What is the purpose of the "F" marked front sight base that I've heard about in conjunction with flattop A3/A4 receivers? Do all manufacturers use it?

Somebody will be along to correct me if I'm wrong, but my understanding is that the "F" marked FSBs, with the higher shelf where the sight post is, are meant for carbine length guns, and not necessarily flattops. Not everyone uses them - Bushmaster comes to mind.

The F marked front sight base is actually meant for use on flat tops. It matches the height of rear iron sights.

The structural requirements of the removable carry handle change the amount of vertical travel available in the rear sight mechanism.  This slight structural difference changes slightly the location of the mechanical zero of the weapon.  This is in turn compensated for by the slight height difference of the F marked FSB.

As I understand it and my info is only worth what you paid for it.

You need to have an F-Marked FSB for flattops, and no not all manufacturers use it (including Bushy)! Check with the manufacturer to be sure they have it, if your looking at the gun in person, it should have an "F" stamped on the left side of the FSB.

Right, wrong, wrong, wrong.

Boy, there is more nonsense about this than almost any other topic. This is becase people do not understand the geometry involved.

F marked is for carbine length, NOT "flat top".

Go to this thread and scroll down to the 10th post (by augee) with the picture and you will understand. Look at the pic in below and you will understand.

The line of sight (rear sight to front sight) must converge with a line coming out of the barrel matching the bullet travel. Because the sights are above the barrel, the line of sight must angle down toward the barrel. You can essentially construct a triangle using the barrel, and the sights. If you move closer to the rear sight, the distance between the lines is larger.

The carbine has a shorter sight radius (distance between front and rear sights), and therefore needs either a longer post, or a higher platform for the post in order to zero it. The F marked FSB allows the use of the same post height regardless of sight radius.

This will likely turn into a "pissing match" between the "flat top" and "carbine length" camps, but if you look at the picture you will UNDERSTAND FOR YOURSELF which is correct, and not just repeat errors that you see others propagating.

ETA: Thanks usmc for importing the pic in the next post. I couldn't figure out how to do that.  



OOO! I did it, I did it!

As I said worth what you paid for it. It must be the 8/3 on the A2 and 6/3 on the detachable carry handle I was thinking about.

At least bring the relevant post here thanks

That's what I'd thought.

Mabe this should be tacked?

So I just got a Bushy Patrolman's M4 A3 with removeable carry handle, what does this mean for me in relation to buis? Do I have to by a special buis because my front sight is not fmarked and I have a carbine length gun?

Thanks.

there are no special buis AFAIK.

try to zero it...if you have problems get a "taller front sight post" from Bushmaster

Thanks that makes it clear for some reason.