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Posted: 12/17/2004 6:07:35 AM EDT
| Well, I just found me a new Colt barrel. The seller has indicated "there are no Colt markings on the front sight base". From what I now understand, for a flat top rifle such as mine, the front sight base needs to be higher than that of an A2 style front sight base. Am I to believe that the barrel I just bought, with the lack of some mark indicating otherwise, will be too short to use with my RRA flat top? What's a solution, replace the FSB? |
   
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Colt flattop carbine and flattop rifle barrels typically have an "F" marked FSB as you already know. That being said, there are exceptions to every rule. Here is a Colt 6920 flattop upper that does not have an "F" marked FSB, although the FSB is the correct height   If your barrel is indeed a Colt (is marked C MP etc on the barrel itself) you could possibly have a FSB that is the proper height, yet lacks the "F" marking. If you find that it's too short rather than replace it you could always try a .040 higher front post for $4.95 from Bushmaster HERE |
| Colt's flattop rifles come with an "F" marking on the FSB, indicating that the FSB is slightly taller. If your Colt barrel lacks this mark, then most likely it was intended for use with a regular A2 upper. The lack of an "F" marked FSB is only a problem if you intend on using a Colt carry handle BUIS. There are other aftermarket BUIS options that are compatible with the standard A2-height FSB. You can also buy a taller FS post from Bushmaster. |
What?!?!? Colt producing an exception to their own rule?!?!? 
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Mongo, I tried to keeping up with Colt's "exceptions" once, but I found myself curled up in a ball in the corner of the room, drooling heavily, and having a seizure Now I just say "screw it, as long as it works I don't care" 
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Yep, that's the one, Joe. Thanks for posting it. Ok, so right now I'll operate under the assumption that I do not know if the FSB is the correct height for my RRA detachable handle/iron sight. When I fit the new barrel on, how will I know it is the incorrect height? What will I find happens at the range? |
I feel your pain. I also quit trying to figure it all out and just started accepting whatever I had in my hands as it was, regardless of all the "rules" that are out there. |
I hear ya. One of my range buddies has a blue label Colt lower with a standard pushpin and .155 H/T pins. I've been trying to talk him into selling it to me for about ten years  But he won't do it, the selfish bastard 
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You should not have any trouble because, as pointed out earlier, it's the Colt carryhandle sight that requires the taller FSB. If you were to have a problem with an FSB that was not tall enough you'd notice that your rifle would shoot too high when sighting in at the range and you couldn't raise the front sight post high enough to compensate. That's why the Bushy .040" higher post is a good, cheap, and quick fix. Just an FYI - the F stamped FSB measures 1.975" from the (inside) top of its barrel opening to the flat machined surface where the post screws in while the non-flattop FSB measures 1.935". |
Ah, yes. Thanks for that clarification, that point was lost on me. No worries then with my RRA carry handle. Tech support team may now close this incident, issue resolved. |
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So now I have a question, If I put a barrel with an F marked front sight on a flat top RRA upper, what kind of rear sight would work? Or should I use a different front sight post? Right now I have an ARMS 40 and a DPMS solid rear sight. I'd rather not get a colt carry handle if that is the fix.... 
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If you use a RRA carry handle (or whoever's carry handle as long as it is not Colt), just get the taller front sight post from Bushmaster. Correct? |
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Actually, I'm beginning to consider the whole "it's the Colt higher sight unit" story a pile of BS. Why? Because my Rock River upper, Rock River barrel, Bushmaster sight unit M4 gery needs the higher front sight blade from Bushie. Also, when I put a current flat blade front sight on my SP1 carbine, and it atarted shooting 6 inches high at 50 yds, I went home, looked at the original round blade I'd pulled off, and found it was .040 inches higher than the rifle unit flat blade I'd mounted. Colt was raising the front sight point on carbines TWENTY FIVE YEARS AGO! And on solid top units at that. So much for the BS that's been repeated so often on this site that it's become a mantra. The best I can figure is rhat the shorter barrelled versions rise higher in recoil before the bullet exits the barrel; but that's based on intuition, not evidence. The only way to determine if you need a higher front sight blade is to take the thing out and shoot it. Now if the "experts" on this site want to keep repeating that it's only due to a variation in Colt detachable top units, y'all have at it. It doesn't change reality
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I'm sooooo confused now.....Do you guys think I may have to shave my front sight post down a bit? If a taller one is needed with a non f marked FSB, it seems like a shorter one may be needed for a f marked one and a non-colt BUIS right? wrong? I'll be in fetal position in the corner until someone replies 
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WTF??? GET OUT OF MY CORNER! HHHHhhiiisssss...... |
| Okay I just found this over at the Colt industry forum, and it would lead me to believe that the f marked base will be okay with other BUISs. Maybe. |
From personal experience I know that ARMS, Troy, and LMT backup irons work fine with Colt's taller FSB. |
CJan_NH, Did you change your sling mount from side/ to under on your 6920, or did your's come that way? OK, back to FSB chatter |
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RedFisher, my 6920 upper originally came with the side sling mount installed. Unfortunately it wasn't compatible with the navigation LEDs of my Surefire M500A so I removed it. I had a few standard mounts in the bin so I tried one and it worked great You have good eyes for noticing that, by the way
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