Neck Tension & Plug Gauges

[PMA3]

In order to offer a proposed solution to a question, most people want as much pertinent information as possible, so here are the basics:

Krieger 6MM 1x8 twist, .236 bore 6X47 Lapua
BAT MB + Jewell Trigger
ST-1000 Stock
BAT 20 MOA rings + NXS 12-42

The rifle has a .267 neck and the loaded round measures .2646 (.2430 +.0108 + .0108) which gives me a clearance of .0024 with .2430 measured bullets and .0019 with .2435 measured bullets...Berger 108 BT.

The FL die was cut with the same reamer as the barrel by my gunsmith. If I use a .263 bushing, the neck tension should be approximately .2646-.2635 (bushing size +.0005 spring back) = .0011 neck tension. To check the neck tension, I bought a series of Vermont GO plug gauges that range from .2410 to .2430 in increments of .0002 ( .2410, .2412, .2414 etc). If we use a .2430 bullet as the starting point and subtract .0011 neck tension,
the resulting number is .2419 which tells me that a .2418 or .2420 plug gauge should just fit snug in the neck.
I cannot get a .2410 plug gauge to fit which tells me I have .002 or more of neck tension!

I also used a .264 bushing, which should give me approximately .001 LESS of neck tension (.2646-2645=.0001)
which is basically NO neck tension, and a .2428 or .2430 plug gauge should fit snugly in the neck (.2430-.0001=
.2429). I still cannot get a .2410 plug gauge to fit the neck!

I measured all the plug gauges with a Starrett 1" mic and they're dead on. I am at a total loss to explain the numbers coming from the above procedure and sure would appreciate any feed back that might explain where I went wrong with my logic.

Paul

 

[bozo699]

Paul,
It looks like your measuring the neck wall thickness X 2 + caliber diameter, thats fine I do that also but if you really want to know measure your loaded round, for arguments sake lets say it's .267, deduct .001 from that number, you need a .266 bushing if you want .001 tension. As far as the pin gauges go, again measure your loaded round, then find what pin will just fit your chamber, again deduct the loaded round from the pin that just fit, say it is .269.
.269-.267=.002 total neck clearance or .001 per side. I don't have the pin gauges (wished I did ) I measure a fired round and add about .001 for spring back to guesstimate my chamber dimension and go from there hope this helps paul.
Wayne.