I have a 1.94 (A2) FSB on an Armalite 16" midlength 1:9 with a M4 flat top.
Armalite doesn’t sell a flat top with a conventional style front sight (A2 or F-marked), their M15A4C has a railed gas block, which I didn’t want at the time.
I hear lots about purchasing the Bushmaster extended front post, but I can’t reason how that fixes anything because the front post will still be far more elevated than it should be. see photos at the bottom of the first post here
Does the extended front post simply allow the front post to be properly secured in its threads, since if you don’t it wobbles?
(BTW I bought the Armalite because it was a good price [180 for the complete barrel versus 260 for a CMMG and about 230-ish? for a BCM, which weren’t in stock at the time] I didn’t know Sabre Defense and RRA made midlengths back then))
If changing the front post doesn’t fix the relation between the post and the sight ears, which is confusing to my eye, then I’d be up for an swap. But Randy at ar15barrels.com told me it’d have to be specially rigged because the front sight bases and the barrels are drilled at the same time in a special fixture.
LMK if you have an unused untarnished F marked FSB.
EDIT: Heard that there’s such a thing as “civilian” flattop carry handles that don’t need the F marked FSB (or F-height, in the case of LMT)…that true? never seen it substantiated nor seen one for sale advertised at such.
I hear lots about purchasing the Bushmaster extended front post, but I can’t reason how that fixes anything because the front post will still be far more elevated than it should be. –It elevates it just enough to remedy the flaw of NOT having an F FSB, which is what you need for a flattop upper since your BUIS sits a bit higher than the sights on the fixed carry handle type upper.
Does the extended front post simply allow the front post to be properly secured in its threads, since if you don’t it wobbles? –Yes, it keeps it securely attached while allowing you to properly zero with your BUIS. The standard post on a non-F FSB would have to elevate to the point of instability to get it zeroed.
Heard that there’s such a thing as “civilian” flattop carry handles that don’t need the F marked FSB (or F-height, in the case of LMT)…that true? never seen it substantiated nor seen one for sale advertised at such. – Not sure if it’s referred to exactly as a civilian carry handle but Ive read somewhere that BM’s carry handle are a bit lower to compensate for the non-F FSBs on their barrels.
Ahhh very good info. I haven’t sen that info from a search before.
If the Colt detachable carry handle is .04" taller, then will that bring my A2 front sight and flat top rear into alignment?
…raising the rear raises the point of impact, lowering the rear lowers the impact; raising the front lowers the impact, lowering the front raises the impact…
If the rear is raised .04", then that mean the front would have to be lowered to maintain the same Point Of Impact/Aim.
So that means, if I am correct, I could just swap on a Colt carry handle. Unless the rear sight base (not the handle itself, the little “tub” with the screw in it) could be swapped out for the one I already have?
I did a test with a micrometer. 0.040" measured from the elevation screw bottomed out upwards, I counted 12 clicks. My carry handle is 1/2MOA clicks.
EDIT: Oh and I had my front post level as it comes from the factory. I raised the rear up 12 clicks, or an effective .04", and fired one HMC .233 55gr. The round could not be seen on the target.
If you want to use a Colt carry handle or any quality BUIS, you must install a 0.040 taller front sight post in your Armalite’s front sight base to bring you gun up to spec.
