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Posted: 2/20/2007 6:20:59 AM EDT
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When do I have to use an F marked FSB? Also, when Do I not? Can I use a carbine length barrel with a flat top upper? A2 upper? Can I use a rifle length barrel with a flat top upper? A2 upper? When does it make a difference? Thanks in advance for the info. |
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It's hard to say. If you're buying a barrel to mate with an IN SPEC m4/flat top upper, you would want the F base. But it really depends. There's a lot of aftermarket crap out there. Combine that with the fact that some FSBs are not marked F when in fact they are supposed to be matched to a flat top.... If I were you I'd post the specific manufacturors that you are considering and the guys here can give their experiences specific to those parts! |
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If you're going to build a carbine, you'll almost certainly need an .04 inch higher front sight point with either a flat top or A2 type upper. Whether you get a barrel with an F base, or get a standard base with an .04 higher front post is irrelevant. The net result is the same. If you're building a 20" barrelled piece, you will almost certainly NOT need a higher front post or base; unless you're going to use a rear sight unit that sits higher on a flat top than standard. On the other hand, an F base on a 20" barrel does no harm, since at worst all it means is that you'll have to screw the front post down a few clicks more than usual. My personal preference is a standard base with the higher post, since it gives you better vision around the target. But then I've been using that combination on my Colt since 1977. You see, before they came up with the F base, Colt put a higher front post on all it's carbines----and that was 20 years before the advent of the M4 and the F base. 
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I beg to differ. My Factory Colt 20" upper came with the F FSB. My contention is that this is the case because it was a flat top upper receiver with the standard carry handle A2/A4 sight. With this set up, my mechanical Zero was correct. I did not need to dial the Front Sight down to Zero the rifle. |
markm, The reason you didn't have to dial the FSP down is because you have the taller Mil-Spec DCH. All Colt, FN and Mil-Spec flat-tops have taller "F" marked FSBs and all Mil-Spec DCHs and BUISs are equaly taller to match up with the taller FSBs. shamayim is correct in that carbines are the reason for the taller front sight. Colt M4 carbines have the taller FSB but this is negated by the fact that their DCH is equaly taller. This balances out on the A4 rifles though. I have heard shaimayim say that even A2 rifles might be comeing with "F" FSBs now, in which case you would have to dial down the FSP for zeroing. |
So my assumption that the F sight base matches the Mil spec DCH is correct? I'm assuming that the Taller sight issue relates to guys using aftermarket DCHs with an F base out front. |
Right.
Incorrect, carbines need the the taller front sight and standard height rear sights. Correct, for rifles with shorter DCHs and taller front sights. You may have to dial down the FSP more than normal to achieve zero on a rifle set up this way. |
Ahhh. You newbies to the AR world make me laugh. And yes, you are a newbie in my eyes if you've been an AR owner for less than 20 years. Sport, I have no doubt that your rifle w/an F base was sighted in properly; which simply means that Colt asjusted the blade in the base before they shipped it. That's one of the reasons Colts cost a bit more. They test fire and do at least a preliminary sight in before the damn thing leaves the factory; unlike most of the other jack leg outfits out there. So what? My only point in the first post was that carbines need a higher front sight point, and it doesn't matter one damn bit whether it's a flat top or not!!! It's the shorter distance between the the front and rear sights, not the flat top that causes the need for a higher sight point. It would appear that for Colt and apparently FN as well, all front sight bases are going to be .04 inches higher than they used to be, no matter what length barrel. That's fine. And it would also seem tha all future military production is going to be flat top. Also fine. So what's your problem? My only contention is that a higher front sight is needed on a carbine and irrelevent on a rifle. |
So you have to have 20 years of experience to not be a newbie? So what if you have 19 years you don't eek in there? I guess its like AARP if you are one day younger than the age then you are screwed... haha just joking, just trying to bust your chops as the newbie I am... This sounds like a dumb question, as I can feel my newbish self being flamed, what does DCH stand for? I already figured out Front sight base (FSB) by long moments of pondering... I am still trying to figure out all the acronyms for the abbreviations you guys use on here. Also how do I know that I have a F marked FSB? I am guessing it would say it. I just built an ar and my front sight says just "1" on it. I guess its a mute point since I am going low profile gas block and scope but... |
FSB = Front Sight Base. FSP = Front Sight Post. RSB = Rear Sight Base. DCH = Detachable Carry Handle. BUIS = Buck Up Iorn Sights. An "F" marked FSB should have an "F" stamped on the left side of the gas tube port in front of the gas tube roll pin hole. But I don't know if anyone but Colt marks them with the "F". |
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