Looking For AR15 Front Sight Base to Install on Bushmaster AR-15

arakboss · Jul 14, 2019

I purchased a Bushmaster AR15 a few years back and the seller had installed a Midwest free float quad rail on the rifle. Rather then replacing the front sight base with a low pro gas block, he ground down the factory front sight base so the hand guard would slide over it. I want to restore the rifle back to factory configuration with front sight base and was hoping to find a member here looking to get rid of one. I understand there could be issues with cant since gas FSBs are drilled for pins individually, so I don't want to invest much money ($10ish) in case the replacement FSB doesn't line up right. I would need the complete FSB with sight post parts as I am missing those parts.

JeremyK · Jul 14, 2019

So you need a standard A2 front sight base?

arakboss · Jul 14, 2019

JeremyK said:
So you need a standard A2 front sight base?

It's a flat top receiver. So not sure if I need F marked base or not.

JeremyK · Jul 14, 2019

arakboss said:
It's a flat top receiver. So not sure if I need F marked base or not.

Yup, you'd need the f-marked. I think I can help you, but I'm in Utah until August

DirectDrive · Jul 14, 2019

arakboss said:
I purchased a Bushmaster AR15 a few years back and the seller had installed a Midwest free float quad rail on the rifle. Rather then replacing the front sight gas block with a low pro gas block, he ground down the factory front sight gas block so the hand guard would slide over it. I want to restore the rifle back to factory configuration with front sight gas block and was hoping to find a member here looking to get rid of one. I understand there could be issues with cant since gas blocks are drilled for pins individually, so I don't want to invest much money ($10ish) in case the replacement block doesn't line up right. I would need the complete block with sight post parts as I am missing those parts.

It's called a Front Sight Base (FSB) and it won't "line up right" in most all cases.
The FSB/barrel are the only unique parts on an AR.

This won't be a cheap fix, but it's doable.
Contact @Velzey and he will fix you up.

If you try to do a pinned FSB on the kitchen table with no jig or expertise, you will most likely be like a pup chasing its tail.

eldbillbo · Jul 14, 2019

Actually if you find a bushmaster front site base it might line up however other brands will not. Companies have their own dies set up for drilling. I have mated Bushmaster FSB with other Bushmaster barrels same with colt. however for example a colt will not fit a bushmaster and Vs versa. I have some spare FSBs however I don't I dont know what they came off with the exception of a couple colts that I am hanging onto.

arakboss · Jul 14, 2019

eldbillbo said:
Actually if you find a bushmaster front site base it might line up however other brands will not. Companies have their own dies set up for drilling. I have mated Bushmaster FSB with other Bushmaster barrels same with colt. however for example a colt will not fit a bushmaster and Vs versa. I have some spare FSBs however I don't I dont know what they came off with the exception of a couple colts that I am hanging onto.

I suspected that might be the case. I am going to measure some pin hole spacing on my Bushy FSB vs some AR Stoner (BCA) FSB barrels I have and see how they compare. As far as canting goes, it doesn't seem like there could be that much difference between FSBs or else the barrel gas port hole would be obstructed by the FSB. I would think a tiny bit of cant could be compensated for with rear sight adjustment?

JeremyK · Jul 14, 2019

Meh I think this is being overthought. Just get any old f-marked FSB, stick it on the barrel, eyeball it to get it straight, then tack-weld the sucker in place

arakboss · Jul 26, 2019

I want to throw out a huge shout out to @808hondacrguy for sending me a spare FSB he had lying around. It took a little massaging of the front hole on the barrel to get it to fit right but it's on tight and appears (to my eyes) to be straight (not canted). I installed the rear taper pin first and used that positioning to decide what massaging the second hole needed. With only the rear taper pin in I could tell the FSB was straight and not canted so it should work out. I have to remove the FSB again and replace the free float barrel nut with a factory barrel nut and install a gas tube in the FSB. After that I will take it to the range and see if it shoots straight. Thanks to all for the advice and never say never.

HB of CJ · Jul 26, 2019

You need an AR15 front site assembly. First determine which size barrel diameter version you need. Grab a three pound hammer and the proper sized pin punch and place the barrel firmly supported on an anvil and smack out the factory OEM taper pins. They will only go out one way. One side for one....other side for the other. I forgot which. Liquid wrench soaking overnight really does help here.

Then determine your barrel diameter at the gas port. Dial indicator. Front site post assemblies come in various barrel diameters. No big deal. Just measure what you have and get the proper one. You can use the taper pins over again. You might need to find the proper front end barrel cap. This stamping holds the front of the hand guards on. Round or triangular. You probably need round.

Or triangular. Or none. Depends.

Slide the end cap on first. Then slide hammer the front site base assembly into place. More liquid wrench. Not much. Align the base, then, use the anvil, three pound hammer and the pin punch to drive home the two OEM taper pins. Roll pins are a NO NO. Get taper pins. This assumes the barrel has been precisely notched for taper pins. A time consuming factory manufacturing step to do.

Now the fun part. Most likely if you decide to get a detachable top rear sight assembly and carry handle for your AR and install such with the taper pins; ... sight down the barrel AND the iron sites to see how closely aligned they are. HINT ... Hang a carrying strap on a white wall and pace off about 20 feet. Vise up the upper assembly and look down the bore at the carrying strap hanging on the wall.

Then compare with your exactly centered rear irons.

You will be able to see how KINDA close right to left your iron sights will be with the rear aperture exactly centered. with ZERO clicks right to left. Then bench rest test fire the rifle at 100 yards with the irons and see how far your holes are OFF right to left from being shooting and printing down the pipe. IDEALLY you want to be within just 4 clicks right to left from being exactly on target. Fun fun fun.

Most ideally EXACTLY centered. Usually only with match builds. Time consuming.

If you are off then the corrective process is more fun. Time consuming but easy and fun. BUT .... we will leave such knowledgeable corrective action regarding AR15 builds for another time in the future. Friday nite. I am going dancing!!!!

Hundreds of builds.