Hi
@sacnho
Lets not assume the worst and help you out.
I am going to assume you have a barrel that is finished, chambered that means it has an extension installed and already ported. There is no way to make a head-space assessment w/o a breach
and the bolt.
I would be surprised if any company leaves anyone hanging out to dry. Head-space and other chamber problems are not frequent but do happen so I am glad you are taking proactive action to
verify these simple things.
This is what I would do:
- Contact the manufacturer and tell them your barrel didn't pass the no-go headspace test and ask them what is the actual chamber and reamer used for your rifle. Knowing what reamer and whether
the chamber was cut with a Saami 308, 7.62x51 or any other variant, will help. Also ask them if they can sell you the bolt they used for headspace. bolts are not the same dimensions although
the tolerances should not be that different. The bolt manufacturer can also make mistakes. The critical dimension for the bolt is from the bolt face where the case si supported to the back of the
lugs where these contact the extensions. Any variances here will have to be subtracted / added to what you have now. you might want something longer on this critical dimension.
- Once you have that information you can start doing some additional assessments. It might be as simple as using a different bolt or in fact there is a situation with the chamber or a compounding
effect of both.
- The headpace for the 308 SAAMI chamber is the same as for any of the other offspring calibers and taken from the datum of the shoulder all the way to the base of the bolt when in battery.
This is why the same gauge is used for 243, 260r, 7mm/08, 338F and 358W among a few others.
In terms of SAAMI the cartridge will fit withing 10 thousands tolerances from 1.630 to 1.640 with a 1.633-1.635 being pretty typical dimension.
Old military rifles with 'loose' NATO chambers will go up to 1.640 and beyond some rendering a dangerous situation specially for thinner walled 308 ammo vs. nato ammo.
- you might find out the dimension of your no-go gauge but will not help you determine the level of safety w/o more gauges so lets not get too deep into more values
that you might nor might not be able to measure. By getting a new bolt with just 3 extra thousands in that critical dimension you might have reduced the headspace from, lets
say for example, 1.640 to 1.637 and enough so the chamber will not take the no-go gauge. Still might not be the most accurate chamber but you will feel better about this.
- While you wait for guidance from the manufacturer and/or a new bolt you can do some additional checks if you want. You might be repeating this and if nothing else
practicing and learning what to expect. The best way to do this with the barrel off the receiver and a stripped bolt w/o the ejector plunger although you might leave the
extractor as you will be inserting the bolt manually. And this is the perfect way of doing it because you get a feeling for the bolt rotation and friction and if there is a difference
and it will jiggle and you will feel it move in and out when in battery.
- First thing is to make sure the gauges are very clean and dry as well as the chamber. Because you are getting a loose fit vs. tight chamber effect I am going to say this is not the
issue but is always good to check this to make sure no debris are interfering with your assessment.
- Aside from the gauges get yourself a couple of rounds of different brands including some NATO surplus and also a dummy formed by your own dies. If you can make the dummies as close
as the real even put some grains like rice or buckwheat to simulate a compress charge and seat a typical bullet to standard SAAMI load. This dummy might come handy other times in the future.
- If you have more than one bolt do these steps separately for each bolt and take notes. If you are getting a new bolt you will repeat this anyway.
- If the manufacturer is sending a new bolt from a specific brand or you order it aside you might choose to wait until you check chamber again.
-The first thing is you do the go gauge pretty much like you have done before. The bolt will rotate and close with your go gauge.
- 2nd try your dummy round slowly and then the bolt should rotate into battery and might feel the slight jiggle like with the go gauge.
- If you want try other rounds see if you appreciate any differences in the fit. you might prefer to avoid this because the issue here
is too much headspace and possibly well past the point this brass will fit snug.
- Last will come the no-go gauge and if this closes but you feel fit but no jiggle this means the current headspace is the same or pretty close to the no go gauge. I think this is what you
have experienced. At this point the chamber might be safe to use with most brass even commercial. Military brass is thicker at the web to counter the effects of 7.62x51 chambers.
- Keep in mind that while a chamber that closes in the no-go gauge might be still within SAAMI max spec but also the chamber will stretch a little after some use and in any case you will
be stretching that brass that will not last as long and perhaps potentially brass could fail by head separation after several reloads. Also there is a good chance accuracy and consistency
will suffer than in a shorter chamber.
- You cold acquire a field gauge to see if the firearm passes this test but to be honest even if this passes you will have to repeat the test after some use and it is less than ideal for a brand
new barrel anyway.
- If you do not have a field gauge but you have some calipers this is what you can do. With a tiny piece of scotch tape add this piece to the base of the no-go gauge. you might have to clean the
gauge with some alcohol for the tape to stick. then with a razor cut around the base so what you have done is adding 1.2 to 1.5 thousands to that gauge. This is something I do with the actual
brass to headpsace from the rounds I am shooting but you are well past this point so all we are trying to do here is to figure out if the no-go will still close this way. Please measure the
scotch tape with your calipers to make sure about thickness. this is not a precise gauge but will add some minor length. then proceed slowly to check again and you should not be able
to close the bolt. If you do, do not fire this barrel and talk to the manufacturer or you might choose to wait until you get the bolt or whatever they suggest.
- I could give you more directions on how to achieve a tight yet reliable chamber but unless you have a lathe and other tooling this will not be possible. A chamber can still be fixed in most
cases but will require now to disassemble the bolt, cut extra length form the tenon and index the extension to be aligned with the gas port after torquing that is a lot more tricky and then
ream a few thousands using the same chamber reamer.
- So see what they say and test other bolt or whatever the manufacturer suggest. You might also ask someone who has one or more 308 bolts if you can borrow them to measure critical dimensions
and test head-space.
-Also while you wait keep in mind that while a chamber might pass a head-space test it doesn't mean that the clambering is done right nor it is safe to use so do a visual inspection with a flash light
will show any obvious issues and for others will need to spend more time. Do a drop test of the round and make sure the round doesn't jiggle side to side and also the tenon is long enough to
give support all the way ahead of the rim. A mistake cutting a tenon too short could provide perfect head-space yet lead to a potential catastrophic situation. I ran into a couple of commercial barrels
chambered like this. One was actually fired and brought to me. The shooter was lucky. The manufacturer messed up big time. This should not happen with a decent manufacturer but one never knows.
- Where are you located? I am happy to bring a couple of bolts of different makers for you to test. Otherwise ask around the local gun shop or local range, you never know.
Good luck.
