"F" marked front sight base questions

I know an "F" marked front sight base is for use with flattop upper receivers. It is used so that back up irons will properly zero. At what range does it become an issue to not use an "F" marked front sight base? When does it affect your point of impact? Considering most companies do not use "F" marked front sight bases, (RRA, Bushmaster, etc...) as a matter of standard practice, will it hamper accuracy? I know it all comes down to the question,"what is the intended use of that particular AR"? Any information would be most useful. Thank you.

I'm not entirely sure I understand the question(s), but...

Typically most people will zero an AR at 50 yards, and some will zero at 100 (and yes there are still some that will zero at 25). Zeroing at 50 yards, without an "F" FSB but using a BUIS intended for one, you will most likely run out of vertical movement in the post itself and will have to get a taller post to make it work.

What most people report, that don't know/understand about the "F", is that they were trying to zero, kept backing out and backing out the front sight post, and by the time they got the rifle zeroed they either had a wobbly post or it came out completely.

Which isn't to say that all of this is guaranteed. I have had non-F FSB rifles that zeroed with a Troy rear BUIS just fine, but they are generally the exception and not the rule.

Also, as long as you're using the carryhandle that came with the gun you should be fine, as most of the companies that sell non-F rifles also sell them with a shorter carryhandle to make up for it.

Someone asked this on another forum and one of the replies was that not all manufacturers mark their sight and it is possible the rifle has an "F" spec sight with no markings.

http://lh5.ggpht.com/_Kybyq3n1q3w/Sb...FshelfColt.jpg

If your sight has no markings on it then you measure it so you know what you have.

Rob S, you answered my question. So it is when you zero at closer in ranges, there is the potential to run out of height in the front sight post, when not using a non F marked front sight base with a BUIS. That is where the taller post comes into play. I think I got it. Thank you.

You can also just get the extended post and put it the existing front sight base. I'm anal so don't but lot's do and they work just fine.