I'm looking to rebarrel and want to keep my bullets the same length they are now because they are just the right length to reach the lands and still cycle through the mag. How do I determine what my throat length is no so I make sure to order the right reamer??

Just load up three dummy rounds with the bullet seated to the desired seating depth and send them to your reamer maker and tell him what you want.
You might think about seating them, (your dummy rounds) a little shorter, so you have room to lengthen your round as your throat lengthens.

Hal

Once the barrel is pulled, you can drop the desired bullet into the old barrel and take a measurement from the base of the bullet to the breech end of the barrel using a depth mic and then using a separate throat reamer and that bullet, simply advance the new throat and bullet the desired distance.

Or, you can do the above measurement and transpose it to the case as in the photo, measure the overall length of the bullet and case, and like HalH mentioned, make up a dummy and send that to the reamer maker.

Thanks, I have two dummy rounds loaded up to take to the smith for the rebarrel. Would this formula give your throat length if you seat your bullets to just touch the lands? (base to ogive) - ( case lenght). My ogive length was 2.927 and case length after trimming measured 2.615. So is my throat length .312?

It depends on where you took the ogive measurement from. Are you using a Stoney Point measuring tool? Remember, whatever way you measure it, in order for the dummy round to work, it has to represent the distance from the bolt face to the leade.

You can take a fired case and just barely resize it, enough so that you can push a bullet in it with your fingers, and tight enough so that it will not fall out.

Start a flat base bullet into the case mouth with the pointed end going into the neck first.

Carefully chamber the cartridge, completely closing the bolt. The bullet will seat the flat base against the start of the rifling.

Carefully extract the dummy cartridge. Carefully because you do not want the change the depth of the backward seated bullet.

The amount the bullet protrudes from the case is the length of the throat. You can measure this protrusion to the case mouth and get an exact measurement, in thousands, of the throat.