Quoted:
Quoted: This begs the question: Why didn't Colt, or whoever designed the flattop receiver, take into account the height of the receiver to match a standard FSB? Pure genius was behind that move. |
It has nothing to do with it. Its probably the most common misconception about F marked front sight bases. You need the F marked FSB even on an A2 rifle with a 16 inch barrel. The fact that its a flat top doesn't mean anything. Its all about the barrel length and that standard height rear sight. I.e. the rear sight on an M4 with an LMT rear BUIS is the same height as the rear sight on an A2 upper receiver.
You would have to move the height of the rear sight which isn't practical considering the repeatability of training. So they changed the FSB. |
More or less correct. Look, it doesn't have a damn thing to do with it being a flat top upper reciever. It's all about the positioning of the front sight base. When you build a carbine, the closer front sight poiint requires an .04 inch higher sight. It aleays has---all the way back to the XM177s of the Viet Nam War era. My SP1 carbine came w/an .04 inch higher post in 1977.
Prior to the M4 becoming the standard issue weapon for the services, but with plenty of M16s still in service, Ordnance shops kept a few .04 higher sight posts in stock for the occasional carbine that might come into the shop. When the M4 became standard, somebody (Colt? Army Ordnasnce?) decided it would be easier to simply raise the base that .04 inch. That way, armorers wouldn't have to keep two different sight posts and decide what to use on what weapon.
The F was stamped on M4 barrels at the factory simply to distinguish them from any carbine barrels that Colt still had in stock w/the "old" standard height bases that take the higher post. I suspect that Colt unloaded the older non F barrelled pieces with the higher post on foriegn contract carbines; like those they sold to the IDF.
Why didn't they simply lower the reart sight point? Because ALL government issue M16 based rifles and carbines have exactly the same rear sight height---all the way back to the very first Air Force M16s. But just to make things interesting that's exactly what most of the civie market did do
You buy a Bushmaster, DPMS, Rock River or many other clamp on issue type rear sight unit, and you'll find that the base of the rear sight is.04 inch thinner than a Colt or other milspec piece. That was so they could avoid having to use either a higher post or F base on their barrels.
It's been reported that recent production Colt and FN rifle barrels are now getting F bases on the them---and why not? All that means is thast you'll have to screw down the front post 3 or 4 clicks further than with a standard base; no big deal.
Bottom line? If it's a 20" rifle either a standard or F base is usable. Carbine type, either an F base w/a standard post or a standard base w/an .04 inch higher post; unless it's a flat top w/an aftermarket issue type .04 lower rear sight unit. The aftermarket auxilliary type sight pieces? Some are issue type and some are .04 lower, and I have no idea who has what.
And if all this is confusing, welcome to the club.