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View Full Version : Is an M4 barrel really actually truly 14.5" long?


TED
05-10-19, 04:08
I found a dimensional drawing online that claims that it is really only 13.85". I know sometimes things can be "nominal" or "proximal", and so forth.

Is an M4 barrel really actually truly 14.5" long?

57232
georgeib
05-10-19, 04:19
It is when you include the barrel extension.
mig1nc
05-10-19, 05:20
I believe the ATF measures from the closed bolt face.

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GH41
05-10-19, 06:03
I believe the ATF measures from the closed bolt face.

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Correct.
Todd.K
05-10-19, 10:46
First, that is not the drawing that M4 barrels are made with...

Second, you failed to add the portion of the barrel that the extension threads over. So that adds .620 to the length.
markm
05-10-19, 11:30
First, that is not the drawing that M4 barrels are made with...

Second, you failed to add the portion of the barrel that the extension threads over. So that adds .620 to the length.

:lol: Maybe it's LWRC's crayon spec sheet.
Clint
05-10-19, 13:04
True, but the "nameplate" length is generally measured from the breech face.


I believe the ATF measures from the closed bolt face.
mig1nc
05-10-19, 15:08
True, but the "nameplate" length is generally measured from the breech face.Thanks, Clint. I always learn something new.

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jpmuscle
05-10-19, 15:49
:lol: Maybe it's LWRC's crayon spec sheet.

You still hate them don’t you?


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MegademiC
05-10-19, 20:28
There was a thread a while ago about this.
A lot are near 14.5 but most were a bit longer.
TED
05-10-19, 23:37
I believe the ATF measures from the closed bolt face.

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Yes, that's right the ATF measures from the closed breech face.

So then, from the closed breech face what is the length of an M4 barrel without a muzzle device?

This could be an important detail for people when doing a pin and weld muzzle device.

TED
Iraqgunz
05-11-19, 01:11
The ATF procedure for measuring barrel length is to measure from the closed bolt (or breech-face) to the furthermost end of the barrel or permanently attached muzzle device. Permanent methods of attachment include full-fusion gas or electric steel-seam welding, high-temperature (1100°F) silver soldering, or blind pinning with the pin head welded over. Barrels are measured by inserting a dowel rod
into the barrel until the rod stops against the bolt or breech-face. The rod is then marked at the furthermost end of the barrel or permanently attached muzzle device, withdrawn from the barrel, and measured.
Clint
05-11-19, 07:30
Here is an example calculation for our 14.5" MPC barrel (https://www.blackrivertactical.com/concrete5/store/#!/BRT-14-5-OPTIMUM-MPC-Barrel-**-PRE-ORDER/p/134491035/category=1852006).

The conservative approach is to target the total length just over 16" and keep the extra length gained by the ATF method in the bank.

Pro Tip: Most advertised "16 inch" AR barrels are actually 16.125" from breech to muzzle.

https://dqzrr9k4bjpzk.cloudfront.net/images/812059/1069973485.jpg