Reading SAAMI spec sheets

[precision.riflecraft]

Is there anyone here who can help me learn to read SAAMI spec sheets?

I'm trying to figure out the headspace dimension of .300 PRC, from the case base to the datum line. I want to use my headspace comparator to measure my new Lapua brass against the SAAMI spec using my .420 bushing.

This drawing indicates 2.204" (so far, so good), but that's followed by - .007 (55.98 - 0.18). Anyone know what that second part means?

 

[RegionRat]

It means that the length dimension range to the B diameter ( the 0.420" diameter) is 2.204" - 2.197"

 

[std7mag]

2.204" minus 0.007".
So your distance as above could be anywhere from 2.197"- 2.204".

The numbers in parenthises is metric. 55.98mm minus 0.18mm.
So 55.80mm to 55.98mm

 

[Dusty Stevens]

Thats not a plus/minus tolerance thats only minus. So they are saying it can be .007 shorter but zero longer

 

[fkimble@charter.net]

The comparator is not an exact measurement instrument. It is for comparing the difference in two measurements. ie: new brass vs fired brass, or fired brass vs sized brass. If you have a GO guage you can measure it on your comparator to get a differential or offset number to use or use to "set/zero" your comparator to zero.

Frank

 

[precision.riflecraft]

It means that the length dimension range to the B diameter ( the 0.420" diameter) is 2.204" - 2.197"

Thank you! That makes sense.

 

[precision.riflecraft]

2.204" minus 0.007".
So your distance as above could be anywhere from 2.197"- 2.204".

The numbers in parenthises is metric. 55.98mm minus 0.18mm.
So 55.80mm to 55.98mm

Cheers, the previous commenter explained the - .007, but your response clears up the parentheses too. Much appreciated.

 

[precision.riflecraft]

Thats not a plus/minus tolerance thats only minus. So they are saying it can be .007 shorter but zero longer

Cheers, that makes sense given the purpose of headspacing.

 

[precision.riflecraft]

The comparator is not an exact measurement instrument. It is for comparing the difference in two measurements. ie: new brass vs fired brass, or fired brass vs sized brass. If you have a GO guage you can measure it on your comparator to get a differential or offset number to use or use to "set/zero" your comparator to zero.

Frank

Thanks Frank, that's a good point. I wanted to compare my virgin Lapua brass to this SAAMI spec before firing it, then comparing the fired dimension to that of the virgin.

 

[Doom]

To be technically correct the dimension shown is not headspace. Heasspace is defined in the minimum Chamber drawing. The dimension in question is simply the dimension of the cartridge to allow the maximum cartridge to mate with the minimum headspace chamber.

 

[6bra1k]

so lets clear up several things
new brass is TYPICALLY UNDERSIZED. to ensure it always fits the first time. you may take a REFERENCE measurement..but that is all it is
the the MINUS number is "0" (zero) when setting headspace when chambering a rifle 2.200****
your chamber can be anywhere between the 2 numbers and its headspace is in spec. custom rifles are typically at min/0.
EDIT
MY IDEA IS CORRECT, THE WRONG DRAWING IS BEING REFERENCED,,,CHAMBER DRAWING IS THE CORRECT ONE

 

[BoydAllen]

There is no guarantee that any comparitor will hit the shoulder exactly on the datum shown in the drawing. They are used for making comparisons, hence the name. If on the other hand you have a GO gauge then you can make comparisons to it.

 

[AlNyhus]

There is no guarantee that any comparitor will hit the shoulder exactly on the datum shown in the drawing. They are used for making comparisons, hence the name. If on the other hand you have a GO gauge then you can make comparisons to it.

This ^^^^^^ -Al

 

[AJC]

To be technically correct the dimension shown is not headspace. Heasspace is defined in the minimum cartridge drawing. The dimension in question is simply the dimension of the cartridge to allow the maximum cartridge to mate with the minimum headspace chamber.

You mean the chamber spec not the cartridge.

 

[Edd]

Thanks Frank, that's a good point. I wanted to compare my virgin Lapua brass to this SAAMI spec before firing it, then comparing the fired dimension to that of the virgin.

The specs in the drawing above are for the brass. They are the SAAMI minimum and maximum dimensions for new brass.

The chamber dimensions are shown in a different drawing.The headspace specs are shown in the chamber drawing. The actual headspace dimension spec is 2.200" + .010"

 

[RegionRat]

Thanks Frank, that's a good point. I wanted to compare my virgin Lapua brass to this SAAMI spec before firing it, then comparing the fired dimension to that of the virgin.

It is easy to give advice to spend other people's money..... but if you are going to stick with this game and dedicate to good calibers, that advice from @BoydAllen to get yourself a chamber Go Gage is going to come in handy as you cycle more examples of chambers/bbls over the years.

Having a reference standard to set up your comparators can be very handy.

With just one gun, your idea still works fine and is an important concept to grasp early. How brass behaves with cycles is important. If you were to zero on your virgin Lapua, then check after several cycles of neck-only until closing the bolt indicated the brass was getting tight, you would see the behavior.

Just be sure to maintain that virgin setting if you don't have the Go Gage, in case the tool is disrupted in the time it takes to cycle the brass.

Also learn to take micrometer readings to the nearest 0.000X" on the 0.200 line and the shoulder as best you can and keep good notes. There is some taper in most cartridge designs there, so play with these concepts for a while since it makes the micrometer anvil contact important. Good Luck.

 

[Walt Krafft]

Edd made a VERY important point. The first print is of the cartridge. All of the dimensions are designed so it will fit in a chamber. The print for the chamber has completly different numbers. A GO headspace gage will be of the minimum chamber dimension.

 

[NZ_Fclass]

I suggest that 2.204-2.207 is the correct interpretation. It's not a minus, but the range of tolerance in the dimension.

 

[Doom]

You mean the chamber spec not the cartridge.

Yep, edited post.

 

[Doom]

I suggest that 2.204-2.207 is the correct interpretation. It's not a minus, but the range of tolerance in the dimension.

Sorry but it is a negative tolerance. the dimension is 2.204" maximum to 2.187" minimum. SAAMI Cartridge Dimensions are Maximum, so no positive tolerances, except one that is the grove in front of the rim..