I'm sure this has been covered over and over again, but I can't find it so here goes:

I know the F marked FSB is for flattop uppers - is there enough adjustment to use the F FSB on an A2 upper?

For a carbine?  Yes.

20" rifle.

You may have problems then.  I would seek a different barrel if it were me.

You'll be fine. A 20" rifle with an F marked FSB can be used with either an A1/A2 upper or a flat top with plenty of adjustment to spare.

The longer sight radius makes a difference.

I have a mid-length rifle with an F marked FSB with an A2 upper and the base of the front sight post is only a mm or two inside the shelf of the FSB. I also have a flat top 20" with a non F marked FSB and the base of the sight post is almost even with the shelf of the FSB. Both are zeroed at 50 yards with XM193.

The only time you may run into problems is if you use a carbine(and maybe a mid-length) with a non F marked FSB and a flattop.

On my midlength I had to crank the front post way down to get it sighted in.

Guess I'll find out!  Let you know if/how it works.

Contrary to some of the nonsense you may have read on the error-net on the subject, an “F” marked front sight base will work just fine with a 20” barrel.  (In fact, it can be beneficial if you plan on using an Improved Battle Sight Zero.)  

The front sight post on a Colt front sight will bottom-out in the sight well at approximately 24 clicks below flush.  On an “F” marked front sight base the sight “shelf” is a whopping 0.040” higher than on a standard front sight base.  That means you might have to turn your front sight post down approximately 6 clicks further with an “F” marked front sight base than on a standard front sight base.  (6 whole clicks difference.  Oh, the inhumanity!)  So unless your front sight post is set at 18 clicks below flush with a standard front sight base, using an “F” marked front sight base will be a non-issue.

Quoted: Contrary to some of the nonsense you may have read on the error-net on the subject, an “F” marked front sight base will work just fine with a 20” barrel.  (In fact, it can be beneficial if you plan on using an Improved Battle Sight Zero.)   The front sight post on a Colt front sight will bottom-out in the sight well at approximately 24 clicks below flush.  On an “F” marked front sight base the sight “shelf” is a whopping 0.040” higher than on a standard front sight base.  That means you might have to turn your front sight post down approximately 6 clicks further with an “F” marked front sight base than on a standard front sight base.  (6 whole clicks difference.  Oh, the inhumanity!)  So unless your front sight post is set at 18 clicks below flush with a standard front sight base, using an “F” marked front sight base will be a non-issue. www.box.net/shared/static/uei1uar8kw.jpg www.box.net/shared/static/2d520zm04w.jpg

Nice pictures.  I've read several anecdotal experiences of folks with F marked FSBs that bottomed out with fixed carry handle uppers.  I'm not questioning you; the numbers, it does seem logical that it should not be a problem.

Molon,

thanks for the pics and detailed explanation.  that's exactly what I needed!

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.

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.

I take all this to mean that I can go ahead and stuff a Denny's Operator barrel
into the front of my ArmaLite A2 middy carbine, and won't have to worry about
shooting down aircraft overhead, or blowing a hole in my foot the first time I
pull the trigger, eh??    

Quoted: I take all this to mean that I can go ahead and stuff a Denny's Operator barrel into the front of my ArmaLite A2 middy carbine, and won't have to worry about shooting down aircraft overhead, or blowing a hole in my foot the first time I pull the trigger, eh??

I don't know.  Have you shot down aircraft or blown holes in your feet with your current barrel?

my Colt 20inch w/ A2 has a F marked FSB.

no problem at all

Thanks Frens, so refreshing to hear that I won't be having a problem with the FAA,
or my podiatrist.  Order for GTS Operator barrel to be generated soon.  Nothing
really wrong with that Wilson 4140  1/9 tube, it's just that the barrel is the weak
link in the ArmaLite middy chain and the F front sight issue was causing concern.

I have a 16" Sabre middy with flat-top upper (Troy BUIS) that has the F marked FSB and I'm probably 2-4 clicks below flush.

I just got a 14.5" Bushmaster M4 flat-top that doesn't have the F marked FSB, and it appears as though it is standard. (Has a crown on the left side.) I plan on slapping a Colt, CMT, LMT, or Bushy carry handle on there. Should this barrel already have the taller sight post?

No, but you might need one.

...why does Bushmaster use the standard FSB on the M4 upper then? Especially since they do the M4 feedramps and 4150 barrels.

Because it works just fine 99% of the time, or at least it did 100% of the time on the 5 BM carbine barrels I own.

It's an issue with BM, but not much of one IME.

I agree never had any problems with my Bushmaster.

Just out of curiosity, how did you zero them? 50 yard zero, or 25m zero?

I'd like to know as well.

Even with my .04 taller sight a 50 yard zero puts my front sight almost at the top.

My 20" gave me no issues with zeroing as much as the carbine has.