Install alignment of FSB

[HKGuns]

I am building up an A2 upper and it is full of "firsts" for me.

I have the upper assembled and a 20" Colt barrel waiting on a nut and delta ring.

I installed the A2 rear sight, which was a bit of a challenge. I had to make my own slotted wood dowel to get the spring compressed on the bottom, so I could get the cross pin through. Went pretty smoothly after I figured out how to make my own tool.

I don't want to drill on the barrel, so I got a JP clamp on FSB and am trying to figure out the best way to ensure I don't cant it on myself.

What methods have you used to ensure you don't cant your FSB when installing? Any and all suggestions appreciated.

https://hkguns.zenfolio.com/img/s/v-10/p2840579354-5.jpg

[1168]

I am building up an A2 upper and it is full of "firsts" for me.

I have the upper assembled and a 20" Colt barrel waiting on a nut and delta ring.

I installed the A2 rear sight, which was a bit of a challenge. I had to make my own slotted wood dowel to get the spring compressed on the bottom, so I could get the cross pin through. Went pretty smoothly after I figured out how to make my own tool.

I don't want to drill on the barrel, so I got a JP clamp on FSB and am trying to figure out the best way to ensure I don't cant it on myself.

What methods have you used to ensure you don't cant your FSB when installing? Any and all suggestions appreciated.

There are jigs for this, but the first thing I’d try would be to put the upper upside down on a table before clamping down the FSB. Then shoot it or boresight it, and see how it goes.

[HKGuns]

There are jigs for this, but the first thing I’d try would be to put the upper upside down on a table before clamping down the FSB. Then shoot it or boresight it, and see how it goes.

I'm not sure I'll do another A2, so buying a jig wouldn't be impossible, but not something I'd like to do for a one time use. Didn't know there were jigs for it though, so I'll have to search them out and see how much they go for......

I plan to align the gas port with a bore scope, maybe that will get me close enough.

[georgeib]

There's an old thread on this. Not sure how much it'll help, but here it is anyway.

https://www.m4carbine.net/showthread.php?26630-FSB-alignment-jig-pics-inside

[georgeib]

I'm not sure I'll do another A2, so buying a jig wouldn't be impossible, but not something I'd like to do for a one time use. Didn't know there were jigs for it though, so I'll have to search them out and see how much they go for......

I plan to align the gas port with a bore scope, maybe that will get me close enough.

One method that won't require a more specialized jig would be to use a dimple jig to dimple the barrel opposite the gas port and then use that dimple to align the FSB.

[grizzman]

If I were assembling an A2 upper, I personally wouldn't consider a clamp on FSB. I also wouldn't drill a barrel for an FSB.

I'd send the barrel to a reputable smith to have the FSB pinned.

[prepare]

I am building up an A2 upper and it is full of "firsts" for me.

I have the upper assembled and a 20" Colt barrel waiting on a nut and delta ring.

I installed the A2 rear sight, which was a bit of a challenge. I had to make my own slotted wood dowel to get the spring compressed on the bottom, so I could get the cross pin through. Went pretty smoothly after I figured out how to make my own tool.

I don't want to drill on the barrel, so I got a JP clamp on FSB and am trying to figure out the best way to ensure I don't cant it on myself.

What methods have you used to ensure you don't cant your FSB when installing? Any and all suggestions appreciated.

https://hkguns.zenfolio.com/img/s/v-10/p2840579354-5.jpg


https://youtu.be/HWpvAuT4NIE?si=U4crnLsvdVDSsbrQ

Where did you get the A2 sight?

[joedirt199]

Since it is a clamp on, you could just sight it in with the front sight adjusting for windage. I did this with a standard fsb that had only one set screw in after removing the sling ring. I kep the handguard off and kept the rear sight dead center, then moved the front to zero it. I will pin the front base of the sight and leave the rear setscrewed with red loctite.

[HKGuns]

Where did you get the A2 sight?

Fulton Armory was the only place, that I could find of decent repute, that actually had the complete sight in stock.

Thanks for the video, I will check that out.

[HKGuns]

Since it is a clamp on, you could just sight it in with the front sight adjusting for windage. I did this with a standard fsb that had only one set screw in after removing the sling ring. I kep the handguard off and kept the rear sight dead center, then moved the front to zero it. I will pin the front base of the sight and leave the rear setscrewed with red loctite.

That is a very practical suggestion. Thanks.

I'm giving the extendo Rod a chance. I just ordered the necessary parts off McMaster Carr. Thanks for posting that Prepare.

EDIT: While I enjoy the process of building this rifle and learning, its quickly turning into a situation where instead of ordering the A2 stripped upper, I should have snagged one of those Colt AR15A4's when they were in stock at Arms unlimited for a great price.

[Jim D]

BRD makes a quality fixture that uses the indexing pin of the barrel to align the FSB against, which should help you perfectly center over the gas port.

Rumor is Colt would install barrels into the uppers first, then index of the carry handle to install the FSB... which means the sight isn't canted in the least, but also that minor clocking in the receiver means the gas port isn't necessarily perfectly centered under the aperture in the FSB. On a carbine gas lengths system with .062-.070" ports you have plenty of wiggle room... but on .093/.0995" sized rifle length ports, I'd rather index off the gas port/ indexing pin than off the rear sight.

[HKGuns]

BRD makes a quality fixture that uses the indexing pin of the barrel to align the FSB against, which should help you perfectly center over the gas port.

Rumor is Colt would install barrels into the uppers first, then index of the carry handle to install the FSB... which means the sight isn't canted in the least, but also that minor clocking in the receiver means the gas port isn't necessarily perfectly centered under the aperture in the FSB. On a carbine gas lengths system with .062-.070" ports you have plenty of wiggle room... but on .093/.0995" sized rifle length ports, I'd rather index off the gas port/ indexing pin than off the rear sight.

I found the fixture you reference and I'd probably pony up the 300 to buy one if I thought I would do another. the parts arrived today for the home made carry handle version and I have it assembled and think it may work. Still need to check Gas Port alignment with my borescope, but suspect this will get me where I need to be with this one.

For anyone reading this thread in the future, the fixture mentioned above is linked here: BRD Front Sight Jig (https://brdengineering.com/products/223-slash-5-dot-56-front-sight-base-jig-brdejig3)

ETA: Either the part numbers changed since that Youtube video was made, or I ordered the wrong part, but the "Pointer" McMaster Carr sent doesn't fit into the hole of the inside tube, so I improvised and used one of my pin gauges as an elegant alternative.

https://hkguns.zenfolio.com/img/s/v-10/p2915696060-5.jpg

Thanks regardless, as your suggestion may indeed help someone in the future. I also appreciate your expert insights.

[jsbhike]

If you lose confidence in the clamp on you can drill and tap the taper pin holes in a used A2 sight base for 4 #8(finer the thread the better) or #10-32 set screws.

It is also easier to drill and tap the hole in the bottom of the rear band and use it to zero windage before securing the 4 set screws.

Obviously dimpling or milling flats for the set screws add to keeping everything snug.

[HKGuns]

If you lose confidence in the clamp on you can drill and tap the taper pin holes in a used A2 sight base for 4 #8(finer the thread the better) or #10-32 set screws.

It is also easier to drill and tap the hole in the bottom of the rear band and use it to zero windage before securing the 4 set screws.

Obviously dimpling or milling flats for the set screws add to keeping everything snug.

Do you have a source for used FSB's by chance? I'd like a few if for no other reason than to have options.

[jsbhike]

Do you have a source for used FSB's by chance? I'd like a few if for no other reason than to have options.

Just keep an eye open on equipment exchanges and when demil parts kits hit the market every so often. Surplus oriented shops and maybe some gun shows may also be good places to look. Knob Creek used to have plenty back in the good old days.

The only place I am seeing take offs via google search right now is a site that specializes in clones and their take offs are more than several vendors have virgin fsb's for.

[556Cliff]

BRD makes a quality fixture that uses the indexing pin of the barrel to align the FSB against, which should help you perfectly center over the gas port.

Rumor is Colt would install barrels into the uppers first, then index of the carry handle to install the FSB... which means the sight isn't canted in the least, but also that minor clocking in the receiver means the gas port isn't necessarily perfectly centered under the aperture in the FSB. On a carbine gas lengths system with .062-.070" ports you have plenty of wiggle room... but on .093/.0995" sized rifle length ports, I'd rather index off the gas port/ indexing pin than off the rear sight.

This is true, and I suspect Colt's reason for doing it that way and the reason that the FSBs are not installed before barrel installation is because their very expensive automated barrel nut torque and alignment machine doesn't allow room for the FSB to be in place through that process. I also suspect that they have the FSB-less barrel and upper receiver set up together in some very fancy jig to be put into the machine that does not allow any movement between the barrel and the upper so that the gas port doesn't go off center during the torque process.