With a dedicated .22 barrel like those from Model 1 Sales or CMMG, I understand that, compared to a 5.56mm barrel with the same exterior "length", the .22 barrel is actually slightly longer from muzzle to bolt face due to the way the breech end sticks further back into the upper receiver.  I was wondering if anyone could tell me precisely what the difference in length is.

I expect it's probably enough that a barrel with the external length of a 14.5" 5.56mm barrel, with a permanently attached FH, would probably make 16".  It's only 0.2" difference...

But the real reason I'm asking is because I want to find out if a proper XM177E2 clone with fake moderator by Brick could be made to the correct length.  An original XM177E2 has an 11.5" barrel plus the moderator; to make a non-NFA clone with a 5.56mm barrel requires a barrel about 0.6" longer and of course the moderator permanently attached.  So the question is, does a dedicated .22 barrel have as much as 0.6" greater muzzle-to-bolt face length than a 5.56mm barrel with the same exterior length?

More.

You are calculating wrong if I'm reading right. A 16" .223 /5.56 barrel will be about 1.5 inches
longer than a 16" .22 barrel. This is because the measurement is from the end of the barrel to
the end of the chamber, not to the bolt face. A 16" .22 barrel will actually look like a 14.5" .223 /
5.56. I'd contact CMMG, they won't set you up wrong....If you want the exact length of an XM177
showing to match up side by side with the real thing, you will need a slightly longer .22 barrel to
make that happen.


Spec

(I think this is what they are saying - Spec and Frog, correct me if it's wrong, please)

The 16"/.22 appears shorter than a 16"/.223.  This distance is seen in the area between the FSB and the flash hider, which is important to your project.  The barrel showing beyond the FSB (including FH threads) on a 16"/.223 is 6 5/8".  It is 5 3/4" on a 16"/.22.  A 7/8" difference.  This is long enough to make an XM177E2 .22 visually replicate the original.  The area of concern is the distance between the FSB and rear of the moderator.  On an 11.5" barrel this was about 1 1/2".  When you put a moderator on it comes up legally short, so most people increase that measurement to 2"or more making the barrel 12" or longer.  Although it looks good, it is still off when compared to the original.  I think this it what you are asking.

I set up some pictures for you to give you an idea.  Also note that the 16" OAL inludes the threads hidden by the FH.  They go to about the middle ring in front of the wrench flats.

This is where the moderator would sit on an 11.5"/.223 barrel compared to the 16"/.223 barrel it's sitting next to.  It is too short.  (DD 16" M4 carbine barrel/Brick Moderator/Ring)



This is the proper placement (1.5" in front of the FSB like above) compared to a 16".22 barrel.  It is plenty long enough and will come out about 1/8" over the 16" legal length.  Even longer with the 1/8" of grenade ring spacing. (Tac Sol 16" Carbine barrel/Brick Moderator)



Of course the above are nominal measurements only to show it works.  I'd recommend sending it to Heatnbeat since he would understand what you want and can make sure the measurements come out right. He may be able to turn the exposed barrel profile down to .625, too.

1.5" of the .22 barrel is inside the receiver. So a 16" .22 barrel is the same length as a 14.5" 5.56 barrel. So, I would think you would need a 12.6" .22 barrel to come out at the proper, legal length. But it's still early and I may not be following you.

Hey, wait a minute, aren't you supposed to be hurting?

Good to see you're doing better, Dave, hope you continue to improve until you're fully recovered.

Ted

AFSC2W171Z: Dr. told me to drive the computer all I can stand. Just don't
drive cars, trucks, boats, motorcycles or fly airplanes unless I'm a passenger.
Thanx for your well wishes.  2-3 weeks of this.

Dave

Thank you, this is exactly what I was looking for.  Also thank you RLR350 for your very thorough reply.  Your second picture is exactly what I needed to see.  Rather than turning down an M4 profile barrel I was thinking of this one; only flaw is that it appears to be .750 under the FSB.  If I wanted to be 100% correct there are also custom profiles available from Ballistic Advantage, LLC.


I don't believe that's how the BATFE looks at it.  My understanding is that they put a measuring rod in with the bolt closed, and when it stops, that's how long the barrel is.

Scary, but you may be 100% right...
http://gunwiki.net/Gunwiki/ProcMeasureBarrelLength

Thanks for confirming, CMMG!  

Oops, My Bad Thanks for teaching me, always willing to learn....

Dave

So it's closer to 1" than 1.5"? Good to know. I was always told 1.5" That's why the 16" M4 looks to be 14.5"

Looking at it another way, since I forgot my rod, the Atchisson BCG is 1 1/16" shorter ( back end to breechface) than a .223 BCG (with the bolt rotated shut). Assembled, the .223 breechface is about .100 behind the front edge of the upper receiver, the breechface of the .22 is well back in the port; thus the external barrel length of a .22 with an Atchisson bolt is 1 1/16" longer than a .223 with the same external appearance.
M-261s and Air Force is 1/2" closer to .223 length.

This information explains a lot. We've all relied on estimates of between 1" and 1.5".
I never really knew the way the BATF measured. Guess I thought it was magic. Now
we've got the real numbers and method, We need to find a way to incorperate them
into the sticky thread so we don't have to guess when we forget.

Dave

The way it's officially measured is with a rod down the bore to the bolt face (closed, of course). Closre the bolt, drop a rod down, mark, and measure.