i posted this elsewhere and i got the usual one irrelevant reply…
last year I bought two Colt AR6520 barrels (16" Lightweight, Chrome Lined, M4 feedramps, 1/7 twist, 5.56 NATO) and they have standard FSBs.
I thought i would take a chance and hope that they would zero without a problem. unfortunately POI is still high even as the FSP starts to come out of the base.
what would be the recommended fix for iron sight carbines using these barrels?
replace with a F marked FSB
pros: correct fix for a flattop
cons: finding a place to do it, moderate cost
replace with taller FSP (like bushmaster sells)
pros: low cost
cons: more of a workaround than a true fix
replace with low profile gas block, extended rail, flip up front sight
pros: looks nicer
cons: very very high cost, less durable than a standard FSB, not optimal for iron sight carbine
Try the tall front sight post first. It’s the cheapest. PM me if you want a free sight. I think I have one that I pulled out of a NON-F FSB that’s in decent condition.
You’d have to find an “F” that’s made for the skinny barrel, and then have someone re-drill for the pins.
replace with taller FSP (like bushmaster sells)
pros: low cost
cons: more of a workaround than a true fix
Nothing really wrong with this method. You MAY wind up with the sight sticking above the ears, which is the only reason I’d be concerned, and even then it’s a minor concern at most.
replace with low profile gas block, extended rail, flip up front sight
pros: looks nicer
cons: very very high cost, less durable than a standard FSB, not optimal for iron sight carbine
My choice, and what I did. Although I’d shave the stock FSB down to fit under the rail and re-pin. Much better solution IMHO.
If you’re going to use a rail anyway, it’s a minimal cost increase for the rail itself. Obviously there is the cost of the front sight itself.
It shouldn’t stick up that high. It took me a while to figger out why my POI was so low with the upper I pulled my tall sight post out of… It really wasn’t obvious until I measured the height of the post.
You’re right, it shouldn’t, but things can be on both ends of the spectrum. Just like some rifles have enough travel in a non-F to work fine, others can really need that thing jacked up there.
