"F" FSB & Troy Flip-up

[mudrock]

It's not a flattop thing, it's a carbine barrel sight radius thing.

I heard somewhere that "F-marked" means "for flat-top." Seems that might be wrong.

Don't know if I get what you mean by sight radius. Is it something like retaining the same relation between the line through the sights and the line through center of barrel.

You seem to be saying "F" really indicates "correct when used with carbine-length gas system" like a 14.5" barrel with fixed hider. And I guess the standard front sight is correct for a 20" barrel with rifle-length gas system.

I'm making a lot of inferences and assumptions in the above.

My soon-to-be first AR build, 16" mid-length, falls between carbine and rifle length gas systems. Maybe this "F-marked" vs "standard" front sight issue is irrelevant to my project (maybe not?). Even so, I'd like to understand the reason for these different sights.

[AR15barrels]

But is it Mil-spec?

Not after he adds the descending dimples to it.

[AR15barrels]

I heard somewhere that "F-marked" means "for flat-top." Seems that might be wrong.

You seem to be saying "F" really indicates "correct when used with carbine-length gas system" like a 14.5" barrel with fixed hider.

I know it's confusing, but both are correct at different times and for different reasons.
When you add tolerance stacking into the mix, both can also be proved wrong in some cases.

The only thing that matters is that you can zero within specs and hit what you aim at.
If this requires a taller post or an "F" FSB, so be it.