Redding's Tech Line's Fallacies

[Bart B.]

Came across this site then decided to check it out. Did this one first:

http://www.redding-reloading.com/tech-line-a-tips-faqs/145-dealing-with-headspace

Almost barfed when I read:

Headspace is one of those concepts that is both very simple and yet is extremely important to achieve gilt edged accuracy. For bottlenecked cartridges, headspace is simply the distance between the head of the cartridge case (the end where the primer is inserted) and the front/face of the firearm's bolt when the case's shoulder is positioned against the front of the chamber.

That's "head clearance." If it's zero or less and binds, bolt heads don't twist back into battery the same for each shot. It opens up groups as top ranked competitors well understand.

Then again reading:

The goal of every conciencious reloader should be to use proven and practical reloading procedures in order to ensure that the bullet/case combination is as perfectly aligned with the center of the bore as possible. The more centered up the cartridge, the more accuracy we can expect. However, if the cartridge is laying loose in the bottom of the chamber because of a generous headspace dimension, it's obvious that the bullet will be pointed closer to the bottom of the bore rather than the center. Consequently, accuracy conscious shooters will want to reduce headspace to the absolute minimum i.e. where the shoulder of the case is against the front of the chamber wall and the bolt/breech face of the firearm is very close to or even lightly touching the head of the case. We can do this by adjusting the shoulder of our fired cases in the sizing process.

They must think in line ejectors and firing pins don't drive cases forward well centering their front end and bullet in the barrel. Even when there's lots of clearance. What about extractors pushing cases up off the chamber bottom and against the chamber wall somewhere else?

There are other fallacies in this article as well as other articles.

Hornady also says headspace is the gap between bolt face and case head:

http://www.hornady.com/ballistics-resource/internal

To Hornady's credit, they correctly state the firing pin pushes rimless bottleneck rounds forward into the chamber shoulder. In spite of their subscribing to SAAMI standards as members agreeing to them wherein SAAMI says:

HEAD CLEARANCE
The distance between the head of a fully seated cartridge or shell and the face of the breech bolt when the action is in the closed position. Commonly confused with headspace.

HEADSPACE
The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.

 

[Slamfire]

These guys are selling products so this is an advertisement. Something they forgot is that minimum head space ammunition is undesirable for semi autos, particularly Garand mechanisms. Which would be M1 Carbines, M1 Garands, M1a's, Ruger Mini 14's. All gas guns with free floating firing pins have slamfire accounts on the web. The Garand mechanisms all have slamfire accounts with sensitive primers, what is more perverse, is this mechanism will also slamfire out of battery given a combination of a tight, overlong, cases and a sensitive primer. By selling tight cases, someone is going to copy this for their Garand, and blow the back end off their receiver.












In terms of accuracy, I don't shoot benchrest so I cannot test the concept that a tight case is the most accurate case, but I do shoot Mid Range and Long Range. What I have found, in one rifle particularly, that I must have clearance between the case and chamber or I have to break position and knock the case out with a cleaning rod! Crush fitting cases to a chamber is a guaranteed path to a jam.

 

[stoky]

What makes this less straight forward than it seems like it ought to be, is that the datum for the measurement is at a particular diameter on the case shoulder. In the case of the 7.62x51, it's been changed (at least twice), trying to wring more accuracy out of it for match stuff.
Now SAAMI says 1.638" (max) for .308, NATO says 1.6455"(!) for 7.62x51.
This isn't the only case of ambiguities. The older the parent case, the more it seems to have been mucked with and things get even murkier with "improved" cases with a seemingly ubiquitous squaring of the shoulder and shortening of the neck.

 

[Bart B.]

SAAMI minimum, 1.630"; maximum, 1.634"

See page 110 in

http://saami.org/specifications_and...ns/download/Z299-4_ANSI-SAAMI_CFR.pdf#page=13

In the case of the 7.62x51, it's been changed (at least twice), trying to wring more accuracy out of it for match stuff.

What's the details of that?

Im not aware the USA arsenal's ever changing their original M118 or M852 match case specs. It doesn't matter where the datum is on the case or chamber's 20 degree shoulder. Normal tolerances can cause the shoulder's actual contact point with that of the chamber anywhere on their shoulder.

 

[stoky]

You're right Bart. I was referring to Field gauge dimensions, which aren't salient to cartridge specifications.
Some of what I recall came from Gus Fisher's A Little Bit of Everything To Do With Headspace:
http://m14forum.com/gus-fisher/66601-little-bit-everything-do-headspace-2.html
One admonishment was not to shoot surplus 7.62x51 ammunition in rifles with the chambers cut to minimum .308 dimensions for accuracy.
The datum changes came from an Armalite PDF (IIRC) that's apparently no longer available.

 

[Bart B.]

I don't comprehend Gus Fisher's comments about 7.62 ammo in 308 and 7.62 chambers.

I've measured dozens of lots of both service and match 7.62 ammo for case headspace; none measured greater than 1.630". SAAMI and MILSPEC minimum for their respective chambers.

Of course, 7.62 match ammo bullets were 200 to 300 fps slower than ball ammo. It shot 173's at 2550 fps and the ball ammo 147's at 2850 as they chronographed at 26 yards. Both at pressure of 50,000 cup. That USMC Master Gunnery Sargent doesn't know that about the arms and ammo he works with?

All the military 7.62 match grade rifles were fitted with bolts making chamber headspace 1.630" + .0005". If that couldn't be had with their box of bolts, the barrel was relegated to normal training and service use, a new barrel clocked into the receiver then re-bolted until headspace met specs at 1.630" where best accuracy happened Just like bolt guns.

What's the problem about surplus 7.62 ammo in 1.630" headspace commercial chambers?

There's no field gauge spec for SAAMI 308 chambers.

 

[JimKirk]

It goes back to what I said a long time ... your ammo has to fit your gun ... that is also the reason I own multiple dies set up for different guns in the same caliber ...