I may have posted this info before but if so, I couldn't find it.
In any event, I wanted to make a couple of experimental bullets for my 7-30 Waters TC barrel so that's a good excuse to talk about how to measure a rifle throat.
The 2 basic methods are cerrosafe or an upset slug. I used to rely on cerrosafe because that's what the experts recommended, but after getting cerrosafe stuck in the chamber a couple of times, I decided I liked the upset slug method better.
You'll need a dummy case (no live primer) and a pure lead bullet. If you don't have a bullet mold in the right caliber, you can use a smaller caliber -- it doesn't matter if the bullet is too small because you're going to upset it to fit your throat.
I made a couple of pure lead bullets in a 7mm mold. The bullets were cold and wrinkled but that doesn't matter because I'm not going to shoot them. I filled the dummy case with rice up to the base of the neck so that the bullet could not fall inside the case. On other occasions I've poured lead into the dummy case up to the base of the neck to accomplish the same purpose.
Then I chamber the round with the pure lead bullet just barely started in the case. A little oil on the bullet may assist extraction later on. Then I inserted a brass rod through the muzzle until it came to rest against the bullet, and gave the rod a few smacks with a hammer to upset the bullet.
Dummy case with pure lead bullet ready to chamber.
If I had been using a too-small bullet, I would have chambered the empty case, dropped the too-small bullet down the muzzle until it came to rest against the case, then used the brass rod to upset the bullet.
Even if you do everything right, sometimes the upset slug will stick in the chamber and have to be pounded out. Sometimes the slug gets damaged trying to get it out.
If that happens, just do it over until you get a slug that's in decent shape.
An upset slug (top) and a cerrosafe cast (bottom) of the same 7-30 chamber. You can't tell from the crappy photo, but the upset slug was actually quite decent and I had no problem getting consistent measurements from it, except on the bore which I'll discuss later.
To measure the bore, I like to use an expanding ball gage, inserted at the muzzle. I think I paid about $10 for this set of Asian gages some years back. It does take some "feel" to use the gages properly, but once mastered, they work well enough. However, the bore tends to erode just ahead of the throat, so it's likely the bore will be larger at the throat than at the muzzle. I decided not to worry about that.
Some people measure the bore at the throat by measuring the chamber casting or chamber slug with a blade micrometer. I tried doing that but it wasn't working because of the twist and the small caliber -- it's too difficult to fit the blades in the twisting rifling impressions. Also, the pure lead is easily dented by the micrometer. You can get a reading, but is it a reliable reading ? I did not trust the blade mic readings and decided to ignore them.
Some people like to use pin gages to measure the bore but in my opinion they are not very accurate. It takes significant clearance to insert a pin gage without force and without damaging the barrel and that clearance throws off your measurement. If a pin gage can slide into the barrel easily, it's prolly at least 0.001" smaller than the bore and maybe 0.002" or 0.003" smaller. That's not very accurate.
Bore diameter at muzzle = 0.2785"
Chamber neck diameter = 0.3085"
case neck thickness (WW 30-30 case) = 0.011"
Throat/leade diameter = 0.286" tapering down to 0.285"
Groove diameter near throat = 0.285" (it's prolly eroded a bit near the throat, I think the rest of the barrel is closer to 0.2845")
Distance case mouth to rifling = 0.14"
Loaded with a 0.286" bullet, the WW case neck OD should be around 0.308" for a snug fit in the 0.3085" chamber neck.