I am building an M4gery with a dedicated 22LR barrel and am going to setscrew a FSB @ the carbine gas position.  It is a flattop upper with a carry handle so an F marked FSB would usually be appropriate, but I read somewhere that a non-F works best with 9mm and being 22LR flies somewhat similar to 9mm out to 50 yards or so was thinking a non-F might be the ticket here as well.

Am I thinking correctly?

In response to your question, I played around with the JBM ballistics website to see how 22LR trajectories compare with M855.

With a 40-gn bullet @ 1070 fps, and a sight height of 2.6”, it says you need about 12.5 MOA of elevation to zero at 50 yards.

For M855 to zero at 300 m, you need about 8 MOA at rifle velocity or 9 for carbine;  let’s just call it 8.5 MOA.

Hence the 22LR wants an extra 4 MOA as compared to the baseline M4 or M16A4.

IIRC the nominal difference in height between an F and an A2 FSB is 0.032”, which with a carbine sight radius gives 8 MOA.

So I suppose either one would work, maybe depending on what ranges you have in mind.

So it would fall midway between the two it seems.  Probably would rather have a little more elevation than less if I stretch it out a ways, eh?

TY for the 411

Let’s say the farthest you envision shooting the 22LR is 100 yards.  The trajectory program says you need 6 MOA to get there from a 50-yard zero.  That’s only 8 clicks on the rear sight, so you have plenty of elevation travel to spare. If you use an F base up front and it turns out to be a bit higher than you wanted, you can make up for it by raising the rear.

But if you’re gonna try stretching it out to 200 yards, then things look different. Program now says you need 24 MOA, which takes one full turn plus 8 clicks, and that’s about as much travel as the sight has.  No room for fooling around — can’t waste any clicks on the bottom end, so yeah maybe the A2 base. If it turns out to be lower than you want, you can always get an extra-tall post.

Sure, happy to help. One more tidbit:  you know how with 5.56 the distance that needs the lowest setting of the rear sight is somewhere around 100m?  The corresponding distance for an AR in 22LR looks to be around 40 yards.