Re: Neck tension question
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JimGnitecki</div><div class="ubbcode-body">I did some measuring via digital caliper last evening. Very interesting results.
First, Federal Gold medal 308 175g comes with only .0019" neck tension. Hornady 175g comes with about .0026". These have funcitoned well in my LMT MWSE semiauto, so I guess .002" neck tension IS enough.
But here's the really interesting part. I measured my pre-firing and post-firing handloaded Lapua cases.
The pre-firing neck wall thickness was .0159" (consistently) and post-firing neck wall thickness is .0160", again consistently.
The neck OD of the brand new Lapua cases was .3364" (very consistently).
I used a .336" bushing in my Redding sizing die.
This implies projected neck tension = .336 - .308 bullet - 2 x .0159 wall thickness = .0038".
However, the actual measured neck OD after bullet is seated is .338", not .336". So, neck tension actually is .0018", not .0038" as planned.
I guess the case stretches enough furing the bullet seating operation to gain .002" in diameter.
That makes sense, as the case wall thickness of only .0159" is no match for the solidity of the .308 bullet, and one of them HAS to give when they are forced togetehr in an interference fit.The .002" gain in neck diameter is only .002 / circumference of the .338 circle = .002 / 1.0619 = two-tenths of one percent.For virgin annealed brass, that is not hard to do.
The case neck epxands during firing to "fill the rifle chamber", and then relaxes just a bit. The measured neck OD after firing in MY rifle is .3456". This is consistent with the post-firing OD I got with both the Federal Gold medal and the Hornady.So, the case neck diameter of the Lapua cases was expanded by .009" during firing, because the chamber in my rifle is that diameter (or a bit more I guess)at the neck, since it is a semiauto rifle and MUST feed reliably with no chance of firing out of battery or bullet setback.
The case diameter just below the shoulder grew .0028" and near the base it grew .0039", after shrinking back a bit after firing. This seems reasonable too.
I learned too that my combinaiton of chamfering via Wilson / Sinclair trimmer with chamfering tool with micrometer set for 2.005", followed by HAND deburring with the Wilson hand tool, must have been, in inadequately controlled COMBINATION, too agressive. I say this because the case OAL actually SHRUNK between virgin and post-firing length by an average of .005", and varied by .0115" from shortest to longest! I think I did the chamfering right, using the micrometer and locking the setting. But, that hand deburring must have been aggressive enough to not just deburr, but to actually "encounter" the chamfer. Once it encountered the chamfer, any subsequent additional "deburring" became aggressive (because then deburring a "knife edge" surface), and unpredictable and unmeasured, SHORTENING of the case length! That could not have been good for consistency, so it is a msitake I will not repeat. From now on, ONE turn of the deburring tool only!
I don't know whether to stay with the .336" bushing, or to go to a .335" bushing to get just a bit more neck tension (closer to .0028" than the current .0018"). I can't see .0028" being too MUCH neck tensino for a semiauto, and it would set my mind at ease on preventing bullet setback in a fully loaded magazine or during feeding.
Opinions?
Jim G</div></div>
A collet bullet puller, your press, and a fish scale in lbs.