View Full Version : Headspace Question
Howdy,
I've yet to build an AR but have studied the process. I think I have a grasp of headspace and the importance of it.
That said I want to have some spare bolt parts for my DDM4 and of course a bolt.
I assume I would check a spare bolt exactly like I was building with go-no go and maybe a field gauge. Correct?
Also would I have a better chance of matching parts out of the box from same manufacturer of the AR? Should I get a Daniel Defense bolt and or BCG to go with my DDM4 instead of any other brand?
May as well buy some gauges...any favorites?
Thanks!
W
gunnerblue
12-15-20, 10:28
A field gauge alone is sufficient for checking headspace. Brownells carries it.
While not necessary, some prefer to match barrels and bolts from the same manufacturer. If both parts are within spec, it shouldn't matter.
Also would I have a better chance of matching parts out of the box from same manufacturer of the AR?
None of the reputable manufacturer's parts would require head space checks. A headspace gauge is useful on well worn bolts/extensions. After 25 plus years on the AR platform, I may have used my headspace gauge twice.
gunnerblue
12-15-20, 10:52
None of the reputable manufacturer's parts would require head space checks. A headspace gauge is useful on well worn bolts/extensions. After 25 plus years on the AR platform, I may have used my headspace gauge twice.
Agreed. A field gauge is cheap insurance when it comes to Murphy's law, though.
I just built two guns an 11.5in 5.56 and a 16in 223Wylde both with Faxon gunner barrels and used PSA/Toolcraft black nitride. I use gauges on the 11.5 but the 16 I used a BMC upper and didn't check before installing the barrel. Didn't feel like trying to heat up the upper to get the barrel out. But took both to the range and both worked flawlessly. Good parts are good parts.
About one in five barrel/bolt combinations in my classes and ones that cross my bench fail a go gauge.
If you don't gauge or use the right ones, you'll never see it.
A no go is not as important as a go and field. Those show the minimum and maximum.
About one in five barrel/bolt combinations in my classes and ones that cross my bench fail a go gauge.
If you don't gauge or use the right ones, you'll never see it.
A no go is not as important as a go and field. Those show the minimum and maximum.
Is disassemble of the bolt (removal of the ejector) required to properly check headspace using the go and field gages? Particularly the Brownells gages?
Of course the correct answer is that you should check headspace.
On the flip side, is there evidence of catastrophic failures caused by improper AR-15 headspacing?
If the barrel and bolt are manufactured to spec (aka quality components) the the worst case would be having both at the minimum or maximum amount of tolerance. If the headspacing is slightly short, pressure would theoretically be higher and if slightly long, brass life will suffer for reloaders.
If headspace is drastically short, the bolt won't close on a live round, if drastically long, cases could separate (headspace would have to be ridiculously long). Case separation can absolutely cause catastrophic failures.
I have seen plenty of pics of ARs that suffered catastrophic failures due to out of spec loading/ammo but none blamed on headspacing.
Note that my post only applies to .556/.223 AR-15s.
Andy
Its easy enough to dremel away a bit of metal from the headspace gauge, using a cutoff wheel, so the ejector does not make contact with the gauge.
I use a live round as my "go" and a proper gauge as "no go" Of course you can get by without gauging, but why not do it and know your shit is in spec?
If a couple/4 friends chip in $10 each, just pass it around.
Mark
Is disassemble of the bolt (removal of the ejector) required to properly check headspace using the go and field gages? Particularly the Brownells gages?
I suggest you remove the ejector and extractor from the bolt before checking headspace on any rifle or carbine. The pressure from the ejector may have an affect on your sense of feel while applying pressure against the bolt. Removing the parts gives you an opportunity to closely inspect machined surfaces and thoroughly clean the bolt. I highly recommend thoroughly cleaning the chamber before checking headspace, so you don't get a false reading.
It doesn't take much pressure on the bolt to get a good headspace reading. I've seen a few headspace gauges wrecked when people used them to check headspace on a fully assembled rifle.
Is disassemble of the bolt (removal of the ejector) required to properly check headspace using the go and field gages? Particularly the Brownells gages?
I say it's necessary to disassemble the bolt to check headspace. The brownells field gauge is a Forster product. I will admit that it can be a pain if you don't have experience dealing with the ejector roll/spring pin.
All of my gauges are modified so bolt disassembly isn't required. When a bolt/barrel combo fails, I always follow up with other gauges to verify my gauge is not off. I also check the calibration on my gauges every 6 months.
Of course the correct answer is that you should check headspace.
On the flip side, is there evidence of catastrophic failures caused by improper AR-15 headspacing?
If the barrel and bolt are manufactured to spec (aka quality components) the the worst case would be having both at the minimum or maximum amount of tolerance. If the headspacing is slightly short, pressure would theoretically be higher and if slightly long, brass life will suffer for reloaders.
If headspace is drastically short, the bolt won't close on a live round, if drastically long, cases could separate (headspace would have to be ridiculously long). Case separation can absolutely cause catastrophic failures.
I have seen plenty of pics of ARs that suffered catastrophic failures due to out of spec loading/ammo but none blamed on headspacing.
Note that my post only applies to .556/.223 AR-15s.
Andy
I have experience with AR's that have kaboomed that had short headspace. I checked them and warned the owners before the guns blew up. Sometimes there were often additional issues present that lead to the blown up gun. Ammunition can sometimes be the sole cause as well.
Diagnosing a kaboomed gun afterwards can be impossible in some cases depending on how the parts were damaged. Being conclusive about tolerances with bent and broken parts is often a futile effort. I have experienced success in proving it out in a few cases.
Short headspace can cause reliability issues when the gun gets hot or fouled. It can have extraction issues and failures to go into battery. Not all ammunition is loaded to the same dimensions or pressures so some ammo won't show a problem, and other ammo will. The point of using headspace, throat, and chamber gauges is to ensure the gun will function with a wide diet of factory loaded ammunition. It also ensures the gun will run hot, cold, wet, dry, and so on.
Short headspace can also lead to accuracy issues with some ammo, primer popping, bolt face etching, destroyed brass, and can increase the chances of bolt lug or cam pin hole failures.
Many people assume that a certain brand ensures a part is in tolerance. They fail to understand that the AR is a "recipe" and all the parts are supposed to work as a system. How those parts interface is more important than individual dimensions of a part. Two parts can be made within the plus or minus and still be good, but when mated don't work right or gauge out. This is a tolerance stack issue that people swear won't happen with "quality parts". I see it every day. Why is that? Because I check.
Or you can just run it and hope for the best like many people do.
I'm fine if people don't follow my advice. It's their gun to do what they want.
Im not directing this comment at you, but I'm honestly surprised that members of this well respected forum parrot the same "good enough" advice that other less respectable places do.
Pressingonward
12-20-20, 10:36
I suggest you remove the ejector and extractor from the bolt before checking headspace on any rifle or carbine. The pressure from the ejector may have an affect on your sense of feel while applying pressure against the bolt. Removing the parts gives you an opportunity to closely inspect machined surfaces and thoroughly clean the bolt. I highly recommend thoroughly cleaning the chamber before checking headspace, so you don't get a false reading.
It doesn't take much pressure on the bolt to get a good headspace reading. I've seen a few headspace gauges wrecked when people used them to check headspace on a fully assembled rifle.
I'm having trouble picturing how a headspace gauge can get wrecked when used on a fully assembled rifle - can you explain further? Thanks!
Thanks!
I assume companies like Daniel Defense and BCM use in house manufacture of ALL parts and have have mills set up and calibrated to match.
So while a BCM BCG may work fine with a DD barrel perhaps a DD BCG would be a better starting point....
Obviously the problem is multiplied when buying parts from different manufacturers over time. Even if high quality slight variance could stack up to problems....
W
I'm having trouble picturing how a headspace gauge can get wrecked when used on a fully assembled rifle - can you explain further? Thanks!
The ejector or extractor can raise a burr on the gauge when you force the bolt into battery. That can make the gauge read incorrectly.
The ejector can load the gauge so it reads incorrectly.
If the extractor cutout (the groove that grabs the rim of the casing) is located incorrectly it can give a false headspace reading.
Using gauges in a dirty gun can also wreck gauges.
Thanks!
I assume companies like Daniel Defense and BCM use in house manufacture of ALL parts and have have mills set up and calibrated to match.
So while a BCM BCG may work fine with a DD barrel perhaps a DD BCG would be a better starting point....
Obviously the problem is multiplied when buying parts from different manufacturers over time. Even if high quality slight variance could stack up to problems....
W
It is a safer gamble to follow that philosophy, but it's not a guarantee you won't have a problem. I see factory uppers and complete guns that fail headspace all the time.
Some companies will act fast to remedy the issue, while others just tell you headspace isn't important and to just cram ammo into the chamber and "run it".
I say it's necessary to disassemble the bolt to check headspace. The brownells field gauge is a Forster product. I will admit that it can be a pain if you don't have experience dealing with the ejector roll/spring pin.
All of my gauges are modified so bolt disassembly isn't required. When a bolt/barrel combo fails, I always follow up with other gauges to verify my gauge is not off. I also check the calibration on my gauges every 6 months.
What about these gauges, are they the correct measurements? They're also ground off on one edge[ATTACH=CONFIG]
What about these gauges, are they the correct measurements? They're also ground off on one edge[ATTACH=CONFIG]
I can't see the picture.
I can't see the picture.
Click on it.
For some reason;
The go is a--- 1.4636 instead of 1.4646
The No go is- 1.4696 instead of 1.4736
I have experience with AR's that have kaboomed that had short headspace. I checked them and warned the owners before the guns blew up. Sometimes there were often additional issues present that lead to the blown up gun. Ammunition can sometimes be the sole cause as well.
Diagnosing a kaboomed gun afterwards can be impossible in some cases depending on how the parts were damaged. Being conclusive about tolerances with bent and broken parts is often a futile effort. I have experienced success in proving it out in a few cases.
Short headspace can cause reliability issues when the gun gets hot or fouled. It can have extraction issues and failures to go into battery. Not all ammunition is loaded to the same dimensions or pressures so some ammo won't show a problem, and other ammo will. The point of using headspace, throat, and chamber gauges is to ensure the gun will function with a wide diet of factory loaded ammunition. It also ensures the gun will run hot, cold, wet, dry, and so on.
Short headspace can also lead to accuracy issues with some ammo, primer popping, bolt face etching, destroyed brass, and can increase the chances of bolt lug or cam pin hole failures.
Many people assume that a certain brand ensures a part is in tolerance. They fail to understand that the AR is a "recipe" and all the parts are supposed to work as a system. How those parts interface is more important than individual dimensions of a part. Two parts can be made within the plus or minus and still be good, but when mated don't work right or gauge out. This is a tolerance stack issue that people swear won't happen with "quality parts". I see it every day. Why is that? Because I check.
Or you can just run it and hope for the best like many people do.
I'm fine if people don't follow my advice. It's their gun to do what they want.
Im not directing this comment at you, but I'm honestly surprised that members of this well respected forum parrot the same "good enough" advice that other less respectable places do.
Thank you for the information. I understand forensic examination after the failure is difficult.
With all due respect, you are introducing additional variables, particularly throat and chamber dimensions. Neither is checked with a head space gauge.
You are certainly not directing your post at me, the first thing I typed was that headspace should always be checked. That's hardly parroting "good enough".
Andy
Thank you for the information. I understand forensic examination after the failure is difficult.
With all due respect, you are introducing additional variables, particularly throat and chamber dimensions. Neither is checked with a head space gauge.
You are certainly not directing your post at me, the first thing I typed was that headspace should always be checked. That's hardly parroting "good enough".
Andy
Andy,
I should have worded my chamber gauging better. My apologies. I know they are generally separate gauges and processes. It can be done with one gauge but that's not optimal. I own a gauge that does throat, neck, chamber, and headspace in one gauge.
I use those various gauges daily and in my classes. Most people don't think think an AR needs to have headspace checked at all.
Click on it.
For some reason;
The go is a--- 1.4636 instead of 1.4646
The No go is- 1.4696 instead of 1.4736
The 1.4636 is a .223 GO.
1.4736 is 5.56 Field.
Andy,
I should have worded my chamber gauging better. My apologies. I know they are generally separate gauges and processes. It can be done with one gauge but that's not optimal. I own a gauge that does throat, neck, chamber, and headspace in one gauge.
I use those various gauges daily and in my classes. Most people don't think think an AR needs to have headspace checked at all.
I honestly appreciate your input. Knowing that headspace can cause problems is worlds away from an explanation of the problems you have witnessed. I learned a lot from your post.
Andy
I'm having trouble picturing how a headspace gauge can get wrecked when used on a fully assembled rifle - can you explain further? Thanks!
If the charging handle slips out of your hand or you intentionally release the bolt and allow it to quickly go into full battery, it pounds the headspace gauge. A headspace gauge is a precision ground measuring instrument and slamming it between two metal surfaces can cause undue changes to the gauge. Change the dimensions of the gauge between the base that engages the bolt face and the datum line .002" and you may as well toss it in the trash.
If the gauge is not damaged, you may damage the chamber in the barrel.
Pressingonward
12-21-20, 00:14
Yup, that would definitely have the potential to damage the gauge (or the rifle for that matter). Didn't even consider that someone might try to close the bolt with the buffer spring - when I checked headspace on mine I left the bolt fully assembled, but had the upper released from the rear pin and pivoted open so I could actuate the bolt by hand.
My gauges have a notch cut out for the extractor, but I like the idea of also grinding out a recess for the ejector pin...not that I'll be building enough ARs to justify the time it would take to do it.
Here is my 2 cents.
Debris from the manufacturing process may not be completely removed before assembly at the factory and there may be a small piece of milling somewhere in the bolt assembly. There could be storage grease, oil, etc. somewhere in a slot, nook or cranny on the bolt. Before I assemble a rifle out of new components or install a new component on a rifle already in service, I thoroughly clean it. If I take a new BCG out of a package, I take it completely apart and clean it thoroughly before installation, checking headspace, etc. I've seen low round count rifles fail to eject spent brass and got them up and running by removing the ejector and clearing debris from the bolt where the ejector lives.
Headspace is not a daily check, so I don't mind taking a little extra time when building or rebuilding a firearm. If you just drove from Oregon to Camp Perry and a piece of debris in the BCG fouled the weapon during competition, you are not going to be happy. If you are using the rifle or carbine for self defense, it is even more important you don't have a malfunction.
If I worked in a factory and built 50 rifles per day, I understand that I would not have time to completely disassemble the BCG and clean/inspect it before assembling the firearm. If I am only building or repairing a few rifles per day, I don't mind taking extra steps to reduce the chances of a malfunction during an important moment for the user.
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