I'm putting together my first M4gery build, and wanted to know. Do I need a
"F" marked FSB to use a flat top upper?

Some do, some don't.  Some use a non-"F" marked FSB and a +0.040" front sight post.

The flattop receiver/"F" marked FSB is your surest bet, though.

Quoted: The flattop receiver/"F" marked FSB is your surest bet, though.

Mongo. You and I finally have a disagreement

First of all, the higher front sight point doesn't have a thing to do with whether or not the piece is a flat top.  It's purely a function of need when using a carbine type barrel with the shorter gas tube/handguards.  And Colt used the standard base height with a higher front sight post, for years, all the way up to the M4 series.  My 1978 produced SP1 carbine came that way, and so did all the XM177 type SMGS. The shorter barrelled pieces, whether flat top or old style, have always needed a higher post.

The only reason for the F marked barrels is for GI armorers to only need one height front sight blade for both carbines and rifles.  I had speculated on this as the reason for some time, and we recently had a post from a GI confirming it.

Now I like Colt uppers and barrels as well as anyone, but I sure wouldn't pay the extra they get for them just to get an F marked sight base.  Not when I can get the higher post for the standard base from Bushmaster for just a few bucks.

Quoted: Quoted: The flattop receiver/"F" marked FSB is your surest bet, though. Mongo. You and I finally have a disagreement First of all, the higher front sight point doesn't have a thing to do with whether or not the piece is a flat top.  It's purely a function of need when using a carbine type barrel with the shorter gas tube/handguards.  And Colt used the standard base height with a higher front sight post, for years, all the way up to the M4 series.  My 1978 produced SP1 carbine came that way, and so did all the XM177 type SMGS. The shorter barrelled pieces, whether flat top or old style, have always needed a higher post. The only reason for the F marked barrels is for GI armorers to only need one height front sight blade for both carbines and rifles.  I had speculated on this as the reason for some time, and we recently had a post from a GI confirming it. Now I like Colt uppers and barrels as well as anyone, but I sure wouldn't pay the extra they get for them just to get an F marked sight base.  Not when I can get the higher post for the standard base from Bushmaster for just a few bucks.

This is all news to me and contrary to the ARFCOM lore.  It makes more sense than the ARFCOM lore, so I am more inclined to believe this.  Thanks for sharing.

I have always had a hunch that the difference in gas ops between the full rifle and short carbine type ARs caused the carbine muzzles to have risen further at the time of bullet exit from the barrel, which would explain the need for the higher post.  Someone else recently pointed out that the shorter sight plane of the carbines was the reason; which may also play a part in the need for the higher post.

Whatever the actual reason, the  "flat tops need a higher front sight because of  diiferent rear sight height" is an urban legend, unfortunately spawned on the site quite some time ago.  Sure, it's possible to use a rifle height sight on a non F marked barrelled piece,  The base of the front sight will be a bit above the base, but nothing one's eyes can't adjust to after a few tries.

The bottom line, though, is that all carbines, of whatever type and manufacure, should have the higher post.  Colt has always built them that way, either w/a higher post, or more recently the F base, and the other makers should.

Then why does Colt's use the "F" FSB on flat top 20" rifles?

Quoted: I'm putting together my first M4gery build, and wanted to know. Do I need a "F" marked FSB to use a flat top upper?

No, you do not.

Quoted: Then why does Colt's use the "F" FSB on flat top 20" rifles?

Ask them

My guess is because they don't build 20" rifles for the Armed Forces any more, and they're not making any more of the original height bases, so they're putting the F marked bases on what they sell to the civilian market because that's all they make these days.

Remember, Colt has always built civie market pieces with whatever was easiest and cheapest for them.  The total flat side lowers of the SP1 and 2 series came about because they had a bunch of blank forgings to use up. Once they were gone, "Hey, the new Colt AR15s finally have the rib on the side", etc,etc.  If they're not making 20" rifles for the military any more, I really don't see them doing special runs of the original bases for the civie market 20" riufles.

No reason for them not to do so.  Just means you have to screw down the front sight a little further into the base to get the rifle sighted in.

The fact remains, if you have a carbine type AR, you need a higher front sight point than if you have a 20" standard rifle barrel. And it doesn't matter if the thing has a flat top or not

Quoted: My guess is because they don't build 20" rifles for the Armed Forces any more, and they're not making any more of the original height bases, so they're putting the F marked bases on what they sell to the civilian market because that's all they make these days.

Colt's still gets M16A4 contracts.  Those rifles have a "F" FSB.  Colt's Flat top rifles have "F" FSB, fixed carry handles do not.

Reason being that the rear sight is higher, so the FSB also needs to be.  Take a measurement from the bottom of a M16A2 receiver to the part where the rear sight bottoms out on.  You should get about 2.58".  Take the same measurement on a M16A4 or a M4, and you will get about 2.65".

Last I knew the front sight post on the USGI M4, M16A2, and the M16A4 were all the same part number.

Quoted: Quoted: My guess is because they don't build 20" rifles for the Armed Forces any more, and they're not making any more of the original height bases, so they're putting the F marked bases on what they sell to the civilian market because that's all they make these days. Colt's still gets M16A4 contracts.  Those rifles have a "F" FSB.  Colt's Flat top rifles have "F" FSB, fixed carry handles do not. Reason being that the rear sight is higher, so the FSB also needs to be.  Take a measurement from the bottom of a M16A2 receiver to the part where the rear sight bottoms out on.  You should get about 2.58".  Take the same measurement on a M16A4 or a M4, and you will get about 2.65". Last I knew the front sight post on the USGI M4, M16A2, and the M16A4 were all the same part number.

I'll take your word for it that the flat top 20" barrelled pieces being produced have a different total measurement than the earlier produced fixed handle, but that wasn't the original question. Whatever the configuration of the upper receiver, flat top or fixed sight, carbines need the higher front sight, whether it is acheived with a higher post, or a higher mounting point on the base.

Now when I did have an M16A2 clone at my house, and measured the dimensions from the top of the long range rear aperture to the bottom of the upper, and compared it to the same measurement on an M4gery, built on a Rock River upper receiver and barrel and a Bushmaster sight assmbly, the total difference in dimensions was 4 or 5 THOUSANTHS-----surely not enough to justify a higher point for the front post.  The A2 clone did quite well with the standard front post, while the M4gery, like the XM177'd SP1 I own, needed the higher Bushmaster front post.

My SP1 carbine came from the factory with a higher (and skinnier) round post in 1978, and when I replaced it with a current type square profile piece last year, I needed the Bushie higher unit to avoid having the collar sticking up above the base.

So at the risk of being overly repetitive, I'll restate my original point.  If you have a carbine type AR, flat top or sighted, you will almost certainly need a higher front sight point.  Whether you acheive it with an F marked front sight base or a higher post is irrelevant. You can pay for a Colt F base barrel or go aftermarket and use a higher post. Either way works

At the risk of myself being repetitive, as they say........  I don't know much about clone/Brand X stuff, but with the real deal the detachable carry handle is taller then a fixed, so you may need a taller FSB.  That is what Colt's found when they put the flat top into production, and that is why they use taller "F" FSB's with their flat top rifles and Carbines.  Taller front sight posts were not an option, in that the Army was not going to allow another part number.

Quoted: CJan_NH posted these pictures a while back showing a M16A4 (flat top) [20" rifle].  It also is build with the "F" (Flat top) front sight base. I hope he does not mind me reposting his pictures. img4.photobucket.com/albums/0603/Ekie12091941/Colt_M16A4_2220_small.jpg img4.photobucket.com/albums/0603/Ekie12091941/F.jpg

S. Hood took those pics while he was in Kosovo (notice that they are hosted on my account).  He got bored and took a bunch of cool pics for me, these are my favorite:

www.avtomats-in-action.com/pro17.html

And here is a close up of the FSB on my M16A4 upper I got from Dennis Todd:

 

More pics here:

www.ar15.com/forums/topic.html?b=3&f=118&t=234528

so, is there something wrong if I would convert my A2 in a A4 without changing the FSB (non F marked)?

Quoted: so, is there something wrong if I would convert my A2 in a A4 without changing the FSB (non F marked)?

You may or may not need a taller front sight post.  I still haven't found any consistency in the "F" marked FSB application and I think it has as much to do with how each shooter uses their sights as it does with the equipment being used.

Quoted: Quoted: so, is there something wrong if I would convert my A2 in a A4 without changing the FSB (non F marked)? You may or may not need a taller front sight post.  I still haven't found any consistency in the "F" marked FSB application and I think it has as much to do with how each shooter uses their sights as it does with the equipment being used.

also, can I keep my bolt+bolt carrier (all colt) even if I put a non-colt upper?

Quoted: so, is there something wrong if I would convert my A2 in a A4 without changing the FSB (non F marked)?

it has absolutely nothing to do with using a flat-top upper, it depends on the rear sight you select.  I have used a Colt carry handle, a Mark Brown CQB rail, DPMS detachable rear sight, GG&G A2 flip up and a Larue detachable.  And I have used them with a Colt M4 barrel, a Colt 16" lightweight barrel, an RRA 16" midlength, a Colt 20" A2 barrel, an FN A2 barrel and another FN A2 that was cut to 18".  Of all those combinations the only rear sight that needed a taller front sight post was teh Larue, and the Larue worked fine with the two FN barrels but needed the taller post when used with the Colt 20" and the RRA midlength.

Based on that experience, it is clear to me that there is no easy way to predict what combination of barrel, upper and rear sight will need the taller post or F front sight base until you actually get aroundto testing them on a range.  If you are buying a new barrel and one is available, then getting the F marked base is good insurance, but is not required.  Trying to replace the front sight base on a barrel you alrady own is far more trouble than it is worth.

Interesting.

My 16" w/A2 upper has the standard FSB and the front site post is about flush when zeroed. IIRC, it's only 2 or 3 clicks from perfectly flush.

My flattop 20" w/detachable carry handle needs the taller front site post. It is about tapped for elevation and is sitting about .035" higher than the FSB flat that is marked "UP".

shamayim, I'm not sure I buy your idea, although I know you know your stuff.

What I have in front of me just isn't coming up with the same results. Kinda the opposite, actually

WIZZO

My recently purchased sabre 20" flat top upper has the F marked upper and I really doubt it has anything to do with left over Colt parts. With my zero it's always a few clicks up or down from flush with different ammo zero's.

The thing I don't see mentioned here is that the +0.040" front sight posts just about stick above the sight protector ears on the front sight base.
If you need a taller front sight post with your particular configuration, by all means, use a barrel with the "F" marked front sight base.
It's not as much of a problem to use a shorter front sight post in an "F" marked base than a taller front sight post in a shorter front sight base.

Randall Rausch
www.ar15barrels.com