Originally Posted By Ed_Victory:

A fantastic look at the elusive gray model, thank you for sharing.

Originally Posted By hrt4me:
not the best pics, but these are both my issued M4s as I relinquished the old black for my new grey (different collapsible stock, different pistol grip, different RDS optic and mount, different colored Geissele railed forearm, different colored Inforce wml, new afg...)

https://www.AR15.Com/media/mediaFiles/1182/FBI_M4s__black___grey__jpg-1114013.JPG

Originally Posted By hrt4me:
newly-issued grey M4 contains M16 hammer and H2 buffer (never before seen one marked this way)

https://www.AR15.Com/media/mediaFiles/1182/FBI_M4__grey__with_M16_hammer_jpg-1114054.JPG

https://www.AR15.Com/media/mediaFiles/1182/FBI_M4__grey__with_H2_buffer_jpg-1114056.JPG

Originally Posted By HCM3156:

Yes.

The standard issue FBI carbine has the black rail.

The grey is on a modified carbine with a Troy or Maxim PDW type stock used by a unit that primarily works out of vehicles.

SWAT gets OD green railed carbines.

The color coding of the guns by assignment / program  is for internal logistical / armorer purposes. If you are responsible for hundreds or thousands of guns little things like color coding can make your life easier.

Originally Posted By hrt4me:
the grey carbines actually have slightly shorter barrels, making them approximately 1.5" shorter overall with collapsed buttstocks than the green or black carbines

Originally Posted By HCM3156:

It's a big organization. There are certain left and right limits but within those limits there are variations and mixes depending on the office, team, PFI (Primary Firearms Instructor) and individual user preferences. Guns get sent in and re-built into updated configurations, guns get mixes of old and new accessories depending on what is available at the time.

You have the four colors down. Guns are color coded by "program" for administrative purposes.

HRT operates on the fight club rule.  The heavy 11.5" barrel was requested back before free float rails were in use. For an 11.5" with an A frame and non FF 7' rail it's a good set up. Do they still use those barrels under the FF rails ?  Don't know.

Regional SWAT guns will have green rails and select fire lowers with ether select fire or G-SSA triggers. Locally it is operator choice but I can't speak to very office.  They will have a laser and a light, brands vary. I've seen both L3 and Steiner, Surefire and Inforce IR. Beyond that they vary by what the team and office will allow or provide. Some Prefer the T1/T2, some prefer the SIG Romeo for the circle dot.  Some SWAT rifles are painted, most aren't.

Locally the SWAT guns have either the aimpoint or SIG, rear mounted PEQ 15, surefire mini scouts and unity TAPS switching.  SIG vs Aimpoint is preference. Those favoring the SIG normally cite the circle dot reticle based both on prior exposure to eotech and use of the 6 o'clock hash mark at CQB ranges.

Agent rifles will vary depending on what was available  when they were built, what the office/PFI allows and Agent preference within the left and right limits of policy. A mix of Aimpoint and SIG optics is normal.  Agents are limited to the single cell gen2 inforce WML.

For example, the troy rails were replaced with Geissele, vs Troy BUIS which are still common even though new guns are getting magpul Pros.  Same with the fab defense AFG vs the magpul AFG, Vtac sling vs magpul sling etc.  

Guns may have GI, Ergo, Hogue, or magpul pistol grips.  Vtac or magpul slings etc.  Geissele and Radian ambi charging handles are authorized as well.

The general trend is away from Troy, FAB, Ergo and Viking stuff to magpul  sights, grips and slings. The magpul CTR continues to be the defacto stock.

Locally they try to get every Agent a long gun but this is not true of every office, especially in bigger cities.  In some places only SWAT and those in specific assignments get long guns.

There is a Personally Owned Weapon (POW) rifle program.  It was revived a few years ago. The original configuration was basically a 6920 with an 11' Troy Alpha rail. That was updated to a 6920 with a 13" Geissele Mk4  and an optional SSA trigger about 2 years ago.  Wait times for these rifles were sometimes a year or more as Colt apparently built them in batches.   The new POW rifle will be a 16' Geissele Super Duty with a 13' Mk4 rail.