[rccalzadilla]

What does it mean to have an AR15 M4 with an "F" marked front sight?

[Quentin]

It's slightly taller than the A2 front sight base (FSB) which is used with an A2 upper receiver (integrated carry handle/rear sight). The F-marked FSB is used with flat top receivers - A4 or often called A3.

[rccalzadilla]

That's good because I ordered a flat top, upper receiver and the front sight is marked with an F.
Will be using a Magpul Gen.2 MBUS Back-Up Rear Sight.
thanks

[Quentin]

That should work fine. The MBUS is designed for the F-marked FSB. If you had an A2 instead, odds are you'd have to unscrew the sight post too much to get the extra height.

[Sniper03]

RC

The F Mark on your rifle is a GOOD THING!
Because if the factory puts the wrong Front Sight Housing on the CAR Rifles, Wrong being the A2 Rifle Front Sight Housing,the operator has a serious problem adjusting his or her elevation on the Front Sight Post. The symptom is they can not get the impact to come down far enough to zero elevation using the iron sights. And when the even get close if they ever do, the Front Sight Post is almost ready to fall out of the Front Sight Housing and the Front Sight Detent is hardly engaging the Front Sight regarding the click adjustments indents on Front Sight Post.
Several years ago if this happened we would add metal to the top of the Front Sight Post, dress it off and blue it. Now they can buy an extended Front Sight Post from Brownells for example which will fix the problem. So as Quentin stated the F is the correct sight for the CAR Rifles.

03

[Quentin]

What makes this a problem is a few makers like DPMS, ArmaLite and Bushmaster put the shorter A2 FSB on their flat tops then sell you a rear sight that's also shorter to correct it. The difference in height is 0.04" I believe. But many people buy 3rd party rear sights which tend to be designed for the taller FSB. Bushmaster does sell a taller front sight post you can screw into an A2 FSB to correct the problem.

Actually Colt is to blame for this problem, they should have used the A2 FSB for both A2 and A4 uppers and just shaved down the A4 carry handle 0.04". Then 3rd party would have followed Colt's lead and we wouldn't have this minor headache.

It's not a big deal as long as you understand why you can't zero with the iron sights you bought. The Bushmaster fix is only about $6 but of course all this doesn't apply to you, RC.

[rccalzadilla]

Thank you, thank you.