So on my rifle, I had a gap of about .002”-.003”, bought some +4 rollers that brought it up to .006”... within "spec". Measured via slamming the bolt home to a closed chamber, and dry firing (dry firing makes no difference on gap for my rifle btw). But If I use a round, a sliver bear one, and let the bolt slam home, I get a gap around .009-.010. The HK manual doesn't specify if gap is checked empty or loaded or dummy loaded.
Below I’m explaining how I see this works… I make some assumptions, and it seems to make sense to me, but don’t “use” this to determine if its safe to shoot because I’m not 100% sure of what I’m saying is so (thus the post and questions)
Most diagrams I seen, show, or seem to show, the bolt resting on the barrel when closed with a round in the gun. My rifle clearly does not, or the round I'm using is out of spec, BUT it seems, to my experience most gun’s bolts do not rest directly against the barrel, my CETME is the same way. Too me it seems more logical to check gap with a round in it, as that is how it will be set up when it is fired, not empty. As from my understanding the gap is a measurement of how the bolt is locking up, too little gap, too much lock up; too much gap, very little lock up. From what I have researched, low bolt gap will lead to cycling problems in the extraction area, where too much and it won't lock up, causing a violent rearward movement of the bolt and burst casings.
A very slight movement of the bolt translates to a larger movement of the bolt carrier in these guns (thus how the mechanism works), so therefore my bolt is very close, but not touching the barrel, since gap changed only .004 with the bolt resting against the barrel vs with a round. I read the lever ratio is about 1:4 between the bold and carrier, so my bolt sits about .001" from the barrel with a round in it. I would understand that this would give you and idea how far the bolt is from the barrel, and thus how much of the cartridge case is supported or unsupported. I believe headspacing and gap are different things, but related to each other. I think to get a real accurate measurement of “headspacing” by which I mean the distance from bolt to barrel, you would have to measure with out a bullet, and then with, and take the difference and divide by 4.
But I don’t know what the safe range is exactly. Just guessing looking at headspace gauge lengths: 308 win. GO-1.630” and NO-GO 1.634” (Field Reject 1.638”) the difference of .004” (or .008” with F.R.) which, assuming GO means the bolt is touching the barrel or nearly so, that means it can have .004” distance from the barrel to bolt, or unsupported case. For 7.62x51: GO 1.635, Field Reject is 1.6455” difference of .0105” much more than 308, and therefore why the 7.62 have thicker brass. I’m assuming all rifles gaps change with a round chambered, which I don’t know if that is true, also my extractor does not extend past the bolt face, I have heard that some do?
(by my assumptions) This would mean that, for example, that as long as your change in bolt gap is less than .012, then your headspace is in range for 308 (bolt-barrel distance would be 0.000-.004”), if change is less than .02”, but total gap is less than .02” (well .0105*4=.0420” but total bolt gap needs to be less than .02 for proper and safe operation) then it would be in range for 7.62. If there is no change in bolt gap (but still have bolt gap) then you would have a space between the barrel and bolt of 0, meaning the bolt is resting on the barrel, so either the round is short, or the chamber is long (or extractor extends form the bolt face, good I would think if it doesn’t extend more than .004”… I think). If no change in gap, and no gap, then something is worn out of spec. Remembering you still want you gap to be in the .004-.020 range for proper and safe operation of the locking mechanism.
To me, though, thinking about it, IF im right, then this system has a fairly large room for different length or cartridges, basically making the rifle “automatic headspace adjustment” within a range, though small, always locking the cartridge in tight. Even though the 308 and 7.62 have different GO gauges length, the CETME system will adjust to it, and could be checked as I said above.
This would also play into the ground bolt not being safe, as you may get a good headspace (change in gap) the parts may not be locking up tight, allowing play in the system which could, when under pressure, cause the headspace to increase rapidly leaving too much casing exposed, initiating self-disassembly.
I'm mostly curious about this as I read about 7.62x51 vs 308 in guns, one of the concerns being the headspace is different between the two, the Nato chamber can have more headspace than a 308 before it is no longer safe. It seems a lot of this is discussed in respect to typical operating guns (ie M14, FN FAL) but the CETME/HK are different in this respect as they can, in a way, vary the headspace they have depending on the cartridge. I’m trying to better understand how headspacing on CETMEs works.
Maybe... this may be asking too much, those out there with cetme's particularly ones on the edge of gap tolerance, tell me what their gap is without a round, and (if you feel comfortable and safely... maybe do it at the range) see what your gap is with a loaded round or better yet a dummy round? Also if your extractor extends past the bolt face and if so by how much. I'm curious if what my rifle does is normal or not, and what it does to gap, if anything.
Ugh... if you read all of this kudos to you... lol my head is starting to hurt... I'm not 100% about my speculations, so correct/debate me on any point I've made... plus I’m a bit tired... I want to understand this system more.