OK, After studying up on head-spacing the AR-15, I find everyone tells me "Disassemble the BCG." Half of this, the claw extractor, is easy. The other part, the pin ejector, has me beat. That pin just does not want to move.

So I headspaced my rifle with the ejector still installed. Everything seems fine. It closed, with a firm push, on the go gauge. It would not close with an equal or greater push on the no-go. Again, this is with the ejector still there. Do I really need to remove it?

I would think I'm fine, but I don't want to bungle a safety issue, so I came here to ask. Have I goofed, or is this just fine?

Welcome aboard!

You really need to remove the ejector to properly test headspace.

Unless your headspace gauge has a severely rebated rim on it. These allow the testing with the ejector in place.  Most are of the full case diameter though.

Go ahead and buy a bolt disassembly tool. It's yet more bucks spent but it compresses the ejector and spring taking pressure off the pin. And really makes life so much simpler when trying to put the ejector back in in the bolt.

This is the one I use and I can't imagine trying to do the chord without it;  

https://www.adcofirearms.com/itemdetails2016.cfm?inventorynumber=78

Agree with Bumblebee Bob.  I bought the bolt disassembly/assembly tool from right here on ARFCOM for $15.00.  I check headspace on every rifle I assemble, regardless if the manufacturer of bolt and barrel say they already have done it.  I have found only one out of spec bolt out of the few assemblies I have built and checked.  Finding just that one defect justifies the $15 for the tool plus the $60 for the three gauges, and the 3 minutes it took to check it.  Lots of people on here say it is unnecessary, but for me, it's a personal choice for my own peace of mind.

Or dremel a grove in the gauge for the ejector, and not be bothered with pulling the ejector.

I just remove the extractor spring and reassemble to check, no tools required except a roll pin tool to push out the extractor pin. YMMV

get a copy of the M4/M16 tech manual free online.  study it thoroughly to gain a complete understanding of your rifle.  then disassemble the bolt carrier group and then correctly check headspace.  GI gauges do not require disassembly of the bolt but commercial ones do.  different designs between the two.

I still don't understand why no other company is making GI style head gauges. There CLEARLY is a market for it. I'm surprise even a "second hand" seller hasn't just started milling out ejector pockets in them. Seriously.... if I had a shop, I'd be buying Clymer's (or Forster's for cheaper) in bulk and selling modified ones at $75/ea all day long (I'd be first in line for modded "no go").

I don't use a specific tool to remove the ejector pin.  Just clamp a scrap piece of brass in a vise and push the bolt down onto it.  This depresses the ejector, removes tension from the pin, and allows you to drive it out.

Do the same for reinstalling.

Best practice is to remove the ejector and extractor unless the gage is notched and does not need the ejector removed.
I strip my new bolts and check the springs and holes anyway, so I just leave them disassembled in a zip lock bag so I can check the headspace when I match them to a barrel.
I also put a firing pin in the bag after checking FP protrusion.

So on the advice here, I got a tool for the process, and that pin still won't budge. So rather then break a few pin punches, I'm going to get the dremmel and notch my gauges.

Still, it would seem .... That if the damned old thing will compress when a round is in the chamber, it ought to compress for my gauge, making the whole business unnecessary. I'm still not clear on why it is. Can anyone clear that up for me?

I use one of the fixtures that hold the bolt and a starter roll pin punch (very short – not as easy to bend)
The ejector must be compressed.
Occasionally I see a new bolt where the pin was peened at one end and must be driven out from the other side.
Odds are you are doing something wrong.
Proper support, good punch, and hitting it hard enough is all I have ever seen needed.

Thanks PFC. I finally got it out, and didn't break my punch, though it was tough going. Soaking it in oil for a long time helped, though I do think I'll need a new punch soon..... Darn things really are fragile, and that pin really, really didn't want to move.

I used long roll pin punches for years, as soon as I saw a set of the short starter punches I got it.
You also need to replace the pin, especially if you deformed it any.

Mechanically you're correct, it doesn't make a difference if you remove the ejector or not. The spring compresses and your gauge measures the same.

The book says to strip the bolt so everyone blindly follows the rules.

I've measured both ways and never found a difference. My guess is that the removal is due to the fact the ejector puts force on the gauge which makes it want to turn right and will cause the hardened gauge to scratch the chamber (bare steel chamber) or possibly damage the gauge in a chrome lined chamber. Examine some spent brass and you'll see scratches from the barrel extension lugs and the two 'vampire bite' marks on the neck, now imagine that same side load applied to the gauge.

I suppose there's also a small chance the gauge won't self center on the datum line with the side force present. I haven't seen evidence of it, but that doesn't mean it's not possible.