With all these builds going on, does anyone check headspace after building or are we relying on the milspec parts to be in spec and not worry about it. In theory, if the parts are milspec then the headspace should be fine but I am curious if any checks the headspace. Thanks...

I headspace check most of my builds, especially if I have used parts involved.  I use a new, stripped bolt with a used bolt carrier and GO & NOGO gauges.  Better to be safe than sorry.

I headspace mine.  Yes the HS gauges are kinda steep, at nearly $60 for a set of 3, its cheap insurance to make sure the gun works and is safe.  Plus, I feel that it is easier to HS when the barrel is not installed, and the bolt is stripped down, so you can insert and turn by hand.  

As long as your are buying quality parts, it's almost a waste of time.  I do mine cause I blew the clams on a bunch of headspace guages, but none have ever been even close to out of tolerance.  It's nice for peace of mind, but generally unnecessary IMHO.

I want to make one point! even if the headspace was off what can be done to correct it? nothing! but buy new parts. when you buy a new barrel assembly ask to have a bolt sent with it already headspaced bolt cheacked then build it without worrie.  Id think the issue mainly lies with used worn out parts that are past there service life that could cause problems, Iam shure you been to gun shows and seen all the junk they have!

With the M16/AR varieties and considering HS gauges, as a novice builder I would only buy the field gauge.  If it closes on the field, time to re-barrel.

Can someone explain the headspacing gauges?  I recently bought a set and my bolt closes on the GO and NO-GO, but not on the FIELD.

Quoted: Can someone explain the headspacing gauges?  I recently bought a set and my bolt closes on the GO and NO-GO, but not on the FIELD.

By using light hand pressure, you should be able to chamber the GO gauge.  You may be able to chamber the NO-GO, but should NOT be able to chamber the FIELD gauge.  All of the above are doen with light hand, or finger pressure.  They are NOT done with the charging handle/ charging the weapon.

So, what is the difference between the GO and NO-GO?

Quoted: So, what is the difference between the GO and NO-GO?

Here you go:

Forster go,  1.464
Forster no-go,  1.466
Colt Field,  1.4736

Can someone explain the practical application of the gauges?  It would seem that if the bolt closes on the NO-GO, then there is some wear and if it closes on the FIELD you have a dangerous situation and if it won't close on the GO it is out of spec.  

Should a new bolt and barrel close on the NO-GO?

Quoted: Should a new bolt and barrel close on the NO-GO?

It shouldn't, but I've seen plenty of times where people have applied a little extra pressure and said that they do.  

Quoted: Can someone explain the practical application of the gauges?  It would seem that if the bolt closes on the NO-GO, then there is some wear and if it closes on the FIELD you have a dangerous situation and if it won't close on the GO it is out of spec.   Should a new bolt and barrel close on the NO-GO?

You pretty much have the idea.

A bolt should close on "GO".

A bolt should not close on "NO GO"

A bolt that closes on "FIELD" is unacceptable for use.

The difference between NO GO and FIELD is that the first is to set the limits for a new or reconditioned rifle.  Nothing that closes on the NO GO should leave the factory.  The FIELD, on the other hand, is to find rifles that have worn to the point that they are no longer fit for service.

Headspace guages are to be used gently.  It is not your goal to see what is the biggest guage you can force into the chamber.  Rather, you chamber the guage, then push the bolt forward, and if you feel any resistance, you stop, and consider the chamber as not having accepted the guage.

What about a new upper assembly that won't close on the go gage?

previous posts are all pretty much correct. I've seen .223 gauges frequently close on the no-go guage without a problem, and this is on brand new barrels.  It all depends on who is chambering the barrels, you can ream them deep, shallow, use a different style i.e. "Wylde" or even variances in barrel extensions, many things can effect headspace.  You should try to do it by hand, with the extractor and ejector removed from the bolt, the bolt should be reasonably new.  Many tech manuals say to try to close the carrier/bolt combo by ramming it home, but this is not the proper way to "feel" the headspace.  If the go gauge "go's" then put into the lock position and rock up and down, you will be able to feel the .001", .002" or whatever amount of play there is in the headspace.  Then try the "no-go" gauge, it it does not go...good, if it does, feel the play again, then try the field guage, if it goes, the barrel is trash.  You can try a new barrel extension, but it probably will not help, time for a new barrel.  If the "field gauge" does not fit, but the "no-go" does, you are still in good shape, just take the weapon out and shoot, checking with the "field" gauge every one or two thousand rounds or so.  Remember .223 gauges are tighter than 5.56mm so you are still pretty safe with the no-go.