I originally posted this in the optics forum, but after thinking on it last night, it's
really a question about M4 Barrel length:

Hi folks,

I was looking on Trijicon's website at the TA01NSN tonight (here), and it states
the following:

"The TA01NSN was designed specifically for the US Special Operations
Command SOPMOD M4 Carbine.  To meet their requirements, the reticle
bullet drop compensator and range finding stadia lines are based on
the trajectory of the 62 grain bullet from a 16 inch barrel,
flattop mount (Picatinny rail Mil.Std.1913)."

I thought the SOPMOD M4 Carbine had a 14.5" barrel - am I missing something ?

I plan on getting one of these ACOGS, but I am trying to decide on whether
or not to go with a 14.5" barrel (with permanent Phantom), or a 16" barrel.

The Trijicon web page for the TA31RCO also states the following:

"The TA31RCO is an Advanced Combat Optical Gunsight (ACOG) designed
for the M16M4 weapon system (16" barrel)" - with regards to their
scoped designed for the "USMC Rifle Combat Optic (RCO) for M4 (16)".

Does this mean that the USMC is using a 16" M4 variant, and that the Trijicon
listing for the TA01NSN for that Army's M4 SOPMOD may mistakenly say 16" when
it means 14.5" ? Are they both wrong ?

Any thoughts ?

Quoted: NSN description is incorrect if I remember correctly and the RCO description is correct. To be honest though, it really doesn't matter. C4 www.GRTactical.com

I thought the RCO was a 20" gun.

Thanks for the info C4iGrant -

So the Marine Corps version of the M4 does have a 16" barrel ?

You really don't think the 1.5" barrel difference matters with regards to the bullet drop scales
on those two ACOG scopes ?

Quoted: Quoted: NSN description is incorrect if I remember correctly and the RCO description is correct. To be honest though, it really doesn't matter. C4 www.GRTactical.com I thought the RCO was a 20" gun.

Trijicon has one RCO listed for a 20" (the RCO A4), and one for a 16"(the RCO M4).

I missed the key part, M4 as opposed to A4.  My bad.

I just called Trijicon, and the customer service rep said that those scopes were designed to 'split the difference' between 14.5" and 16" barrels.

Kind of a strange answer. Not sure I agree with that - but oh well.

Quoted: I just called Trijicon, and the customer service rep said that those scopes were designed to 'split the difference' between 14.5" and 16" barrels. Kind of a strange answer. Not sure I agree with that - but oh well.

They may have been trying to incenuate that the TA01NSN was designed to measure bullet drop efficiently between 14.5" and 16", seeing as how I'm sure they factored in the civilian market as well as military sales. I'm not sure of the exact number of TA01NSN's that have gone to the military, but I'm sure a very respectable number have been sold on the civilian market as well. Best guess anyway.

ETA: M4A1's ALL have 14.5" inch barrels as issued. If soldiers have put their own 16" uppers on then that's a different matter; I seem to remember reading a post a few months ago that had a pic of what looked like a joe in Iraq with a 16".

Thanks for the info.

Anyone have any idea how these ACOG range-finding reticles would work out in an 18" barrel ?

Is Vahan's ad below a misprint, or does he really have Colt 16" SOCOM uppers?

See the 17th ad down:

www.autoweapons.com/photos/accys/colt.html

BTW, my Colt SOCOM barrel is 14.5"

16" barrels are a commercial thing because of the NFA.