How to figure neck thickness w/o micrometer?

[Winny94]

I dont currently own a case neck micrometer, so my question is; since you can get ballpark recommendations for bushings online for a given cartridge and brass mfg that are close enough to get decent loaded rounds, will the loaded round neck thickness - bullet dia. / 2 formula give me the true neck thickness, or does the neck thickness actually change when sizing/expanding?

Use case:
Cartridge: 6mmAR (6mm Grendel, 243 LBC, 6mm Predator, etc.)
Platform: AR-15
Brass: Lapua 6.5 Grendel necked down to 6
Dies: Redding FL Type S
Bushing: .268 to begin
Bullet: 107 SMK
Powder: 4895
Expander: Either turn downed ball or pin gauge

Im relatively new to "precision" reloading, so thank-you for your patience and advice

 

[jlow]

Measure your bullet diameter, sit bullet in a case, measure diameter of the neck where the bullet sits, minus the two and divide by two. Accuracy would depend on what you use, more accurate with a micrometer, less with a caliper. Neck thickness changes very little after seating. Remember when you size a neck, it really also does not go to exactly the sizer dimensions of the sizer due to spring back/work hardening.

 

[dcali]

Yes, (loaded_diameter - bullet_diameter)/2 will get you plenty close for the purposes of bushing sizing.

 

[gunsandgunsmithing]

In reality, neck thickness doesn't matter. It's the loaded round diameter that does matter. A loaded 6 Grendel with Lapua brass will measure .272 with a .2430 bullet seated. Neck thickness is just a number related to the setting of a neck turner when turning necks. Yes, you can assume .272 minus .243= .029/2= your .0145 neck brass thickness. Neck clearance is another story...you'll want at least .002 for which I'd do a .274 neck chamber. A .275 wouldn't hurt a thing here.
Keep in mind, there will likely be a place at the neck shoulder junction that doesn't get sized down to quite that when necking down. It'll go away upon firing and all will be fine, ime from then forward but it may make bolt closure pretty snug on virgin brass, depending on your chamber.
Given the above numbers, in a gas gun, you should be fine with a .268 or .269 bushing. Less tension if fired single shot. I wouldn't go bigger than a .269 in a repeater, as the bullets can get pushed back in the case under recoil.

 

[Rocketvapor]

The 4 diameters that matter with a neck:
1) As fired virgin/annealed
2) As fired aged/work hardened
3) Neck sized, bushing or mandrel
4) Loaded
Keep track of those for each chamber.
Measure at several points along and around the neck.