Headspace with .308 Winchester question.

[amlevin]

First the "players".

Rifle=Remington 700 5R Milspec
Brass=Mixed but mostly Winchester and Lapua (segregated by brand)
Loads=in the upper end of published load range for powder used


Now he question. How much can one expect brass to take the shape and length of a rifle chamber. How close to the actual headspace in the rifle would one expect to see when measuring a fired case using a Hornady Headspace Tool?

Cartridge drawings I have show the "headspace" dimension to be 1.634" when measured from case head to a point on the shoulder that is .400" in diameter. When I measure my fired cases I get 1.627 +.000/-.001" depending on when I annealed the case last.

Is this average for this cartridge, to see a fired cartridge to be .007" below the "design headspace"? Never really gave it much thought as I just routinely bump shoulders back .0015"-.002" and everything seems just fine.

Based on the number or rounds down this barrel (LOTS!) if anything I would have expected the measurements to be up at or greater than design headspace.

An inquiring mind wants to know, problem? No problem? Any information is welcome.

 

[fdshuster]

amlevin: "Based on the number of rounds down this barrel (LOTS) if anything I would have expected the measurements to be up at or greater than design headspace".

I may not understand your question (won't be the first time), but headspace length never changes, except for the few thousandths that occur with firing & proper sizing. The number of rounds fired effects throat length, not case headspace length.

Headspace length will vary from one chamber to the next, depending on the dimensions on the reamer & the gunsmith who cut the chamber. I have 6BR chamber lengths, all cut with the same reamer (mine) that vary from 1.152" to 1.159".

Allowing for the usual .001" of springback my fired case dimensions are close to being identical to those shown on my reamer print drawings.

 

[memilanuk]

What you are seeing with that comparator is a relative measurement, not absolute. The 1.627" is possibly to a very slightly different point on the shoulder than in the print, and some headspace comparator inserts have a radiused edge on the hole, etc. I never really 'got' this until I took my comparator and measured my actual headspace gauges (marked 1.630" and 1.634") and came up with 1.624" and 1.628" (or there-abouts). As someone put it to me at the time... I'd probably trust the tolerances of the machined steel headspace gauges over the (much cheaper) anodized aluminum inserts.

Other inserts, such as the Sinclair ones which use a regular shoulder angle rather than measuring from an arbitrary diameter on the shoulder, drive this point home as well. All you need to worry about is a) what is the measurement (headspace) of a fired case and b) what is it after sizing. That, you can control. You can't do anything about the chamber, and as long as it passes a GO gauge (and preferably not a NO-GO) don't worry about it.

Monte

 

[243winxb]

1.630" SAAMI minimum chamber. See drawing. http://www.saami.org/PubResources/CC_Drawings/Rifle/308%20Winchester.pdf It may take 3 full power firings, neck sizing only, to get a true measurement from your fired brass. Or when the bolt closes hard on chambering.

 

[broncman]

I have a factory savage 308 ftr rifle. I use the redding shellholder bushings and have to use .006 to bump .0015-.002 on my brass. I have wondered what the saami spec is for factory brass vs saami spec for chambers?

 

[gotcha]

Memilanuk, Very well stated. ........... Not trying to appear the expert. Just learned this lesson through experience & input from others more knowledgeable than I. Good input from 243winxb too.

 

[PALarson]

Headspace is a nominal figure, as has been stated as long as a go gauge passes and hopefully a no go gauge doesn't. Surprisingly even if the head space is excessive proper fireforming brass and sizing properly (in other words to size to the chamber, not SAMI specs one would be fine. The Hornady headspace tool would assist in accomplishing this. There is some schools of thought that if one would need to back off the sizing die to maintain the chamber head space then shims should be used. I am not sure if this procedure is needed, if I have a situation of this type I back the die off until the shoulder is bumped about .002 and have been very successful with this method fwiw.
Paul Larson

 

[Catfish]

Having the exast head sapce is one thing I never worried about. If the round chambers and goes off the case will head space itself. When I set the head space while chambering a barrel I use a dumby round that I loaded myself and I have had no trouble yet. When to move brass, whether sizeing or fireing, it will spring back some. When you have to bump the shoulder back it is caused by the bolt flexing back when the round is fired, this happens when fireing rounds at near max preasures.

 

[bigedp51]

Below might be slightly off topic but it is funny when talking about cartridge headspace and Mr.Salazar's comment.

"I have very often found that unfired brass gives great accuracy despite the sloppy fit and often poor neck concentricity. I wish I could explain that, but I can't. Along those same lines, a world-class long range shooter whose opinion I value greatly once told me that the case should fit the chamber
"like a rat turd in a violin case." ;D


"ThetRifleman's Journal"
Short Headspace on New Brass
by Germán A. Salazar
http://riflemansjournal.blogspot.com...new-brass.html