Stoney Point cartridge length gauge, or whatever it's called now, for about £50 - 60, that does exactly the same thing, but looks really spiffy.
You choose.
tac in Port Orford Oregon, about to go to a HUGE bar-b-q.