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Posted: 9/8/2003 1:30:28 PM EDT
This might be an odd question, but is there anybody in Pittsburgh that can check headspace on my 20" upper for me? I just replaced the bolt carrier, and would like to check it before it blows up on me. I can't afford guages due to my recent unemployment.

And 'smiths or board members in Pitts who can do this?

The Cheat
Link Posted: 9/8/2003 2:01:21 PM EDT
Cheat; The bolt carrier has nothing to do with headspace. Cheers!
Link Posted: 9/8/2003 2:22:18 PM EDT
Sorry I ment Bolt carrier assembly The Cheat
Link Posted: 9/8/2003 2:38:08 PM EDT
It won't blow up.
Link Posted: 9/8/2003 3:17:34 PM EDT
Ok, then what will happen if the headspace is not correct? Pardon my ignorance, this is my first build. The Cheat
Link Posted: 10/20/2003 1:42:17 AM EDT
[Last Edit: 10/20/2003 3:38:15 AM EDT by Tweak]
The_Cheat, I'm glad you asked. OK, I was looking for threads dealing with HS for the Troubleshooting FAQ and found your unanswered question. To answer your question there is a good chance (say, greater than 50%) that your HS is fine. The HS of your rifle is prolly good too.[:D] [b]But[/b], you do not know if the parts (barrel/bolt) are correct until you check the HS. If the HS is off the rifle may not function perfectly. A short chamber (tight HS) can cause failure to eject due to the higher chamber pressures the interference fit creates. Very worst case scenario, and the chances are incredibly remote, is a kB. On the flip side if the chamber is long (long HS) your accuracy can suffer, the life of your brass may be shortened, you may even see shortstrokes because the case cannot fully seal the chamber when the case obturates. You could see case head separations or blown primers and, on the other end of the sprectrum from tight HS kBs, there is a very remote chance you could get a kB because of a case head separation. Now, all this fearmongering aside, the reason to check the HS on your rifle is that the HS on every fiream must be checked after it's assembled. Since you are in effect the assembler it is your responsibility to check it. As I stated earlier, if you do not check it you do not know it's status. Only a puke would build a firearm that sloppily. You wouldn't replace the engine in a car then hand it back over to the owner without bench testing it at least would you? The people that say, "don't worry about it the parts are all in spec" are saying, in effect, "it's a crate motor, of course it'll work." So, now on the next pile of BS I've been reading on these HS threads. There seems to be an idea that you cannot use .223 gages in 5.56 chambers. This is complete garbage and shows a complete lack of understanding of the factors in play. HS gages measure one thing and one thing only, the length from the face of the bolt to the datum line on the shoulder of the chamber. In the case of the 5.56/.223 the datum line is the 0.330" diameter about halfway up the shoulder. While .223 and 5.56 are different chamberings they are the same cartridge hence the datum line is in the same location. The problem with using .223 gages in 5.56 chambers is that the HS standards vary depending on the model of firearm. In general, self loading pieces need longer HS to operate than manual loaded pieces do and fully automatic pieces need still longer HS to continue to function especially after they heat up. Self loaders need the extra HS because they are trying to unlock while the case is still stretched. You won't encounter that problem with a bolt action because you can't physically open the action that soon after pulling the trigger. Let me throw this in here now. You cannot use a loaded round or a fired round as a GO gage for the simple reason that you won't know which part ofthe case is hitting the chamber and the GO length is larger than an unfired round to allow the round to expand and seal the chamber and smaller than a fired round because the round did expand to fill the chamber. Clear as mud? If you use a case you might start feeling resistance to the bolt closing as soon as the bolt touches the rear of case. Does that mean the HS is shallow? No, it means that the base of the cartridge swelled on firing and needs to be forced into the chamber to fit. You'll notice that HS gages do NOT look like cartridges, they are cylindrical, not tapered, for exactly this reason. Remember that all important datum line? Whether the case is fired or not, there is no guarantee where the datum line is located on a cartridge case, it's not a critical dimension during case manufacture because the case will swell to fit the chamber during firing and make up any slop. Now, because the military doesn't like to see mistakes made they have a special set of gages that take into account the extra HS the M16 needs to operate. The military also has gages for the M249 SAW (also a 5.56 chamber) and they are even longer than those for the M16. So, the thing to remember is that the GO (minimum) gage, be it military or civilian, is the same size. It's a SAAMI minimum, 1.464" or 1.4646"+0.0002" for the military. So, you'll need a GO gage of this length. Now that we've established what the minimum length of our chamber is supposed to be what is the maximum length? Well, the military rejects HS when it reaches 1.4730"-0.0002". BUT, there is a a FIELD II gage out there too, it is 1.4736" long. You can see there is some latitude on what is too long. FWIW, the FIELD gage for the M249 SAW is 1.50216"-0.0002", quite a bit longer than that for the M16 and yet it shoots the same ammunition without difficulty. New rifles should chamber a GO and and not chamber a NOGO, the military NOGO dimension being 1.4706". Here is where the problem arises, a civilian ".223" NOGO gage is 1.467" and a civilian ".223" FIELD gage is 1.470". Both of them less than what the military considers to be NOGO HS. Almost every military rifle I have checked would accept a civilian GO [b]and[/b] NOGO gage, a very few of those would accept the civilian FIELD gage. What does all this mean? It means that you need a GO gage to check that your chamber is the proper length to load, fire and extract correctly. You need a FIELD gage as it is the longest HS allowable. In between those two dimensions there is a lot of room for discussion. New rifles, or bolt/barrel combos are checked with a GO and NOGO. Rifles that have seen service, and have already been checked for the GO dimension, are checked with the FIELD. A civilian FIELD (1.470") gage is close enough to a military NOGO (1.4706") to be used in its stead. Edited to clarify, quantify, and sassify; A new rifle or barrel/bolt combo must accept a GO gage, military 5.56 or civilian .223. It should not accept the military 5.56 NOGO gage. It might, but most likely won't, accept the civilian .223 FIELD gage. It must not accept the military 5.56 FIELD. It probably will, but may not, accept the civilian .223 NOGO gage. Check annually with a military FIELD gage thereafter.
Link Posted: 10/20/2003 2:46:30 AM EDT
So, now that you know what you need the next topic is how do you use the info? Military HS gages, with the exception of the FIELD gage a solid base and an extraction groove. If you have HSed a lot of rifles you can get by on "feel" if not, you'll want to either modify the bases of the civilian gages to clear the ejector and remove the extractor or remove the extractor and ejector when using the civilian gages. The added tension of the extractor and ejector springs could give you a false reading if you don't know how hard to close the bolt. So, take the upper off the lower, mop out the chamber with a clean, dry patch and retract the carrier enough to drop in the GO gage. Remember, you'll want the ejector out and extractor off the bolt for the truest reading. If you're going to leave the extractor in (I do) I suggest you ensure that the ejector can fully telescope below the face of the bolt before checking the HS. Also ensure that the extractor can move aside far enough to accept the base of the case into the bolt face [b] with the bolt in the locked (pushed into the carrier) position.[/b] Slowly close the bolt by pushing on the rear of the carrier, use only your thumb to do this and push lightly. If you're lucky the bolt will click shut. If you're luckier it will open again when you pull on the charging handle. The bolt should close fully without force and more importantly open without force. I've seen a couple of jokers in my short time here that locked the bolt to the rear, dropped in the FIELD gage and pushed the bolt release. If you do that every rifle you check is going to have excessive HS. [:D] So, if the bolt closed fully without resistance and you didn't need to take a screwdriver to the underside of the bolt carrier to get the bolt to unlock your HS is [b]at least[/b] minimum length. Repeat the procedure with the civilian FIELD gage, it's being used as a pseudo NOGO, or use the military NOGO if you have one. The chamber may (very rarely) take the civilian FIELD and should not take the military NOGO if the barrel/bolt is new. Go ahead a press a little harder on these gages than you did the GO gage just to be sure. Once again, if you have to pry the bolt open then the gage is too long for that chamber or the HS is too short for that gage. If the chamber takes the civilian FIELD or the military NOGO then repeat the process using the military FIELD gage. Here's the catch, if the chamber easily accepts the military FIELD gage and the bolt opens without difficulty then you have no idea how long the HS is. You do know the the HS is [b]at least[/b] as long as the FIELD gage is. The HS could be 2" long for all you know. So, what do you do if the HS is short i.e. won't accept the GO gage? Well, first step is to recheck the chamber with the gage, make sure the ejector seats beneath the face of the bolt and the extractor has enough range of motion to allow a cartridge to properly seat against the bolt face. Make sure there are no burrs or fouling on the gages or the bolt face. Mop out the chamber with a clean patch again. Repeat every step again paying closer attention. Did you get the same reading? So, you did get the same reading. First step is to swap out the bolt. Same deal, strip it, inspect it, clean it etc. Check again using a different bolt. Here's the catch, there is only 0.003" tolerance on the HS dimension on the bolt. What is the HS dimension on the bolt? It's the distance from the rear of the locking lugs to the face of the bolt. So, if the bolts are proper you can only move the HS 0.003" by swapping bolts even if your two bolts are on opposite ends of the tolerance. Now look at the numbers for the gages again, there is about 0.003" between the gages whether civilian or military so at most you can change the HS by one gage through swapping bolts, all else being equal. If you're more than one gage from where you need to be, and the barrel is chromed, then this is where you're looking at buying another barrel. If the barrel is not chromed then you can use a chamber reamer to move the shoulder in the chamber forward, thereby moving the datum line forward and increasing the HS. If you've never used a chamber reamer before go VERY slow, take tiny passes, and do not push on the reamer while you're turning it. And yes, if you stick a 5.56 reamer into a .223 chamber you will be able see, hear, and feel the difference in the chamber dimensions as the reamer cuts the larger freebore and neck. Before swapping bolts, buying a new barrel, or reaming the chamber make sure that you double check your work. Messing up this part of your rifle can get expensive quickly. If the HS is long i.e. it takes the military FIELD gage you have a couple of options. The first is to swap bolts. If the HS is long it's not going to matter if there is fouling in the chamber or a burr on the bolt face as once this extra length is removed the HS will be DEEPER. If swapping bolts doesn't work you have one last option. You can take the barrel into a qualified smith and he can remove the barrel extension and cut back the end of the barrel and the shoulder that the barrel extension seats against. You will then need to find a different extension that times up correctly so that the alignment pin is at top dead center and properly aligned with the front sight base when the barrel extension is tightened. The smith will have to know before he starts cutting how deep the chamber is and how much he needs to remove off the back end of the barrel to shorten the HS enough. If he cuts too much the GO gage won't go and you're back to square zero. If he doesn't cut enough the HS is still deep and you have to start over again. Finding that magic new barrel extension is easier said than done, I've gone through 5 gallon buckets of them before with no joy. There is a limit to how much you can take off the end of the barrel. As you remove material from the ends of the barrel the distance between the gas port and the shoulder on the barrel extension gets shorter. Changing that dimension causes the gas tube to stick too far to the rear and makes the handguards very difficult to remove. Some people can live with tight handguards but most won't like it when their bolt carrier doesn't close all the way.
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