Head Clearance and cartridges or cases?

B

Bullet94

Guest
Lately I’ve been reading about Headspace. I came across this quote on one of the threads here and I believe it is correct according to SAAMI (see below). I have a question in regards to Head Clearance of cartridges. In factory rifles with standard chambers what is the head clearance supposed to be? Does SAAMI or anyone set standards for this? How do they ammo companies arrive at a case size? I’m not asking how much to resize my cases just wanting to know how some of this works as far as set standards relating to Head Clearance and cartridges or cases.

Sorry if this is not posted in the correct place.


headspace is a characteristic of a chamber( not a case, or ammunition). it is specified on the SAAMI chamber drawing( and not on the cartridge drawing). it is a hard number plus some small amount(0.010 or so). the difference between the chamber min and the cartridge max is called clearance. think about the bolt and the reciever.....by design there is some clearance to allow movement, but in benchrest it is very small.
 
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To answer you in a sort of broad manner.........chambers have to be from
.000 headspace to a plus number in length, say .005 as an example. Then the ammo manufacturer has to make the cases shorter than .000 to be sure that the cartidges will chamber in a rifle that may have been manufactured at the .000 headspace condition. I'm not sure what their exact minus tolerance is, but I can tell you that it is fairly generous.
 
There are minimum and maximum chamber dimensions... (the rifle makers use)
There are minimum and maximum cartridge dimensions... ( the die makers use)

Occasionally you have an overlap that is a problem and you can not chamber a round (pretty rare)...

Reloading dies are made to a minimum and maximum specification...

Often with a maximum size chamber and a minimum size die problems occur when the reloader oversizes and does not realize what they have done (It goes in the chamber fine)...

Occasionally with a minimum size chamber and a maximum size die you can not size a case enough...
 
to answer your question, the same standards that exist in chambers, have a coresponding standard drawing for the cartridge. while in most cases the two never have an interference fit, there are a couple of notable exceptions. .30'06 is an example of a 0.0039 "potential" interference fit. smart ammo manufactures make sure thier ammo does not come with in .004 of the min spec.
( headspace only exists on chamber drawings,not on cartridge drawings. head clearance is the design or actual clearance between the bolt face and the base of the case).

lots of people use the term incorrectly, but "know" what they are talking about.
 
mike in co
The quote I used in the OP I believe was yours. Thanks for using and explaining the correct term (Head Clearance). I had read it in SAAMI’s glossary before and have had people argue with me about it even when given a link to SAAMI’s glossary.


Dennis Sorensen
I hadn’t even thought of the die makers, good point.


Thanks to all that replied.


.
 
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