FN M16A2 FSB vs. A3/A4 FSB - How to tell the difference?

Printable View

06-24-17, 09:46
Celt

FN M16A2 FSB vs. A3/A4 FSB - How to tell the difference?

What markings on the FN M16 FSB ID's A2 vs. A3/A4?

Thanks in advance.

Celt
06-25-17, 08:26
mack7.62

What difference? I thought the F marked FSB's were needed due to the shorter barrels.
06-25-17, 18:29
Iraqgunz

It's probably been discussed before.

https://www.m4carbine.net/showthread...-On-A-A2-Upper

https://www.m4carbine.net/showthread...ked-FSB-Height

https://www.m4carbine.net/showthread...ont-sight-base

https://www.m4carbine.net/showthread...5-F-Marked-FSB

https://www.m4carbine.net/showthread...-and-Rifle-Gas
06-27-17, 15:20
scottryan

Quote:

Originally Posted by Celt

What markings on the FN M16 FSB ID's A2 vs. A3/A4?

Thanks in advance.

Celt

Why do you need to know this?

What do you mean by ID? The F marking or the forge code?
07-29-17, 21:10
Rifleman_04

Quote:

Originally Posted by mack7.62

What difference? I thought the F marked FSB's were needed due to the shorter barrels.

No. The F marked fsb has a shelf that is .040" higher than standard because the rear sight on a flat top upper sits .040" higher than an A2. Has nothing to do with being on a carbine.
07-31-17, 13:28
scottryan

Quote:

Originally Posted by Rifleman_04

No. The F marked fsb has a shelf that is .040" higher than standard because the rear sight on a flat top upper sits .040" higher than an A2. Has nothing to do with being on a carbine.

That is true but isn't complete information.

All carbines, regardless of fixed handle or removable carry handle, have a taller overall front sight arrangement.

Examples:

A Colt 727 will have a standard front sight base, but a taller front sight post.

A Colt flattop carbine will have a taller F marked front sight base, but a standard height post.

A Colt SP1 carbine has a taller round post than an SP1 rifle or M16/M16A1.

The M16, M16A1, M16A2, M16A3, M16A4, M4, and M4A1 all use the same height front sight post, for logistical reasons.

There several different posts that have been used:

Standard Round = M16, M16A1, SP1 Rifle
Tall Round = SP1 Carbine, XM177/GAU-5 Series, 653, 639
Standard Square = M16A2, M16A3, M16A4, M4, M4A1, LE6920, AR6721
Tall Square = 700 series carbines, 9mm SMGs
Short Tapered Square = some 11.5" flattops and export flattop carbines, some 933, 977, 979
07-31-17, 16:58
Rifleman_04

Thanks for the added info.
07-31-17, 21:37
GRA556

Quote:

Originally Posted by scottryan

That is true but isn't complete information.

All carbines, regardless of fixed handle or removable carry handle, have a taller overall front sight arrangement.

Examples:

A Colt 727 will have a standard front sight base, but a taller front sight post.

A Colt flattop carbine will have a taller F marked front sight base, but a standard height post.

A Colt SP1 carbine has a taller round post than an SP1 rifle or M16/M16A1.

The M16, M16A1, M16A2, M16A3, M16A4, M4, and M4A1 all use the same height front sight post, for logistical reasons.

There several different posts that have been used:

Standard Round = M16, M16A1, SP1 Rifle
Tall Round = SP1 Carbine, XM177/GAU-5 Series, 653, 639
Standard Square = M16A2, M16A3, M16A4, M4, M4A1, LE6920, AR6721
Tall Square = 700 series carbines, 9mm SMGs
Short Tapered Square = some 11.5" flattops and export flattop carbines, some 933, 977, 979

What about the 6720 (not 6721) ... ??? I've read where Colt used the standard FSB but a taller sight post. I'm wondering why on that model they are different than say a 6920?