Armalite and DPMS compatibility again

[DM15]

After building my first 308 AR I have a few more questions.  The rifle is working fine as stated in other posts.  I am now concerned (after reading a post below this one) that I may have mixed parts.  The BCG is from Toolcraft so I know its DPMS.  The barrel is not stamped but was sold new as "DPMS style".  So far I'm very pleased with it's accuracy and function.  However, the standard 11 3/4" midlength gas tube comes up short with the cam cutout.  Now that I know this I am wondering if I have a Armalite barrel and extension.  I do not have access to "no go" gauges. Factory ammo chambers fine.  I handload so I adjust the case to fit the chamber.  So far all is good.  I've searched for a direct answer but end up chasing rabbits.

Is there any way to determine if the extension is Armalite or DPMS?

 

[shooterrex]

You have a AR 15 midlength gas tube. The one you want is the Armalite carbine tube. It is longer than the AR15 midlength gas tube.

[DM15]

Yes, I'm in the process of acquiring an Armalite carbine length tube.  My concern is that I may have an Armalite barrel extension.  How can I determine if I have the correct extension for my DPMS bolt?

[H_0_D]

I've also been researching the cross-pattern compatibility, and have found mostly contradictory and demonstrably inaccurate info.

Not all barrels that use the longer-than-.223 length gas tubes are Armalite.  For instance the Brownells retro 308 barrels are DPMS pattern yet use the longer gas tube.  So it's not certain that your barrel is Armalite brand.

Also there seems to be no actual dimensional difference in the barrel extensions.  I think the difference is in how deeply the chamber is cut into the barrel, which impacts the headspace measurement.  My inference is that "barrel extension" became a shorthand way of saying a barrel with slightly longer or shorter headspace.

If your build has already been fired and doesn't exhibit anything unusual, you're probably fine.  But it would still be prudent to buy a $30 No-Go headspace gauge.

[Sisco]

If it passes the headspace test, is functioning and shooting accurately, why are you worrying about it?

[ARTrooper]

My take away from my experience is always check headspace before riding the bolt forward. If headspace is a little to light to fully lock, you could get a live round stuck in the chamber that would be almost impossible to remove. 
 

most gun smiths or just a hun store that builds their own ARs will use a go/no go gauge for you for free to cheap.

 

one thing that was suggested to me was, though I don’t know if it is the correct thing to do. 1st take your lower off. 2nd manually drop the round (dummy round obviously preferred) into the chamber, does it sit about in the position it should? If it does, 3rd step is slowly lower and the bolt carrier group onto the round into the chamber. 4th it should take minimal pressure from your thumb at the back of the receiver to push the carrier all the way until it locks, it shouldn’t take to much force... if it doesn’t lock with just one thumb amount of force, at least you don’t have that thing forced into the chamber by the full force of your spring and buffer. 

[DM15]

I'll try and track down a no go gauge.  Rare in theses parts except buying one.  I really want to know now if it passes now.

3 hours ago, Sisco said:

If it passes the headspace test, is functioning and shooting accurately, why are you worrying about it?

I agree for the most part, however it was never properly HS tested.  Ignorance was bliss but now I just want to know for sure.  Also, I don't want to prematurely wear out any parts due to any ill fitting pieces.