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Posted: 1/4/2007 7:38:23 PM EDT
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What is the diffrence between a F marked and a non F marked front sight base? Is one better than the other? Thanks |
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I'm building an M4gery, and my Wilson 14.5" barrel is NOT F-marked on the FSB. Okay, this has been answered vaguely in the past in many threads. I was always told that the F-marked front sight base (or longer post) was only needed for a flat top (A3/A4) upper receiver with a carbine-length gas system, and with a "standard" or "military" carry handle type sight. Does this issue exist with "non military" rear sights/BUISs like YHM flip-ups, Larue BUISs, MI BUISs, or others? One guy in the archive says he swapped his non-Flattop Colt uppers from A2s out to A3s and they sighted in fine w/no F-marked base. archive.ar15.com/forums/topic.html?b=3&f=18&t=289660 Another says the tolerances have to stack against you before it's even an issue and that the Bushmaster carry handle is made to work with a non-F-marked FSB. archive.ar15.com/forums/topic.html?b=3&f=18&t=287858 Is the F-marked FSB even an issue? |
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Won't speak for everyone, but when I first got my carbine, it was a Double Star upper with a 16" barrel, and non-F marked FSB, and it wouldn't zero correctly with my GG&G A2 BUIS. I ended up switching out the entire upper for a variety of other reasons as well as the sighting issue, and ended up with a CMMG 14.7" upper with F-marked FSB and a MATECH BUIS, which sights in just fine now. I didn't even know about the FSB problem at first, I just could not get the thing to zero correctly, even with the post up as high as it would go. Also, it seems that Bushmaster sells a front sight post that is longer for carbine, and that Colt also originally used this method before the advent of the flat top receiver for carbines. Something abou the sight radius, and the need for a higher post is what I've heard, which first transferred into a different part number for the carbine front sight as opposed to the rifle front sight. I've also heard a story that when the first flat top receivers were produced, they were not as high, and the roof of the receiver became dangerously thin, leading to the specs being changed to add more "meat" to the top of the receiver, and by this point, the carry handle sights had all already been designed and produced, so the solution was to simply raise the FSB. One, or both of these explanations may be true, but in my limited experience, it has made a difference. This of course was in a very unscientific situation, and I did not even know about F-marked FSB's when I decided to switch uppers. HTH, ~Augee |
| Also, how does it affect my new 16" middy with a slightly longer sight radius? I haven't had a chance to sight it in yet, so I don't know what to expect. I no the EOTech red dot (also not sighted it) hits about midway on the front post when looking through the rear sight using an absolute co-witness. I know I need to sight it in soon, but I was curious about the F marked sights also. My front sight is a YHM flip up; it isn't marked with an F, but I cannot determine if it is F height or standard height. |
Well, my new Wilson 14.5" barrel doesn't have an F mark on it, and I'm about to sight it in tomorrow with my new DPMS A3 upper receiver that I just used with it last night. I'll post here to let you know if I'm going to need a longer post or not. By the way, the DPMS Panther Claw receiver vise tool is awesome! Very sturdy and simple to use. My new upper looks perfect. Fighting the delta ring (having to push against it while torqueing on the barrel wrench) sucked, though.  ETA: My DPMS A3 upper has a Cardinal forge mark on it, and (like my Anchor Harvey broken A-H marked DPMS A3 upper) has an "M" above the gas tube hole in the front of the receiver. Anyone know if that factors into the equation? |
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I'm not sure if everyone got their question answered, if so just disregard. But I thought I would attempt to explain it again. For "flattop" uppers, commercially called A3 uppers, or A4 uppers as the military refers to them, an F marked FSB is used to compensate for the fact that a detachable carry handle on an A4 upper is slightly taller than a standard A2 fixed carry handle. The F marked FSB ensures that you do not "run out of" elevation adjustment on either your front or rear sight. |
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Pictures below are courtesy of Molon. Note that the forgings are the same. It's just the shelf height that's different. There have always been different height front sights, based on the barrel being on a carbine or on a rifle. This has to do with trajectory. So what happened a few years ago (~15) is that colt came out with the "F" marked front sight base so that the same sight post can be used on rifles and carbines. There ARE also different height rear sights, "commercial spec" and "mil spec" The commercial spec solves the same problem a different way, they stay with the A2 height FSB and post and just lower the rear sight. The Military stayed with the rear sight the same height and raised the front sight base to allow the use of the same post.  
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Randall's response, and particularly his picture are exactly accurate, but once again (for the skatyeighth time), let me point out that carbine types have ALWAYS needed a higher front sight point. Colt put higher front sight blades on the very first XM177s, the SP1 crbine I bought in 1977 came with a higher front sight post, and the odds are that any commercial current carbine type will either need a higher post (if it doesn't have an F base) or you'll have to screw a standard post halfway out of the base to get it zeroed. Now I can't respond to what's needed for optical sights. Don't like 'em, don't use 'em, but I'll tell you this. All three short barrelled pieces at my house, two flat tops and an XM177E2 clone, need and have the .040 higher posts that Bushmaster (and DPMS and yes, even Colt) sells. Bushmaster's catalog statement that the higher post is only for flat top Colts is total garbage, and after 5 minutes of arguing w/Bushies sales rep one day, he finally kind of halfway admitted it. The best reason that anyone has figured out for the creation of the F base is that when the Army went to the M4 as standard, they wanted to make things simpler for GI armorers who would have to deal with both the "new" M4s and also the older M16A2s that were still going to be around. Until then, the first issue M4s had still had the standard base. We had a recently discharged ex servise guy confirm this about three years back, when the issue was raised. Now from what I've seen posted, it seems that the F base has become "standard", even on the M16s that FN is turning out for the Marines. It makes sense I guess to just use the same height base for everything. You can always screw the front blade down into the base of the rifle length barrel a few extrra clicks w/o harm, and given the variation of height in the different optical garbage that the services are using to replace adequate marksmanship training, it does give you added flexibility. But as far as I can see, if you have a full length rifle barrel, you don't need a higher front sight, and if you've got a carbine, you do. Whether the extra .040 inch comes from a highr base, or a higher blade is totally irrelevant. Thanks Randall, for illustrating this 
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Thank you for the awesome answers. I searched the archives and couldn't come up with anything good like this. Hopefully, this will hit the archives in a couple of months, and then it will be preserved. The good thread title will help with normal searches until it hits the archive too. I am assuming here that all rear sights like the YHM flip-ups and Troy/Larue/MI/etc. fixed and flip BUISs are all standard height (same as a carry handle), so they will have the same issue. The fact that the carbine barrels could already have the taller base on them or a longer post when they ship could explain why people are able to swap a carbine from an A1/A2 to an A3 upper receiver with no issues like related in the archive posts I linked to above. I'm pretty sure my new Wilson 14.5" M4 barrel came with the taller post. I didn't get to sight it in (just 30 fast rounds for function check in the rain) today, so I hope I don't have any problems. If I do, I now know what to do - get the longer sight post.  Thank you all again! 
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I have a DPMS upper Cerro forge and it has the M above the gas tube hole. The upper has the "T" marks. No M4 feedramps which would be marked with a "4" or "M4" above the gastube hole. Mike |
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This thread is timely. Yesterday, I tried to sight in 3 16” carbines. All have Troy BUISs. One is a factory Bushmaster and the other two are CMT/Stag uppers with Bushmaster barrels. The ways things worked out, the factory Bushmaster was able to achieve proper zero. The other two ran out of “up” on the front sight post before zero could be obtained. I’ll be ordering a few longer front sight posts. I knew that I faced the need, but I hoped that all 3 would zero. |
I hope you just cut and paste this from a file on your HD because I'd hate to think that you have written it fresh each of the half-dozen or so times that I've seen you post this explaination. 
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Finally got to sight it in - no issues at all. The post is actually seated pretty far down in there for a zero. It looks like the taller post pictured above. So, the 14.5" M4 Wilson barrels that JSE Surplus sells come with the taller post. Thought I'd update, since I said I would...
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Wish I knew how to do that. Guns I know. Computers? I'm a total idiot. 
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Shrink all your windows down to the taskbar so you can see your desktop. Right click> New> Text Document The file will appear on your desktop and the name will be in blue. Type whatever name you want to give it and hit enter. Now double click into that file. You should have a blank white page. Go back and find any long forum posts that you would like to re-post from time-to-time without re-typing them. Once you find the post, left-click-drag (find first word of text, HOLD down left mouse buton and drag down to last word BEFORE releasing button) to select all the text. While it's still selected (blued-out), right-click ON THE BLUE TEXT and choose Copy. Now go over to that blank page you made earlier. Right-click and choose paste. Bingo, the text you copied now appears in that blank page. Save the text document for future use. Next time you need that text, go open the document and copy the text. Then you can paste is right back into a thread without typing it all again... |
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