Another DUMB question

W

William Suter

Well-known member
OK, here goes. How do you head space an AR? I'm well aware on head spacing a bolt action rifle barrel but don't have a clue on head spacing an AR barrel. Sorry if its a dumb question but I've said time and time again, I'm no AR guy. Even though now I have one.
 
The stripped bolt and headspace gauges are used to check the extension/barrel after they are mated. Normally before the assembly (upper/barrel) occurs. As you need to check(feel) amount of rotation of the bolt in the extension. If you need to replace the bolt, you can measure the depth of the bolt face to confirm headspace is not changing.
 
Unless you’re replacing the bolt or barrel, or both it shouldn’t be necessary. That should have been done when it was built. If you’re just curious or checking, it is as SSS stated. The easiest way is with the proper caliber go – no/go gauges.

Strip the bolt of the extractor and ejector. With just the upper clamped in your vise, insert the go gauge in the chamber, insert the bolt, it should fit in and turn locking the lugs. A slight resistance is fine. Then insert the no/go gauge. It should not fit and turn.
 
Reason for asking is if I ever replace my barrel. But if I change to a different cartridge, like a 22 ARK, then a different bolt will be necessary anyway. If I go that route, I'll just buy a complete upper.
 
Headspace is set (and checked) by whoever installs the barrel extension (generally, the barrel maker). It should be correct, but I suppose it is possible to have a really worn, or out-of-spec bolt.
 
Common to purchase a bolt with new custom barrel or send in your bolt to be HS with new barrel. As I mentioned most bolts are to mil spec and are interchangeable (in cartridge family). I do have a Bushmaster 223/556 bolt that is minimum HS, so it doesn't swap with my other 223's. I actually purchased a couple more bcg so I didn't need to swap around between uppers and I could leave the bushy in one piece.
 
Put the fired case in the case gauge to see if it stretched. If you reload, size the case to put the shoulder in the right place and HS problems go away. GKs each had Howa 243 and HS was different for both by 10 thous. I had to size for the shorter so ammo would fit both rifles.
 
Dane G said:
Could you give more specifics on this method?
Click to expand...
When you fire a round the firing pin drives the round forward against the shoulder. Pressure rises and the case expands agsinst the chamber wall and the case stretches until it hits the bolt face. This is why brass lengthens and needs trimmed. After the pressure drops the brass cools and shrinks enough it can be extracted from the chamber and ejected. I use the hornady head space comparator on a caliper to measure the case length and can compare it to the prefired measurement. Then when you resize the fired brass set the die so it only pushes the shoulder back .002" to .007" (depending on application) from fired length as to keep from over working the brass and giving it excessive headspace when rechambered and fired.
 
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"Pressure rises and the case expands against the chamber wall and the case stretches until it hits the bolt face." Actually only the front part of the case is held by the chamber wall. The rear part can stretch till it gets to the bolt. Can cause a thin case near the head and case separation. When you full length size, you set the shoulder back which then makes the neck longer.
 
The reference datum line for measurement is 0.300" diameter (hence the headspace comparator often sold by Hornady and others). The measurement here is taken to have a readout from that reference line aka datum to the where the rim contacts the bolt face. This is for the .223, 5.56, .222, .221 other cartridge may require a different datum reference diameter.
With actual headspace gauges the same standard is used (0.300" datum reference diameter). Thus the gauges looks like a bullet less cartridge, they usually are offered (are used) in GO, NOGO, Field configuration.
National Match gauges are done in small increments, usually 0.001" or so manufacturer dependent or customer stated.
While it is best to have all three, actually only two are required as a have to have. GO and NOGO which may or may not be the common accepted practice as certain organizations have certain requirement, but for home usage should be fine.

Field is the longest safe length. The primary function is used with bolt barrel combinations, for serviceability quick checks usually assembled although not always.
NOGO is beyond the maximum. If the bolt closes on this gauge it is rejected. Choices are use a different bolt. barrel, or reset the barrel extension.
GO is set at minimum (shortest length).

Note: The ejector, spring. and it's retaining pin should be removed from the bolt to prevent a false reading. Performing this check either with barrel mounted / attached inside upper receiver is acceptable. If the barrel is attached to the upper the gas tube should be removed or not installed yet prevent interface with a fully assembled bolt carrier group. Personal preference is to perform the check prior to assembly using barrel with extension and just the bolt. As a greater sensitivity of feel / control can be observed. Either method is acceptable.
But if checking an already assembled rifle barrel removal is not required. Some gauges do have relief areas cut out /milled allowing ejectors to remain in place

The goal is that the locking lugs fully lock and engage within the barrel extension on the GO gauge. FIELD preference is it barely locks, then lock with slight pressure assistance (although is acceptable if it does freely lock as long as the NOGO doesn't lock). The NOGO should not fully lock even with applying extra pressure by hand. If the NOGO locks that barrel bolt combination is rejected.
 
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