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Old July 23, 2010, 09:43 PM   #1
brmfan
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Am I measuring this wrong??

Okay... got a Sinclair bump gage today to help set my FLS die more accurately. Checked the SAAMI minimum headspace for a .308 and it was 1.6315". After zeroing (General Tool brand digital caliper from Lowes... just bought it yesterday), I checked an unfired SA ball, a Malaysian L2A2 ball round, and a 175 Black Hills match round. All 3 mic'd in at 1.55" ... .08" under the SAAMI minimum. The MAL round is very accurate out of my POF (16"), while the Black Hills (in 168 and 175) not as good. Also, fired MAL cases shows no carbon fouling on the outer case neck wall whereas there is considerably more on Black Hills'.

I assumed that since the BH was a true .308 it would have a bit more headspace slack than the SA & MAL NATO rounds, and would explain the difference in the amount of fouling on the case neck walls. But, to my surprise, they both had the same shorter headspace according to the gage (measured off the shoulder datum). Am I missing something? Could it be just the difference in powder that's causing the fouling variance?

Last edited by brmfan; July 23, 2010 at 10:20 PM.
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Old July 24, 2010, 02:15 AM   #2
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More likely the difference in the hardness of the brass or the pressure characteristics of the load itself. Virtually all factory ammo is undersized so that it can be chambered in a wide variety of chambers found in the real world. If it were all made to SAAMI spec, there are a good portion of guns the ammo would not chamber in.
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Old July 24, 2010, 10:57 AM   #3
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That's correct. There is no difference in new case size for .308 and 7.62×51 NATO, as the military drawings for the original T65 cartridge external dimensions were copied for the commercial .308 version. Only the chambers are different. The commercial SAAMI chamber minimum headspace is 1.630", while the NATO minimum is 0.005" longer at 1.635". The new brass maximum is 1.634". This means it is actually a 0.004" interference fit with the SAAMI minimum .308 chamber. It is normal for chambers to shorten a round a bit when chambering, and the SAAMI minimum extra width of a .308 chamber over a maximum new case width is 0.0011", which allows the case to fatten enough on chambering to let the 0.004" shortening take place. The NATO chambers, on the other hand, allow no interference and are even wider. This is to ensure smooth full-auto functioning. The military doesn't care if the brass is reloadable after firing, so they don't worry about making a longish pressure ring.

Code:
Headspace Gauges
    308 Win      vs.   7.62×51 NATO 
GO -    1.6300"  vs.  GO -    1.6350"   NATO 0.0050" bigger
NOGO -  1.6340"  vs.  NOGO -  1.6405"   NATO 0.0065" bigger
FIELD - 1.6380"  vs.  FIELD - 1.6455"   NATO 0.0075" bigger
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Last edited by Unclenick; July 24, 2010 at 11:40 AM.
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Old July 24, 2010, 12:13 PM   #4
brmfan
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Thanks for the info guys! In a nutshell then, I assume I'm good to go... I just found it very strange that SA NATO surplus from 1980, Malaysian NATO surplus from 1987, and new commercial BH .308 brass all measured almost the exact same headspace, and they were all .08" under the SAAMI minimum.
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Old July 24, 2010, 02:46 PM   #5
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How did you determine SAAMI minimum? I've yet to find any of my tools dead on. I have to stick a good quality GO gauge in and measure that for zero, then go from there. The spec is 1.6340" + 0.0/-0.0070". So it should be between 1.627" and 1.634", or +0.004"/-0.003" from zeroing on a SAAMI GO gauge, or -0.0080" to -0.0010" off a NATO GO gauge.
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Last edited by Unclenick; July 24, 2010 at 02:52 PM.
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