Charger ... Instead of worrying too much about the groove diameter I usually take three different measurements on my cast bullet rifles. 1) Push a snug fitting cast bullet into the muzzle (finger pressure) then mike the diameter of the bullet's nose where it stopped going in. This will give you an idea of the proper size nose required. 2) Tap a .36 cal. buckshot down into the muzzle. It's a very tight fit & will leave a ring of lead at the muzzle to be trimmed off with your pocket knife. Then gently drive the slug completely through to the chamber end with a bore diameter rod. This will give you an idea of the smallest diameter along the bore... And if the bore has any constrictions that might need taken care of. Sometimes the last inch or so ahead of the breech is almost a free-fall. This isn't always bad in a cast bullet rifle. 3)Put a 5/16" bolt into an unsized case which has been fired in that rifle until it bottoms out. Trim off everything that sticks out so that you have a plugged .303 case. Then slip a .32 H&R Hornady swaged HBWC (actually .314") into the rifle chamber (solid end forward) followed by the plugged case & close the bolt on it. What you have here is an extremely soft slug directly in front of a plugged case in a closed & locked chamber. Then take the bore size rod & slide it down the muzzle until it hits the plug. Then get a BIG HAMMER & smack the rod until it no longer moves. What you have done it to expand the soft slug against the plugged case & against the chamber walls. Extract the case & gently tap the slug out the breech end. You should have a slug about 1/2" long that shows you the throat area & about 1/4" of rifling. Measure the throat about .10" ahead of the case mouth & by using that measurement & the measurement of the bullet nose that you stuck down the muzzle you'll have a very good idea of the proper size cast bullet needed.
I have seven SMLE's of different make & Marks. They all take a different size bullet. I have honed out a Lee "Push Thru" sizer for each size needed & seat the gas check in that operation. I use nothing but Remington brass & Remington 9 1/2 primers with from 19.0 to 20.5 grains of SR4759. This gives me from 1575 to 1650 fps. Bullets cast from straight WW metal do just fine in all my SMLE's, including the two groovers.
One of my favorite .303 hunting loads is the Old West 314190 that casts at about 205 grains in my 1:1 alloy of WW:Pb & water quenched. Bnh before quinching is about 7/8 & afterwards about 14. Loaded in front of 34 grains of IMR4350 gives about 1900 fps & is both accurate & effective. Hope I've helped...
oksmle

oksmle..Thanks for the reply, we do just about the same thing. I measure the bore (distance accross the lands) with a Starrett deep hole guage and micrometer. I then use a soft round ball (.375) in this case. Hammer it into the muzzle (being careful not to hit the crown) and push it through the barrel to determine groove, or at least the tighest place. I then sometimes measure the throat with an expanded soft slug doing something very similiar to what you do.

The bore measures .3045 and when the over size round ball was pushed through it measured .3045 with no signs of rifling marks. This means the soft lead striped out of the shallow groves and skated down the top of the lands. I did it three times and only used hand pressure on the rod to push the slug through the barrell. You are right that the throat size is more important than groove size, except I am kind of lazy and have no 303 cases (yet)and was going to size my bullets .313 and just wanted to make certain that I was not shooting them LESS than groove. .313 is the largest die I have and would prefer not to buy a larger one, but if I have to, then I have too. Thanks for your help...