I am in the final stages of putting together an ultralight 20" rifle, and have encountered an issue I can’t resolve with online research. The rifle I am putting together will have a fixed A2 FSB and a flattop upper, with a 20" barrel and rifle-length gas system. I know that the accepted practice with flattops is to use an F-marked FSB, but the only information I have been able to find on this discusses it in the context of short barrels (16" and under) with carbine gas systems, and nobody makes reference to whether this F-marked requirement is still the case with a longer gas system.
I have heard some reports from authoritative sources that the F-marked FSB is required for any AR with a carbine gas system, even ones with a fixed carry handle upper, and the standard A2 is fine for rifle-length gas systems, regardless of whether the upper is flattop or an A1/A2. On the other hand, BCM uses an F-marked FSB on their assembled A4 upper half with the 20" barrel.
I already have a standard (non F-marked) A2 FSB in the correct .625" diameter, but am wondering whether I should play it safe and get an F-marked base to complete the build. If it helps answer my question, I will be running a Daniel Defense A1.5 rear sight and an RRA Stand-Alone A2 rear sight assembly (changing them out based on what I am doing with the rifle at any given time).
I REALLY want to get this FSB right the first time, because it is an iffy, expensive, and time-consuming process to change and install a new A2 FSB on a rifle that has already had a different one taper-pinned to it.
Thanks, and I look forward to your help.
Happy Memorial Day, and a special thanks to all you service members and vets out there!
I was recently building a service rifle upper with a 20" barrel and flattop receiver. I emailed white oak armament with your exact question and they said to use an f marked fsb with the removable carry handle rear sight. I did and it seems to be working fine.
I had a 20" FN MILSPEC 1/7 barrel without the F-marked FSB. I built into an upper using the flat top and encountered no issues. With that said, if the F-marked FSB was an option, I would use it.
The F marked front sight base goes with the detachable carry handle/flat top upper. Their is less room in the carry handle for the rear sight elevation mechanism, so it sits slightly higher. The F height front sight post is made to compensate for this.
I suspect a standard height FSB can be made to work, but if you might use a detachable carry handle, or want it to be GI ‘correct’, the F is the hot setup.
You can’t swap FSB on a barrel that already had one pinned on it and have a replacement be secure. The cross pin holes do not line up. If someone tells you otherwise, they are lying.
You need to use your current set up and if you need more adjustment range on the front sight, get a taller front sight post.
Thanks for the input, everyone! One reason that having the FSB work with a standard front post is so important to me is that I use a .05" National Match front post in all of my ARs for the more precise sight picture, so the extra-tall post is not an option for me.
It seems that I should be safe and go with the F-marked FSB, and I think that is what I will do.
Unless it’s a Colt AR15A4 and then you might get an F marked front sight block or you might not. It doesn’t really matter on the civilian side for rifle length barrels as it does on the carbine length other than for conformity.
You guys do know that Colt has made different uppers with changes to bore center line to the rail in spacing right? If I wasn’t too busy, I could dig up some more specifics. Most of this is really minutia for a 20" gun for most though.
That might have been true at some point but I doubt it’s true today. Now it’s more like, let’s use up these old parts and get this stuff out the door. They are now putting the 14.5" SOCOM barrels with pinned vortex flash hiders on expanse rifles and selling them for $600 as complete rifles. It’s nuts what people used to pay for Colt stuff when I was younger and now they are just giving it away- it’s sad honestly but if I had another safe, I’d buy every damn one. Hell I might.
I wouldn’t be accurately to be able to tell you what they put out today, I’m out of the loop for them, you could be right for current offerings, but I would not know at this time for what they offer.
A non-F FSB rarely is going to give anyone problems with zeroing; and is said to become less of an issue as barrel length increases. I’m pretty sure that the 6720s even come/came with non-F FSBs. So, not that big of a deal.
On a side note. If you like the National Match FSP because of the thin profile, you may enjoy a few sights from this set. I really love the .034" round dot.
Not a myth, and certainly not limited to prototype stuff. Common, no, but I have been able to collect some up. Last time was a little over a dozen years ago, pretty sure I was able to pick up 10 or so from Brownells. I’m not sure how many more of these Colt items they had, but I already had what I thought was enough before I found those. I really never had a plan to use most of them and probably haven’t looked at most of them in over 10 years by now.
