Leade clearance is a more complex subject than can be determined by use of a reversed flat-based bullet of proper caliber. The "freebore" measurement you are trying to determine can only be accurately determined by involving the bullet or its ogival duplicate with the leade origin engagement. I have a Sinclair (now sold by Hornady) Tool for this purpose. It uses a drilled & tapped case of the proper caliber to "soft-seat" an exemplar bullet into the leade origin, from which an OAL measurement can be derived. In most cases, I want about .025"-.035" of freebore in my jacketed-bullet loads, and a "soft kiss" of the bullet into the leade origin with my castings. Barnes Bullets recommends a minimum of .050" of freebore with their all-copper "Condor Cuddler"
bullets.
More complications--and you just KNEW there would be some. Magazine dimensions can influence OAL considerations. If a rifle is a repeater, you likely want it to feed from the magazine, and this goes double for self-loaders. OEM big-maker barrels of recent times have very deep throating, and are made that way to guard against some newb refiller pouring in an Internet-derived max powder charge with
"x" bullet and jamming its ogive into a leade and lighting off a bomb. My experience with the Rem 700s of the mid-70s to mid 80s is that their throating was of reasonable depth to fit most commercial loadings in their respective calibers. My calibers among that population/experience were 222 Rem, 223 Rem, 22-250 Rem, 243 Win, 308 Win, and 30-06 Sprfld. Given one caliber of seating depth, most (but not all) bullets in medium weights gave sufficient safe leade clearance in most rifles.
One particular Rem 788 x 22-250 I bought c. 2005 illustrates an example of an assertion I'll make here that will get the benchresters into vapor-lock most ricky-tick. Assertion--leade clearance a MUCH MORE a matter of safety cultivation than of accuracy pursuit. It is a TWEAK--not a basic foundation. Here goes--I bought the 788 from a guy in Ventura with full disclosure by him that its barrel had A LOT of miles on it. 22-250s can get shot a lot, and that cartridge has a large boiler room on contrast to its bore size--lots of propellant gases going down a small hole at high pressure and great heat will wear things out, sometimes in a hurry. The rifle's price reflected that reality, and I was cool with that--I was buying it for its action to re-barrel.
I get it home and start running my usual Tale Of The Tape on the internals. "Worn" was a good descriptor, to say the least--basically, it was a "Weatherby"--lots of freebore, on the order of .150" with the Sierra 60 grain HP seated at max-spec 2.350" OAL. The mag would allow for 2.400", and not a tenth more. Aight.
I did a pound-slugging on the throat--.225" + a couple tenths. Yeah, it has some miles on it. Aw, WTH--let's make some ammo for it and see what it does. Mild load, 60 grain Sierras atop 34.0 of H-380 and WLR primers in WW cases, 2.395" OAL. To the range......and I'll be da--ed, it shot GREAT. A whole bunch of sub-1" 5-shotters. Stepped up to 36.0 grains (book max), and The Song Remains The Same. I hunted that rifle as-is for 8 years until I finally broke down and re-tubed the critter in 2013-14 (it took a while to get it back. Long story). And worth the wait. This barrel shoots great as well.
Moral of the story--if a load goes south all of a sudden, I doubt that throat depth from flame-cutting is the sole or even primary source of accuracy fall-off. There are a lot of "moving parts" in a reloading circumstance, Bullets change--alloys change--powders change--primers change--cases change. In my case, The Nut Behind The Stock changes--brought on by aging and the ravages of same. There was no particular "need" to re-barrel that 22-250, from the standpoint of pure accuracy. I DID pick up about 125 FPS of velocity in any given load because the barrel (Pac-Nor, 1-12" twist) is tight in all respects. But the rifle's accuracy is undistinguishable between barrels.
Just my 2 cents.