AR15 sight question

Howdy: I hope you folks can help me. I am building an AR. I am using an LMT A3 (flattop) upper and a Colt 20" HBAR barrel. I am going to use the standard triangular front site. I was under the impression that I would need to use a sight that was marked with an "F". The place I ordered the sight from said that I would need an unmarked sight as the "F" marked ones were for the M4 style shorter barreled uppers. I also thought that "F" stood for flattop. Does someone know the answer to this question? PLEASE HELP! Also, I thought the difference between an A3 and an M4 were the feed ramps.

I could be wrong but I thought the "F" only applied to the front sight base. Is that what you are referring too?

And if so you do know that those are unique to each barrel right? (as far as fit, you cant just swap them around.)

Yes. The "F" mark on the front sight Base. I Thought It meant it was for the flattop receiver. The Front sight base is new, not a take off. The barrel is also new. It has not been drilled for the taper pins. I bought the part from a place that deals with new mil-spec parts. The guy told me I needed the unmarked one. not the "F" marked. He said it had to do with sight radius. I have a gunsmith that has a jig to line it up and drill it

My Bushmaster M4A3 didn't come with an 'F' front sight base. I had the front sight really backed out to get a proper zero. I bought a taller front sight to compensate. My RRA midlength flat top did come with the 'F' front sight base.

The F sight base is slightly taller and is intended to be used in conjunction with a flat top receiver.
If you use the standard A2 style front sight base you might want to get a taller sight post. They do make them .040" taller specifically for this.
The sight base has no relation to barrel length.

As far as your second Question M16A3 and M4 refer to complete weapon systems. The M16A3 is a flat top 20" bbl'ed rifle. The M4 is a flat top 14.5" bbl'ed carbine.

The feed ramps in the M4 are machined into the barrel extension and extend slightly into the upper receiver. I retired before the M16A3 saw wide use and never had a chance to take one down and inspect. I don't know if the A3 has the feed ramps or not. I am guessing it does, simply because it wouldn't make sense to manufacture two seperate extensions and receivers.

Pete