I am currently in the middle of my 1st build. I have a BCM bolt carrier group and 16" BCM BFH mid length barrel. I know that you can shoot .223 rem out of a 5.56 nato barrel. The BCM barrel has 5.56 nato stamped into the barrel, so my question is, when buying headspacing gauges I will need to buy 5.56 gauges, correct? Or will I need to buy both .223 and 5.56 gauges?

5.56 gauges will do.

Fwiw, you can rent gauges if you prefer.

Here's my take on it. The Go/No Go for the CIVILIAN world follows the SAAMI specification no matter if you got a .223 or a 5.56 gauge.

Keep in mind the key difference with a .223 and a 5.56 is the lead. A 5.56 has a slightly longer lead which creates more pressure.

SAAMI calls for a minimum headspace of 1.4636" and a maximum safe headspace of 1.4736". If you get gauges, make sure your go is set to 1.4636" and your field is set to 1.4736", and your no-go is set somewhere between that. When I first built my AR years back I did like other people did and disassembled the entire bolt. Today, when I build an AR, I don't even bother disassembly it. I just push the bolt into place by hand pressure until it closes on GO and then I attempt to close on a no go. So long as you don't slam the bolt and use hand pressure and not "hammer" or smack/force it in, you shouldn't damage anything.

You could in fact buy a true military gauge for a go/field, but the last time I saw it the cost of one went for about $600 bucks. I recommend buying the same company as some manufacturers create different length for each and might give a false reading. I recommend Fulton Armory for the gauges: http://www.fulton-armory.com/headspacegaugesetfchambergonogoandfield223556mm.aspx

.223 Rem GO Specs are 1.4636
Forster makes one, Brownells Part # is
319-223-464WB
Mfr Part: HG0223G

5.56 NATO GO Specs are 1.4646
Forster makes one, Brownells Part # is
100-010-713WB
Mfr Part: HG022314646SP

5.56 NATO FIELD Specs are 1.4730 (USGI)-1.4736 (COLT II)
Forster makes this one for BROWNELLS - 5.56MM MAXIMUM HEADSPACE GAUGE
319-418-033WB

You will need to remove the ejector and extractor to get a proper reading with these gauges.

.223 and 5.56 headspace gauges are not interchangeable. Get the gauges that match the barrel stamp. If you want a quick and dirty; you can put a piece of scotch tape ( 0.003" in thickness) over the head of an action proving round (NOT A LIVE ROUND!) and see if the bolt will lock with the ejector and extractor removed. If it does, you are out of spec.

Here's a picture of my 5.56 GO gauge. It's at 1.4636, the minimum SAAMI spec.

If you go on their website: http://www.forsterproducts.com/product/nato-chamber-headspace-gages/

The 5.56 GO gauges are 1.4636, with a + of .0003. It follows SAAMI spec, like any other - civilian - gauge out there.

Point is, it doesn't matter whether you get a .223 or a 5.56 because there isn't really a difference other than the lead. A 5.56 will fit into a .223 and vice versa. It's just that a .223 barrel is rated to a different pressure which makes firing a 5.56 potentially bad. If your barrel is stamped .223, it's best to call the manufacturer and ask if it is safe to shoot 5.56 in it.

Plus you don't need to disassemble the entire bolt. That's a massive waste of time and effort and you weaken the roll pins to it by removing it. You can close the bolt safely on a go/no-go without damage so long as you use hand pressure and not slam it in.

The standard HS Spec for 5.56 is 1.4646.

I listed the gauge part #'s

I don't have to disassemble anything, mine are modified.

These are mine.
https://www.instagram.com/p/BU7vCdfBjEr/

Where can one rent gauges? Asking for a friend.

Honestly I would trust BCM in that they've alreahttp://www.ar15.com/forums/manageReply.html?b=3&f=7&t=714226&r=-1&page=1#dy head spaced it.

Google will turn up a couple of options.

This one is GTG

Unless it has changed US mil-spec for Go is 1.4646”.
IMO 1.4636” is fine for semi-auto weapons that will not be subjected challenging conditions.
I would note that the SAAMI No-go spec is 1.4666”, but US mil-spec No-go is 1.4706”.
Failing a 1.4706” gage is a reject point for a new 5.56mm barrel.

New barrels and new bolts manufactured by known sources should always headspace. There is usually no more than .001" - .002" difference including tolerance stacking. This is probably the greatest reason to build your own.

I have never had new parts from known USA manufacturing sources not work.

LMT
Continental Machine Tool
White Oak Armament
Rock River Arms
Brownells
Bravo Company USA
DPMS
Wyndham Weaponry
Stag

and dozens of other sources can be relied on to provide quality parts.

I would agree they should always headspace as the tolerances are generally known.
Unfortunately that has not been my experience.
I have found some barrels that would not pass a Go gage with a bolt that is in the higher end of the tolerance range.

Were they reputable brands? What were they?

One was a Shaw barrel, the other was an upper assembly from Fulton of my barrels.
One friend’s kit from M1S, probably a Shaw barrel.
I would note these three barrels are 1 MOA or better shooters.
I was able to match bolts to these barrels that closed on a 1.4636” gage and not close on a 1.4646” gage.
One friend’s kits from I think DPMS.
I don’t remember who made the ones back in the 80’s and 90’s.