F-Marked Front Sight Base?

The deck for the front sight post is .040" higher on the F marked front sight base (FSB):


They are meant to allow zeroing of the iron sights on flattop receivers. In practice either FSB may work with either a carry handle or flattop receiver - you just have to try to zero and see. If you have a non F marked FSB with a flattop and it won't zero, you can buy a .040" taller front sight post from Bushmaster.

All in all it's not a great concern. If there even is a problem, there's an easy fix.

If there even is a problem, there's an easy fix.*

For values of easy that may include the front sight post sticking up above the front sight ears.

Not using a F FSB is a sign that the manufacturer is cutting corners. That would be enough to make me look elsewhere.

BSW

If the non F marked FSB's sight post deck is the lower of the two and the center of the bore to the top of the ears is the same for both types of FSB, what circumstances result in an FSP sticking up above the ears?

Non-F FSB and flat top with standard irons makes you unscrew the front sight post to compensate.

And changing FSBs on a barrel ain't easy at all...

BSW

The .040" taller front sight post from Bushmaster did turn out to be an easy and cheap fix for my ArmaLite which has a third-party milspec height rear sight. It didn't stick up too high either so the FSB ears still protect it. That isn't always the case though, I've seen other FSBs where the ears aren't tall enough for the taller post.

Annoying problem that should be avoided if you can.

briansmithwins said:

Non-F FSB and flat top with standard irons makes you unscrew the front sight post to compensate.

Click to expand...

True enough. Or you need to use the .040" taller front sight post.

But it seems to me - help me understand what I may be missing - that since the height from the centerline of the bore to the top of the FSB's ears is the same for both F marked and non F marked FSBs, if your rifle requires the top of the sight post to be above the tops of the ears to be zeroed, it will require that condition with either FSB.

It is possible that you could get an out of spec FSB with short ears, but that is independent of the F marked or non F marked situation.

MMcfpd said:

But it seems to me - help me understand what I may be missing - that since the height from the centerline of the bore to the top of the FSB's ears is the same for both F marked and non F marked FSBs, if your rifle requires the top of the sight post to be above the tops of the ears to be zeroed, it will require that condition with either FSB.

Click to expand...

The heights aren't the same. See the posted picture above again... the "floor" between the ears, along with the ears themselves, are higher.

I've had rifles with unmarked FSBs that required the front post to be higher than the ears. Worked okay on two, but the third one almost had the post fall out because it was so high (for the OP, all three were kits from Del-Ton). Since that instance, F-marked bases have become a requirement for me. Finding a company that does it right from the get-go isn't that hard.

Dionysusigma said:

The heights aren't the same.

Click to expand...

The maximum height of the ears above the centerline of the bore is the same, regardless of the FSB being F marked or not. If your gun requires the top of the sight post to be above the top of the ears to be zeroed, then it will require that same height above the centerline of the bore to be zeroed no matter which type of FSB you use.