Head space?

[T1704]

Hey guys,

I own a couple of purchased AR's, but I'm attempting to build one at the moment. As a novice, I have done a tremendous amount of research and keep hearing about "checking the headspace" before firing. Being a newbie to assembling my own, I wanted to get clarification from the pro's on this site as to what this means. Is this the space between the BCG and the top of the upper; the fit between the bolt and M4 feedramps, or what? Also, I would appreciate any other tips that one could offer in terms of do's and dont's.

Currently, I have a USM4 complete lower (was told it came out of the Hogan facility), an AO stripped upper, a fail zero BCG and BCM charging handle. Still need the barrel assembly, forward assist, etc.

Thanks guys for the help.


Travis

[ray1970]

In a nutshell, headspace is basically the size of the chamber when the bolt is closed. I'm sure someone who is better at explaining things will chime in soon.

[MarkCO]

What Ray said. Measured from the bolt face to the shoulder of the chanber that would face the case mouth. 95% of AR barrels are sold with the extension installed. The headspace is basically set by the proper installation of the barrel extension. It is certainly worth checking, but in maybe 400 or so barrels, I have never found one that was off.

[BPTactical]

Ray nailed it in a basic sense. Headspace is a set specification for a particular cartridge. It is the distance from the "Datum line" of a cartridge which is a point determined by the manufacturer (usually the end of the case or point on the shoulder) to the face of the bolt. It is a critical dimension as too little and the projectile is crowded into the throat which can cause accuracy issues and pressure spikes, in extreme cases the bolt may not close. If too much the case is not supported correctly and can blow out the casehead. It is typically checked with a "Go" and "No-Go" set of gauges whiich are precision ground facsimles of casing. usually the difference between "Go" and "No-Go" is as little as .003 of an inch
On an AR barrel it is set with the barrel extension at the factory.


EDIT- Sorry Mark, I are typing slow tonite

[T1704]

Thanks guys. So, if I pick up a barrel with the extension already installed, it would be unlikely that there would be an issue, correct? I don't plan to buy a cheap barrel and the upper is said to be mil spec, plus a FZ BCG should be trust worthy. My alternative is to have my wife's cousing fire it for the first time while I take cover.... I mean, go purchase refreshments... For him and whomever he happens to bring with him. Once I can find a barrel and accessories, I'll be back to quiz you guys on the gas system and potential concerns there.

[kwando]

Another member showed me how to measure headspace, I'm willing to show you how it's done. If you do it yourself remember to remove the ejector and extractor.

[T1704]

Another member showed me how to measure headspace, I'm willing to show you how it's done. If you do it yourself remember to remove the ejector and extractor.

Chit, now you're complicating things. I can remove them on my Glock, but have no experience with the AR BCG. I have taken the BCG down to the bolt, retaining pin, firing pin, etc. I appreciate you offering and will likely take you up on this once I get the rest of the stuff I'm after. Sure could use a barrel and gas system...

[BPTactical]

Come see me.

[T1704]

Come see me.

I likely will. Just got a BCM barrell today. Please send me a PM with your info.

Thanks, Travis

[BladesNBarrels]

If all you want to do is check the head space, here is a link to a YouTube video that shows the process in detail.
This does require the "go - no go" gauges, which can cost a few bucks.
http://www.youtube.com/watch?v=ccYHs3d5T5o