AR15 "F marked" front sight base?


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Frog48
February 9, 2008, 10:55 PM
I have a Bushmaster M4 style carbine, with flat top and removable carry handle. I've done some searches online, and apparently I cannot use backup iron sights unless I also replace the front sight post with a taller one, because Bushmaster does not use "F marked" front sight bases on their flattop models.

What other companies do not use F marked bases on their flattop models?

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Bartholomew Roberts
February 10, 2008, 10:12 AM
Not true. You can use a backup iron sight with a standard front sight base. In some cases, if the tolerances stack weird, you will have to replace the front sight post with a slightly taller one in order to zero (I had to do this on my Bushmaster). You definitely need to try it before you buy new gear and even if you do need to buy new gear, the solution is cheap.

As for companies that use F-marked front sight bases, I believe Bravo, CMMG, Sabre Defence, LMT, Colt and a few others all use F-marked front sight bases; but like i said - a slightly taller front sight post will solve the problem just as well, IF there is even a problem to begin with.

Don't Tread On Me
February 10, 2008, 10:45 AM
Totally agreed. I got away with it on one of my AR's. It zeros perfectly. Depends on tolerances as Bart said above and also the POI relationship between the barrel and upper and sights. Not to mention, the ammo. I had no problem, but some people do.

They sell a taller front sight post to make up for the difference which is very small. It will come close to being as tall as the sight guards, but it won't stick out.

Another thing I've noticed is, most people's groups at 50 yards are bigger than the difference in elevation adjustment from a F to standard base with back-up sight.

Just keep something in mind. 1] it's a back-up sight, if you have to deal with a slightly modified battle zero, no big deal. It will be "minute of badguy" out to 200 yards easily and then some. 2] it's a carbine, with the short sight radius and nature of the very carbine itself, it's not going to be used to make 600 meter iron sight shots anyway.

It's mostly a non-issue.

Matt-J2
February 10, 2008, 12:55 PM
I'm certainly no expert on this issue, but from pics I've seen(I believe on ar15.com, but maybe here as well), the biggest difference between F marked and non-F marked is the letter F. The height difference was very, very minimal.

ID_shooting
February 10, 2008, 01:32 PM
I have standard on my Bushy Patrolman, it zeroed right in using USGI targets and blasts pop can out to 200 (all the further I shot it). I say don't listen to net rumors till you try it, then, and only then if you expirience issues you know where to go for the fix.

green-grizzly
February 10, 2008, 05:59 PM
I think the shelf where the sight pin is only 0.04" higher than a regular base. In my experience, it works fine with a regular pin most of the time. The higher pin, which is only a few bucks from Bushmaster, solves the problem when it does occur.

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