Im going to be switching out the barrel on my AR soon for the first time.

After looking up subjects on headspacing, it occured to me;

Couldnt a person just use a .223 blank as a 'go' gauge? Or would that person be setting themselves up for a darwin award?

Can you tell im a newbie?

No.

You can take your barrel & bolt/carrier assy to a local 'smith for a nominal fee, or buy a headspace gauge from [url]Brownells.com[/url] part #319-418-033. Retail for 14.94.

FWIW. I have only had one customer with a new barrel that was out of spec, but it is always a good idea to double check headspace.
An ounce 'o prevention and all that stuff.
[sniper]

buy you a set of headspace gauges, buy you a set of complete tools, ar's are easy to build.
if you buy a barrel from a manufacture have them send you a bolt already headspaced to the barrel, then when you install the barrel intstall the bolt, you should be ok but I always check headspace! you can install the original carrier.

You do need a "no-go" or "field guage", but for a "go" guage--in a pinch--you can use a quality factory case from a loaded round--AFTER THE BULLET HAS BEEN PULLED, THE POWDER DUMPED, AND THE PRIMER KILLED AND REMOVED. Don't use a loaded round for this.

However, whether you need a "go" guage is up to you, as the first loaded round you chamber after it's all together will also tell you if your chamber is too short. If it is, the bolt probably won't close easily.

Out of spec chambers on factory barrels are really rare, but it's always good to check every barrel/bolt combination you plan to shoot. You never know which one isn't....

No, the GO gage is longer than a case to allow the case to expand during firing. A barrel with tight headspace could allow a cartridge to fit but the rifle might not function and chamber pressures could be increased depending on the interference. Also, unless the case is held to precise tolerances you won't have any idea which part of the case is binding in the chamber.


Edited to add that I went searching for the max case datum figure, since SAAMI still hasn't answered my letter about buying their books, and found this artice:
[url=http://www.jarheadtop.com/article_handloading.html]Boots Obermeyer's "Handloading for Match Rifles[/url].

Then scroll down to [b]HEADSPACE[/b].

Relevant portion cut and pasted here for the impatient.

[Boots]
"The cartridge case usually doesn't contact the .400 diameter ring at first closing. The normal contact is at the radius of the neck and the shoulder. The gun can actually be closed easily with a case .002 to .003 longer than the chamber at this point."
[/Boots]

Armalite also has a good [url=http://www.armalite.com/library/techNotes/tnote11.htm]article[/url] on their page that, discussing the .308, show the dimensional differences between max case, min chamber, and GO gage.

Cut and pasted here

       [center][b]CARTRIDGE [/b]
[blue]MIN[/blue]       [red]MAX[/red]

SAAMI
1.627       1.633

GOVERNMENT
1.630       1.633          

          [b]CHAMBER [/b]
SAAMI
1.630        1.640

GOVERNMENT M-14    
1.6355       1.6385

GOVERNMENT M-14 Match
1.631       1.633[/center]