F-Marked vs Standard A2 FSB on Flattop AR with 20" Barrel and Rifle Gas

[AR-Tenner]

Hi Folks,

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!

[dmd08]

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.

[sig1473]

[RedDawn]

Well I'm working on building a 18" rifle with FSB and carry handle. This is one of the questions I was encountering too. Thank you for the info

[Iraqgunz]

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.

[eodinert]

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.

[26 Inf]

If push comes to shove, and you run out of adjustment, you can always get a taller front sight post:

https://knsprecisioninc.com/square-0...ofeibev8jr0t04

Myself, I'd use the FSB on the upper.

[scottryan]

Hi Folks,

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!

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.

[scottryan]

The M16A4 uses an F marked FSB so it can use the same front sight post as other variants in US service.

An M16, M16A1, M16A2, M16A3, M16A4, M4, and M4A1, all use the same height front sight post.

[titsonritz]

The "F" marked FSB is made for all flat top receivers, the 20" barrel length is just more forgiving.