If you think that you are going to install a rear backup sights on your Bushmaster (and several other brands) rifle and start shooting, you are wrong.
If you replace the rear carry handle on your Bushmaster flat-top carbine you may have a problem zeroing the rifle. The height of the Bushmaster front sight is lower than backup iron sights. Even if you raise the front sight post as high as possible, your may find that your rifle still shoots well above point-of-aim (POA). The solution is as easy as replacing the front sight post… a $6 solution.
Mil-Spec front sight bases for A-3 (removable carry handle / flat-top) AR-15 rifles are taller than older A-1 and A-2 (fixed carry handle) rifles. They are marked with a raised “F” on the front sight base’s forging (as seen right). Colt and other manufacturers that equip their flat-top rifles with Mil-Spec front sights may offer a removable carry handle with rear sights that are taller than the traditional A-1 and A-2 rear sights, and are the appropriate height for the taller front sight.
There are a few manufacturers whose AR front sight towers are not quite exactly Mil-Spec height. Bushmaster still uses the older, traditional shorter A-2 front sight base on their A-3 (removable carry handle) rifles.
To make up for the difference between the front sight tower and the removable carry handle, Bushmaster (and some other manufacturers) use shorter removable carry handles. The difference between a Mil-Spec and commercial removable carry handle is very difficult to tell apart unless you have one of each to compare, or have a dial or digital caliper to measure.
The Bushmaster front sight post and removable carry handle combination work fine together, however you will run into problems should you decide to replace the Bushmaster carry handle with a backup iron sight (BUIS). 99,9% of backup iron sights are Mil-Spec height. When you attempt to zero the rifle you can raise the post in the front sight tower to the point that it wobbly and about to fall out (as seen right) and the rifle will still be hitting too low.
Pictured right is a comparison of the Mil-Spec front sight post (left) that sits on a taller front sight base compared to the .040″ taller post (right) that is required on the shorter Bushmaster sight base to make it work with backup iron sights. These can be ordered from Bushmaster’s website.
It will require a front sight tool to remove the original post and replace it with the taller one. Simply use the tool to simultaneously hold in the front post retaining pin while unscrewing the sight post out of the sight base. It’s very easy with the tool, and frustrating and time-consuming without it.
It is interesting to note that replacing the front sight post is more critical on Bushmaster carbines, but may not be necessary on full-length rifles with 20″ barrels.
Before you start ordering parts for replacing your removable carry handle and front sight post from your Bushmaster, verify that it is not marked with the raised “F”. Older Bushmasters were not. There is a chance that since Bushmaster is under new ownership that newer rifles may have the correct height Mil-Spec front sight tower and sight post that will work with Mil-Spec backup iron sights.