View Full Version : AR front sight base "F" vs "non F"?

December 15, 2016, 15:56
Ok, somebody please explain the difference and what works with each as far as rear sights. The non F looks to be some .09 thousandths lower than the F type.
Why are there these two types?
I am not all that bright so please type slowly.
Thanks :)

December 15, 2016, 16:26
Non-F is for fixed carry handle/NON-FLAT top uppers.

F is for flat top whether you mount flip up or carry handle.

04" difference in height over bore centerline; F type is taller.

When zero'd... a Non-F with a flat top usually causes front sight post to be above the top of protective ears. Works, just isn't protected as well and looks odd.

Oldie but a goodie:

December 15, 2016, 16:38
I am typing this verrry slooowly so as to not confuse either of us , but mostly because it's the only typing speed I got. :sad:

The F-marked sight base is 0.040" taller and is used on rifles with flat-top uppers with the removable carry handles. So, M16A2=non F-marked, M16A4=F-marked.


edited to add;
Ya'see, too late again! :redface:

December 15, 2016, 18:16
I have a carbine length 16 inch barrel with an A4, flat top reciever, with a non F marked front sight base. I had to get a longer front sight post to make up the height difference to zero.
I have a few non F types on 20 inch barrels with A2 receivers that zero just fine with the front sight post flush with the inside of the front sight base. I also have an F marked front sight base on the same formate with no ill effects. These are my National Match rifles with Rock River upper assemblies. I guess the key element is the barrel length?
Thanks for explaining this conundrum so even I could understand it.

I forgot to say I have a clamp on carry handle marked A8 on the top left side on that 16 inch barreled carbine.

December 15, 2016, 21:22
Barrel length shouldn't be the difference although shorter barrels should lessen the apparent effect.

Also, it's reported not all "F" height FSBs are marked "F".

December 15, 2016, 22:46
You are correct Sir! I just measured the F marked and non F marked on my 20 inch barreled National Match rifles and they are all the same. :)

December 18, 2016, 19:33
I've been looking at a PRI gasblock/flip sight and they tell me that on a .750 barrel, the sight plane is 2.57" above the barrel centerline. 2.32" and 2.36" are what the top of a non-F and F, respectively should be, without a post installed, so what does a typical post height run?