I asked this question in the build it yourself forum but it might make more sense to ask it here. Mods, if you want to delete the other thread, that's fine with me.

Can I make this work with a short front sight post or am I going to need to do some filing? I'm planning on installing an A2 aperture in the A1 upper as well. I just want to be able to shoot out to ~50 yards and hit center mass. I have other guns for precision work.

Thanks in advance.

I use A2 Rear Aperture's in my Retro Carbines and I wouldn't worry much about the F/Non F marked FSB, you will be able to zero just fine.

Will this be a carbine or a rifle? If a carbine, it's almost a guarantee that you'll need a shorter sight post with the A1 type upper receiver and an F-marked sight base. If a rifle, no changes will be needed. Aperture type will be of no effect.

Your correct, but he can crank the post down to achieve zero.... I have done this before.

Thanks for the replies guys. I'm going to be running a 14.5" midlength.

Guess I need some help here.
Been trying to figure out what the difference would be, in using these sights on a rifle, vs. a carbine.
Why would it work on one, but not the other?

OP,

At 50 yds it will be fine more than likely regardless of FSB or receiver style used.  

Rifle length vs carbine length. Geometry!  Sight radius comes into play.  The detachable carry handle aperture wasn't tall enough for some to shoot accurately on carbines at short distances.  Main reason Colt went to F stamped taller FSB was to zero a flat top receiver on a carbine length gas system  / FSB at 25 yards. As stated some shooters had to crank the post nearly out of the FSB to zero.  Now F stamped FSB is the mass produced most popular cost effective product and is installed on most any thing AR, necessary or not.  

Longer rifle length sight radius will zero regardless of FSB height and zero distance on a fixed carry handle or flat top upper receiver.  Colt never had a problem using or zeroing the fixed carry handle upper receivers with carbine or rifle length barrels A-1 or A-2.  Problem occurred with the introduction of the M-4 flat top carbine in mid 90's with shorter 6/3 detachable carry handle.  Matech flip sight will zero with either style  FSB.   Main reason question was asked whether you were building a carbine or rifle.  

Again generally the problem only occurs when using a flat top receiver and detachable 6/3 carry handle shooting very short distances.  Of course if a taller sight post or taller FSB is necessary to make a 25 yard shot by all means have at it.  On flat top rifle with F stamp FSB  the sight post is simply lowered in FSB an additional amount to zero and  until one starts getting into 600-800 yd shooting it's seldon pertinent.

Regardless you need to build and shoot it to see if it will zero or not.  Not much available today without the F stamped FSB pre installed on a barrel assembly but there are exceptions.  Only in retroland where it's all about the small details would anyone even care to look at a FSB.  Authentic retro rifles didn't have F stamped FSB's!  Although some had a F casting,  which are rare today and has nothing to do with 0.040 extra height.  Thinking it's on the GM flavor FSB but may have been the H&R and definately not Colt.

For example I have 3 older build flat top carbines with standard height FSB's and at 25 yds all will zero with standard sight post remaining in the FSB with anyone but a novice / newbie at the helm.

Easiest remedy for a non F stamped 25 yd elevation zero problem is a 0.040 longer sight post.  I bought a 0.040 sight post from BM when problem became apparent for some and have yet to need it and it resides in it's original package even when using 6/3 detachable carry handles.

I have a 16" pencil weight middy that has F-FSB on a slick side upper.  It just sits a little low but has much more room until bottom out.
This is the zeroed height of the unmodified A2 post. I recently swapped the A2 post out for the more traditional A1 type post.

I really like the paint job, I'm not a fan of magpul gear except their mag followers and their handgaurds have a very nice feel no matter what the purists say.

I built this gun almost entirely out of my spare parts bin. I had to buy the lower, barrel, and the front take down pin. Eventually the stock will be replaced. I'm a lefty so an ambi charging handle and battery assist lever are going on eventually. It'll also get a Geissele SSA at some point as well.

Got it out and sighted it in today. No issues whatsoever. Cranked the front sight post down quite a bit but it did zero.

This 10 shot group was at 75 yards with crappy steel cased ammo. Not great but not terrible I guess considering how long it's been since I've used iron sights.


The only issue I had was with casings hitting me in the face and shoulder. I'm a lefty and it was ejecting at about 5 o'clock. I'm wondering if a heavier buffer would help.

I'm a lefty as well and cutting a coil off the ejector spring works almost everytime. Some guns are naturally GTG first time out and eject at 2-3 oclock and some can be fixed with heavier buffers or buffer springs, but the ejector spring mod works everytime.

I'll look into that. Thanks.

Personally, I would never pair a F marked FSB with a fixed carry handle upper.  Have to crank the front sight post too far down in my experience where it really interferes with getting a good and fast sight picture.  I hate having to struggle to center a short post and align it to a target.

Not for me!  I made that mistake once, never again.