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Posted: 8/15/2010 6:06:52 PM EDT
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Can some one school me on the "F" front sight bases? I have a couple that came off of M4s and are not marked "F", but measure the same height. And I have one that was on an M4 barrel but is not "F" height. What's up with that??
Inquiring minds want to know!!! |
| Here's a great thread on the topic from a few weeks ago: http://www.ar15.com/forums/topic.html?b=3&f=118&t=506350&page=1? |
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I was told it was a forging manufacturer specific from a particular barrel manufacturer (E R Shaw) and the F is just a FSB forge code. Wouldn't tell me who the fsb mfg was. I've also seen two identical manufacture factory rifles one had F mark and other didn't. Kinda like keyhole upper, cardinal, or splintered A. It seems on big orders from suppliers to mfg the FSB manufacture will very in the same case. Some separate on assembly and some don't. I was also told the F mark was just a selling point used by certain manufacturers and is almost an inside joke kinda like sendin a newbie to get a left handed monkey wrench. F marked means it's definitely forged and not possidly a cheaper cast version being passed off as forged. What's is 040 about as thick as a sheet of cheap copy paper. The picture they use is blown up so you can actually see the difference.
Dave |
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The "F" was made for a reason and there is an e-mail directly from Colt that explains why it was developed. One of the retro members here took the time to ask them.
A manufacturer might feed you a line of non sense and get away with it because in most cases the "F" is not needed. It is only needed on some carbine length gas systems used with a flat top receiver. For A1 and A2 receivers, regardless of which standard gas system, the "F" is not needed; however it can be used, too. On flat top rifles and mids either FSB will work. On a few carbines with flat tops the standard FSB does not allow enough height on the sight post to lower the POI at close distances (25 yards). That .040 taller shelf rectifies the situation. On the limited number of carbines with flat tops that the standard FSB doesn't work with there are several fixes. One is the "F" marked, higher shelf FSB. One is a taller front sight post on a standard FSB. Others include lower (referred to as commercial) height carry handles used with the standard FSB. So there are a lot of combinations for the limited number of applications that the problem exist anyway. There has been some (very little) confusion abut one of Colts older, standard FSB's. The code on the right side includes a letter and some numbers. (This code was on some mid 80's, 20", pencil barrel configurations Bubbaguns sold). Sometimes that letter is an "F". This is just coincidence and doesn't mean that FSB is "F" height. The "F" for the higher shelf is usually located on the left side of the FSB. To add even more confusion, some manufacturers sell "F" height FSB's that are not marked with "F" - which doesn't matter either way because "F" height works with everything. .040 isn't much, but translates into inches at 25 yards. And for retro builds, since none include flat tops anyway, the standard FSB is used. ETA: The problem is caused because the top wall of a flat top is slightly thicker. When a carry handle is mounted the overall height of the rear sight sits a little higher when compared to a fixed handle receiver. This small difference in the rear sight height is what makes the standard front sight a little too low on those carbines at short distances. When the post is raised enough to compensate, it runs out of threads before it gets high enough. |
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These photo's are from the variations guide.
The "C" in this alphabetical code on the right side of the FSB eventually progressed to the letter "F" . It didn't matter back then, because the "F" FSB didn't exist yet. It does not mean it is an "F" sight base. Colt M16A2 conversion kits/replacement barrels, M16A4 FSB's and late replacement M16A1 barrels have a jibberish forge code: 
This is an "F" height FSB. The "F" is stamped on the left side. The M16A4 front sight base is similar to the late M16A2's, except that the platform the sight post screws into is higher, these are marked with an F on the left side: 
ETA: Heatnbeat, the easiest way I found to measure them is with calipers while the FSB is off the barrel. Go from the top of the front barrel hole to the top of the shelf - similar to the red lines in the picture tel004 posted above. Going to the top of the ears doesn't work since they are the same height (or very close) on either type. |
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Colt's reply to my question on the subject included this:
Compared to the standard rifle sight, the carbine rear sight sits higher relative to the bore centerline, in order to maintain enough wall thickness in the top of the upper receiver. These same upper receiver and sight dimensions are also used in the current M16A4 rifle.
Colt initiated a study of the interface relationship between the detachable carry handle and the dovetail on the upper receiver, to determine how this interface relates to the carbine's ability to accommodate the sight adjustment requirements for targeting. Research indicated that the maximum allowable tolerance was well beyond the allowable windage adjustment for the carbine, and accordingly, two modifications were made. First, the locating relationship between the handle and the upper receiver dovetail was improved, which improved the consistency of the rear sight alignment with the bore centerline. The second modification was to increase the windage allowance. It was soon discovered that some carbines failed to meet the targeting requirements because the front sight post could not be adjusted high enough. To correct this, the front sight assembly was raised up to bring it in line with the elevated rear sight. These higher front sight assemblies were marked 'F' (for Flat Top) on the left side. Both the flat-top rifle and carbine utilize the same front sight post. |
| Thanks for the correction. Seems the problem is with the Colt carry handle. Bushmaster notes this problem also when using a Colt carry handle in their catalog on P. 50. When I inquired about this many years ago I was shown a case of 100 FSB from a FSB supplier and some were F marked, some were not and all appeared to be standard height. Everybody there thought it was a big joke when I inquired. I also use Colt carry handles when I can't afford something better (lighter) on flat tops and have never had the first problem with zero on an AR carbine. I have always seen more issues with windage than elevation but nothing a couple of clicks won't usually cure. Back in the day FSB's were sold according to barrel diameter only. Seen a lot of Rock Rivers that were marked F and were standard height and I own two light weight flat top carbines purchased as kits that do not have F marked FSB, one has BFI in diamond and other C & Bell. Neither is a Colt and neither is M-4. Does seems like at some point they should all be F marked. Sure would have been easier to change a $5 front sight post when a problem. Dave |
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Looks like you have it figured out. The Colt carry handle with the standard front sight is sometimes a problem. The taller front sight post would probably have fixed it or a Bushmaster carry handle with the regular front sight post would have worked, too.
When you look at a Colt carry handle and a Bushmaster they appear to be the same, but there is a slight difference. When both of the sight cradles (I have no idea what it's called, but that works) are bottomed out, the Bushmaster will sit slightly lower rectifying the problem while they continued to use standard FSB's. Basically, Colt and Bushmaster originally used two different solutions for the same simple and occasional problem. As I understand it now all new military rifles and carbines, regardless of gas system, use the "F" FSB. |
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I have not seen one in person.
I have one of these DEZ Arms set screw .750 FSB's and it's "F" marked. The .625 may be "F" marked. If you are looking for one give the lady a call. She has been pretty helpful with me in the past. Looks like they may have some without set screws, too. |
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anybody ever seen an "F" marked .625 fsb? My BCM pencil barrel has one. Are BCM pencil barrels .625 all the way through, or are they raised to .750 under the FSB? Mine is .625" at the FSB and drops to ~.575" out to the flare at the muzzle threads. |
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anybody ever seen an "F" marked .625 fsb? My BCM pencil barrel has one. Are BCM pencil barrels .625 all the way through, or are they raised to .750 under the FSB? Mine is .625" at the FSB and drops to ~.575" out to the flare at the muzzle threads. Gotcha. I searched their site, but couldn't find it. I asked because I saw a BCM lightweight carbine (or was it midlength?) barrel that was raised to .750 under the FSB, then back down to .625 forward of the FSB. |
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anybody ever seen an "F" marked .625 fsb? My BCM pencil barrel has one. Are BCM pencil barrels .625 all the way through, or are they raised to .750 under the FSB? My 16" model 1 sales barrel has this profile. Sure is ugly in my opinion |
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