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chamber drawings

10K views 13 replies 9 participants last post by  jim-analog 
#1 ·
Other than the book that Pacific sells, anyone know of any 308 chamber drawings online? This might be in the wrong section.
 
#4 ·
Note that in the SAAMI drawings, posted above, if you compare the case length from the base to the datum line on the shoulder, and compare the same dimension in the chamber, the stack-up tolerances are screwed up. An in-spec case on the slightly long side will not fit into an in-spec chamber which is on the short side. Both are in spec, but they interfere.

Dunno why, but lots of cases have this same overlap issue on the SAAMI drawings. Where I come from, that's called bad engineering, but maybe they have a special reason.

Art
 
#5 ·
I set my resizing die to get 1.630" at the datum line on the case shoulder (I use the Hornady tool for measuring at the datum line). I always assumed that the resizing dies are designed to give you about that dimension if you set them up according to the manufacturer's recommendation, it has never occurred to me to ask if that were true...so do FL dies push the shoulder back to give something around 1.630"?
 
#7 ·
Greetings,

Great info; thanks RAMMAC! Does anyone have the same type of drawing for 7.62mm? Though I understand the potential issues, I'm not quite sure what is actually different on the cartridge/chamber themselves. Is it all a question of how the round is assembled and how the head space is set? Thanks!

Regards, Jim
 
#9 ·
I always liked this simple one to show the difference between the 7.62x51 and .308 win



especially when you're trying to explain to someone who's spatially challenged GI6
 
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#11 · (Edited)
Gus Fisher did a whole series on headspace and you can find it at
http://m14tfl.com/upload/showthread.php?t=66601&highlight=headspace

Also, if you can get a copy of Jerry Kuhnhausen's book
The U.S. .30 Caliber Gas Operated Service Rifles
A Shop Manual
Volumes I & II
pages 43 & 44 have specification drawings for the 7.62 and .308 cartridges and chambers

A larger picture of the 7.62 specs


That simplified drawing in a previous post of the chamber differences between a 7.62 and .308 chamber can cause a little confusion without some explanation. The headspace values are measured at the same point on the shoulder of the cartridge and the chamber on both the 7.62 and the .308 but the specs are slightly different. The cartridges are basically the same (with a few minor differences here and there) but the chambers are different enough to cause problems with the 7.62 military chamber having longer dimensions.

Both chambers have a range of headspace dimensions that they can fall within;
  • 7.62
    min - 1.6355"
    max - 1.6405"
  • .308
    min - 1.630"
    max - 1.640"

So to ensure you know exactly what kind of chamber you have you should measure the headspace dimension on your chamber rather than take somebody's word for what it is.

There are two worst case scenarios to worry about.
The first possible case is if you have a tight .308 chamber (1.630") and you want to shoot foreign made rejected military ammo that measures the NATO recommended maximum headspace dimension of 1.634" at the datum line on the shoulder. Your cartridge will be 0.004" too long for your chamber. This could result in the neck of the case being compressed around the seated portion of the bullet (due to the neck of the case being pushed in to the free bore area of the barrel which is a slightly smaller diameter than the neck). In turn, this would increase the pressures experienced by the cartridge and chamber and you might blow out the primer which would then allow hot gases to reach the bolt face which might blow up your rifle or force those hot gases in to your face.

The other case is isn't much better. This would be where you have a 7.62 maximum chamber dimension (1.6405") and you want to shoot .308 ammo that is manufactured with the minimum headspace dimension of 1.627". In this case you would have at least 0.0135" of clearance between the case and either the chamber's shoulder or the bolt face or split between the two. In this case you would get a lot of expansion and, depending a lot of variables, you would either get very short life spans for your cases due to working the brass a lot or the case head will separate from the case and gases might blow back in to the receiver and blow up your rifle.

If you want your rifle to reliably fire both 7.62 and .308 then you want a chamber headspace dimension of 1.6355" but even with a chamber dimension of 1.631" (which is pretty common with the SAI NM M1A) you would be able to shoot standard .308 ammo and most military surplus, with U.S. surplus being preferred. Even though most 7.62 and .308 cartridges use the same specs (with a headspace dimension of 1.630" being common) there will be some foreign made ammo that will exceed that dimension and if you have a tight .308 chamber (1.630" to say 1.632") then some of that foreign military surplus will not work.

One final note, don't use any of the new high velocity ammo in your M14 style rifles. The M14 style rifle was designed for chamber pressures that are not as high as what the SAAMI standard is for commercial .308 ammo. Standard .308 ammo is safe but if you use a "magnum" style cartridge in the M1A you might just blow it apart.
 
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#12 ·
Greetings,

Thanks much for the detailed info RAMMAC! I do have the Kuhnhausen book, just didn't think to look there. My main reason for asking is not for my M14s (they are all set up for the "happy medium), but a friends' new AR in .308.

He's relatively new to this and I'm trying to help him along. I don't want to pass on any "old gunny's' tales", hence my request for clarification. What I know of his situation is the the builder set up the chamber and head space for a tight .308. This was due to my friends initial concern about potential accuracy. I've asked for the final head space dimension, but have not received an answer yet. It wasn't until later that I brought up the ammo cost and surplus availability issue. The builder stated that absolutely no way should any 7.62mm ammo be used.

Thanks again, I'll pass this all along.

Regards, Jim
 
#13 ·
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