Sight Post Issues

[csmith]

I took my mid-length AR out for the first time two weekends ago to get it zeroed (50 yds). Zeroing the PRO was a straight forward affair, however when I zeroed my irons (standard "F" marked A2 front, MBUS rear), I ended up with my sight post 1 quarter turn above flush with the wings on the side. I thought it kind of weird, seeing as my duty M4's front sight is well into the base (but for a 25 yd mil zero). I took it (my personal AR) back out this morning and moved both rear and front sight around, purposely ruining that zero to reaccomplish my zero and see if it was just an off day or something. I ended up right back where it was.

My question is what's pushing my front sight up so high? Is the MBUS not meant for "F" FSB's, or is the added length between the sights mixed with zeroing at 50 yds causing it? My main concern is the front sight post obviously being above its intended protection.

[Quib]

Not sure what's going on, but it sounds like you need a .040" taller front sight post.

Scroll down to: A2 Front Sight Post for Flat-Top Colts

http://www.bushmaster.com/products.asp?cat=21

[Bob Reed]

Hello,

COLT Offers a Total of 4 Different Front Sight Post.

Part No. SP64665 Short Tapered Post for Carbines.

Part No. SP64507 Short Square, NON-Tapered Post for Rifles.

Part No. SP61706 Round Post for A1 Rifles.

Part No. SP62447 Tall Straight Post for 9mm SMGs.

If the flange on the sight post is above the base (or is close to being above it) then you can install Part No. SP62447 in order to help keep the flange more flush with the base - but this isn't going to cure the problem that's causing you to raise the post so high to begin with.

Specialized Armament stocks all of the above mentioned parts http://www.specializedarmament.com/catalog/AR_15_Parts-4-0.html

[5pins]

Not sure why you are having a problem, I have used the MBUS with a mid-length BCM upper and had no issues. Are you sure it’s an F marked FSB? What make is it? You could try measuring it and making sure it’s the correct height.

[Gun]

Not sure what's going on, but it sounds like you need a .040" taller front sight post.

Scroll down to: A2 Front Sight Post for Flat-Top Colts

http://www.bushmaster.com/products.asp?cat=21

Don't you mean a taller FSB?!!?

[Quib]

Don't you mean a taller FSB?!!?

Nope. I meant FSP.

A taller (F) FSB would help, sure. But this would be the more complex route verses simply swapping out his current FSP for the taller, .040" FSP.

[csmith]

Thanks for all of the answers guys. I didn't think it was right, but wanted to ensure I actually had an "issue" before I started looking for solutions.


Not sure why you are having a problem, I have used the MBUS with a mid-length BCM upper and had no issues. Are you sure it’s an F marked FSB? What make is it? You could try measuring it and making sure it’s the correct height.

It's a Spike's Tactical upper. Claimed to be, as well as marked, F. I'd measure if I could, but I don't have anything that can measure the .04" difference between F and Standard.

Quib, Bob Reed, thanks for the links. I'll start looking into a taller sight post. Out of curiosity, even though the sight will go into the shelf due to it's added length won't my point of aim still leave the tip above the wings?

I snapped a few pics to show what I'm seeing.

http://i803.photobucket.com/albums/yy316/CASDB/Firearms/TallFrontSight3.jpg
http://i803.photobucket.com/albums/yy316/CASDB/Firearms/TallFrontSight2.jpg
http://i803.photobucket.com/albums/yy316/CASDB/Firearms/TallFrontSight1.jpg

[Quib]

Don't think there's much you can do about the height of the FSB ears.

Installing a taller .040" FSP though, will bring the FSP itself back down into the the FSB during zeroing. So the FSP adjustment notches will then be level with the FSB shelf. Doing this may drop the tip of the FSP down some, but in the cases where I've had the same issue as you have here, my actual zero ended up being mechanical zero (notches level with the FSB shelf).

ETA: After I ran into this same issue with one of my builds, I made it a habit to keep a .040" FSP on-hand in my spares box that I take to the range.

[Bob Reed]

Quib, Bob Reed, thanks for the links. I'll start looking into a taller sight post. Out of curiosity, even though the sight will go into the shelf due to it's added length won't my point of aim still leave the tip above the wings?
Yes, but at least the flange will be closer to flush, if not totally flush with the shelf.

[Eric]

won't my point of aim still leave the tip above the wings? Unfortunately, yes it will.

[csmith]

Thank you for the answers guys.