Canted FSB...

[Razorhunter]

I have briefly mentioned this in another thread a while back, and I don't remember ever getting it figured out, SO,
I would like to know what the best method is, for straightening a FSB is, on a Colt upper, with the standard taper pins???
What is the best way to get the FSB adjusted??? I have not yet been able to determine the best route to take, since the taper pins hold it in the same position every time I re-install the FSB.
How does one go about adjusting the position of the FSB???? Do I need to mill out the taper pin channel somehow, to put the taper pin channels/grooves at a different angle to the bbl or what???

Also, what is the best way to KNOW for sure that I've got the FSB perfectly square/level? Can I hang a string w/ plumb bob from my shop ceiling, and then somehow sight down through my bore, in an attempt to get the perfectly "plumb/level" string lined up with the CENTER of the bore, AND the FSB???
What is the best way to CHECK and CONFIRM that your FSB is SQUARE????
Thanks for any thoughts....

[ar_mcadams]

well I installed a removed fsb once that someone took of a Colt m4 barrel and it went on canted as shit. I didnt know what I was doing and if I were you I would send it to someone like ar15barrels.com or someone that can install it correctly. They have a jig they use to make sure the sight base is aligned with the index pin. If you take any metal out or wallow out the pin holes someone will have to use oversized pins to take up the slack. The more you mess with it the worse it will be to fix.

[Razorhunter]

This FSB was canted from the day the gun was new.
It's slight, but very noticeable.
I have removed the FSB exactly ONCE, and reinstalled it exactly ONCE.
It is STILL canted the exact same amount NOW, as it was before....

[ar_mcadams]

is the upper reciever a colt also? I have seen the index slots in the recievers not in the center making the barrel slightly rotated making the fsb look canted. The only way I know to check that is to try another upper reciever and see if the fsb still looks canted. Just a tiny bit of misalignment on the index slot will look like a lot at the fsb end of the barrel.

[ar_mcadams]

what model upper?

[Snake RAH]

What is your windage setting? Unless you have to dial in more than 15-20 clicks, I wouldn't worry with it.

If you must, you can see if retorquing the barrel nut will correct the excessive windage. You'll need a 1/16" punch, hammer, barrel wrench, torque wrench, vise and receiver block. You'll clear the rifle, remove the handguards, remove the gas tube roll pin, remove the gas tube, then put the upper in the vise via the receiver block, and loosen the barrel nut fully. Clean the threads, and verify that the notch in the receiver for the index pin is in the center. If not, replace the upper only. If the index pin damaged the notch (from the barrel nut being put on too tight), then you can either 1) get a new upper or 2) cut a shim in order to put in between the index pin and the notch so that the barrel sits more upright.

Apply moly grease or anti-seize to the threads, seat the barrel in the receiver (by tightening and loosening the barrel nut several times), then use the torque wrench with the barrel wrench in order to tighten up the barrel nut. Minimum is 30 ft/lbs. Line up the gas tube holes in the delta ring, weld spring, slip ring, receiver and barrel nut scallops so that the gas tube is centered and doesn't bind on the bolt carrier gas key. Once good to go on that, reinsert the gas tube roll pin, put the handguards back on, and go zero.

Like I said, if the cant doesn't require excessive windage to zero, then I personally wouldn't go through the trouble of trying to fix the cant.

[Razorhunter]

Guys,
Ok, I have had more time to check into this, and I have found the least desireable situation to be the case here.
I have a FSB that is canted to the LEFT, (from a shooters, REAR view), which means that the FSB is canted in the least desireable direction. This of course, is because I cannot just smack the FSB in the proper direction, and expect it to hold. This of course, is due to the bbl nut having to be torqued in this direction.
IF I were to smack the FSB with a rubber mallet back into correct alignment, the torquing of the bbl nut would only take it right back to it's misaligned position (I think).
SO, maybe a "shim" could indeed be used, but I'm not sure HOW to determine the proper method of shimming, nor am I sure of how to even determine WHEN the FSB is in the proper position, as I can only eyeball things, due to the lack of a fancy jig. As for a shim, do I just use a tiny piece of metal, and stick it down in between the notch and the pin, ????? Seems like a jerry-rig to me.
What is most likely the problem? An improperly milled notch in the upper???? What is the likelihood that another Colt upper would SOLVE the problem? Any chance COLT would replace the upper in any REASONABLE amount of time? SHIT. DOUBT THAT. The stories I hear of their customer service, ALL SUCK, but that is indeed heresay from ARFCOM. Many of those guys ARE telling the truth however.
I of course, could probably design a jig, and make a shim, but first, I would prefer some suggestions on the best way to go about doing this, now that I have found the FSB to be canted in the LEAST desireable position??
Thanks for all your help guys.

[LukeMacGillie]

Your not the only person with this problem. Ive got to send a whole shipment of SMG uppers back to Hartford because of canted FSB's

All I can do is shake my head:confused:

[PALADIN-hgwt]

xxxxx

[Razorhunter]

Your not the only person with this problem. Ive got to send a whole shipment of SMG uppers back to Hartford because of canted FSB's

All I can do is shake my head:confused:


Luke,
Good to hear that I'm not the only one with this problem, so MAYBE Colt will one day start fixing this problem BEFORE it happens. (Yeah right).
Anyhow, WTF is an "SMG" upper??? xxxxx -Match Grade perhaps???? Dunno???

[LukeMacGillie]

Anyhow, WTF is an "SMG" upper???

http://www.colt.com/law/smg.asp

[Aubrey]

Ken Elmore can fixture align the FSB to the receiver.

[Ned Christiansen]

My unpleasant and in some cases not-worth-it solution would be to knock out the taper pins, align the FSB, and re-ream the taper pin holes for the next-size-up taper pins. I made a fixture for this in about 1981 and have been using it ever since, and I imagine that Colt has something similar but I would think and hope, better. But I've always felt that that little 1/8 locating pin in the barrel extension, so close to the bore center was just barely adequate(making a situation where a little clearance/movement there translates into a lot of it at the top of the FSB). This same phenomenon means that if the locating pin has, say, a .005 chip or something under it in the fixture as the taper pin holes are drilled and reamed, your FSB is going to be maybe .025 off center / leaned over.

I have not personally seen one where the play between the locating pin and upper receiver was enough to make a canted FSB that one could notice. I have of course seen plenty of AR's that had the rear adjusted a fair amount left or right, but that's why they're adjustable after all. If the guns could be made perfectly every time they could just drill a hole in the upper for a rear sight-- but we all know that ain't gonna happen.

I've gone so far in my early days as making a "key" to replace the locating pin, thinking that the pin could easily move metal in its slot in the upper, but although it still SEEMS to me like it could, I've never seen it happen.

The ideal solution, in those cases where you can, is send it back to the factory. Why do we keep getting guns so far from perfect? Because they send them out and don't get any complaints. I know that's an unpalatable solution-- takes forever, etc...... just saying it's a good way to let them know "we ain't taking this sheisse anymore".

While on the subject, may I add that in my view, the many alternatives to taper pins are.....let's see here..... cheezy attemptsto save an operation (taper pin reaming). Straight dowels are very strong but required a perfectly sized hole-- like plus or minus a tenth of a thou-- to ensure a no-falling-out pin fit. Have seen them falling out, and Loctited and staked them in. We're not supposed to have to do that. Roll pins, also not good. Set screws/clamp screws, well, screws come loose.

[Razorhunter]

Ned,
Thank you for taking the time to respond here.

Could you please share a little more insight in regards to the following statement a little better, as I'm not quite sure I understand what you were explaining here:

"I've gone so far in my early days as making a "key" to replace the locating pin, thinking that the pin could easily move metal in its slot in the upper, but although it still SEEMS to me like it could, I've never seen it happen."


-I don't understand what you mean by a replacement "key" easily "moving metal" ???? What in the world does this mean? Your replacement "key", is obviously replacing the locating pin, which fits in the uppers slot.
However, I'm lost after that. What is the purpose of your key, and what do you mean by it "moving metal"?????

Thanks Ned. SO GREAT to have your comments in this thread as well as all the others...

[Ned Christiansen]

Picture a piece of steel 1/8 X 1/8 X 3/16 (or so) with a length of 1/8 diameter sticking out of it, it's all one piece. The diameter goes into the hole in the bbl extension and the other part fits in the slot, just giving more bearing area of steel against aluminum in the slot, plus, the key having been sized exactly to the slot to take out any potential play.

A lot of extra work for some theoretical gain that just wasn't worth it.

[AR15barrels]

You can use a simple 1/2" rod with a point on the end like this to check FSB alignment while tightening.
If you have a flattop receiver, you will need to mount a carry handle in order to hold the rod.

http://ar15barrels.com/tech/fsb-alignment.jpg

[Razorhunter]

Thanks Randall,
I was beginning to wonder if that's all the jig consisted of!
Unfortunately, when the bbl is being torqued, the flat top of the receiver is in between an upper vise block and a piece of plywood, in between the jaws of the vise, however, that is an excellent idea for checking when the upper is not down in the vise....

[Snake RAH]

IMO the left tilt is the BEST situation, as there is almost always some play in the barrel index slot. I've had excessive windage on a few builds, and after reindexing the barrel ended up within one or two clicks of mechanical center on the rear sight. The SECRET, shhhh...

apply counter rotation to the barrel while tightening the barrel nut. takes a second set of hands. stick a dowel or bar thru the fsb and twist it CCW from the muzzle end, while the barrel nut is retightened. Once set, it won't shift. The idea to to remove all play to one side, hopefully getting your FSB vertical enough to suit. I've had to do this several times, and it always worked so far. HTH.

On some barrel/upper combos there is no play at all between the index pin and slot in the upper, others you can twist and feel the shift (before the barrel nut is tight!).

Paladin

I used to do this (with a big slot screwdriver) exact technique, but found the results inconsistent. I also found, once going to an armorer's class, a properly used torque wrench prevents overtorquing to begin with.

Set your wrench for 30ft/lbs, and once seated the barrel in the upper, tighten until your wrench clicks. You've reached the minimum torque, and then check gas tube alignment. Usually (9 times out of 10), I only need to apply another few foot/pounds in order to index the scallops of the barrel nut. That one time out of ten may require more torque (in which I usually replace the barrel nut) or 30ft/lbs was exactly enough to get the barrel nut indexed.

When test firing at the range, 5 clicks is usually the maximum I have to adjust. Resuts, though, will vary depending on rear sight (some BUISs fit differently on different uppers, and therefore zero differently).

[AMMOTECH]

Your not the only person with this problem. Ive got to send a whole shipment of SMG uppers back to Hartford because of canted FSB's

All I can do is shake my head:confused:

The guy that was working at Bushmaster must have moved up to Colt. :p

[C4IGrant]

What is your windage setting? Unless you have to dial in more than 15-20 clicks, I wouldn't worry with it.

If you must, you can see if retorquing the barrel nut will correct the excessive windage. You'll need a 1/16" punch, hammer, barrel wrench, torque wrench, vise and receiver block. You'll clear the rifle, remove the handguards, remove the gas tube roll pin, remove the gas tube, then put the upper in the vise via the receiver block, and loosen the barrel nut fully. Clean the threads, and verify that the notch in the receiver for the index pin is in the center. If not, replace the upper only. If the index pin damaged the notch (from the barrel nut being put on too tight), then you can either 1) get a new upper or 2) cut a shim in order to put in between the index pin and the notch so that the barrel sits more upright.

Apply moly grease or anti-seize to the threads, seat the barrel in the receiver (by tightening and loosening the barrel nut several times), then use the torque wrench with the barrel wrench in order to tighten up the barrel nut. Minimum is 30 ft/lbs. Line up the gas tube holes in the delta ring, weld spring, slip ring, receiver and barrel nut scallops so that the gas tube is centered and doesn't bind on the bolt carrier gas key. Once good to go on that, reinsert the gas tube roll pin, put the handguards back on, and go zero.

Like I said, if the cant doesn't require excessive windage to zero, then I personally wouldn't go through the trouble of trying to fix the cant.

Agree. If your windage is less than 11 clicks, I would leave it alone becuase the alternative is a big rubber hammer. :D



C4

[AR15barrels]

Unfortunately, when the bbl is being torqued, the flat top of the receiver is in between an upper vise block and a piece of plywood, in between the jaws of the vise, however, that is an excellent idea for checking when the upper is not down in the vise....

Sounds like you need one of these...

http://www.dpmsinc.com/store/products/images/?id=1675

[Robb Jensen]

Sounds like you need one of these...

http://www.dpmsinc.com/store/products/images/?id=1675

I've recommended those to people on other forums Randall and they always tell me that I'll bend the take down pin areas of the upper.

Yeah I have 4 of them. 3 .223 AR ones and an AR10 one. I've used it for over 250 barrel removals/installs and it's never damaged a thing. The clamp style leaves shiny spots on the upper receivers.

[AR15barrels]

I've recommended those to people and they always tell me that I'll bend the take down pin areas of the upper.

I guess they know better than you and I...

Perhaps they are thinking of the PRI or Adco blocks that do NOT support the receiver from the inside.

[Razorhunter]

Well that DPMS block is the EXACT model I've got.
When I set it up vertical (as opposed to sideways in between plywood), I started torquing off the flash hider, and then the bbl nut, and it SERIOUSLY looked like I was bending the receiver, and it just did NOT look good at ALL.
I was scared shit-less that I was bending or torquing the receiver too much.
That's why many of us mount it sideways down in between plywood spacers/cushions, which takes much of the stress off the upper when torquing.
I know the Delrin DPMS block supports the upper inside, with SOLIDITY, but when you see that thing start torquing with the high forces applied, it starts to make you worry about bending your receiver!
I suppose you know better than I do though Randall....

[Robb Jensen]

Paul,
If I need more than 60ft lbs I use a barrel nut shim from Randall. I try for as little torque as possible to align the gas tube through the barrel nut. I've used as low as 25ft lbs without ill effect (on one of my own guns).

[AR15barrels]

Well that DPMS block is the EXACT model I've got.
When I set it up vertical (as opposed to sideways in between plywood), I started torquing off the flash hider, and then the bbl nut, and it SERIOUSLY looked like I was bending the receiver, and it just did NOT look good at ALL.
I was scared shit-less that I was bending or torquing the receiver too much.
That's why many of us mount it sideways down in between plywood spacers/cushions, which takes much of the stress off the upper when torquing.
I know the Delrin DPMS block supports the upper inside, with SOLIDITY, but when you see that thing start torquing with the high forces applied, it starts to make you worry about bending your receiver!
I suppose you know better than I do though Randall....

Perhaps your vise or workbench is not solid enough or you are not clamping the vise down on it good enough.
If you are torquing more than 80 ft-lbs, you should be using my barrel nut indexing shims instead of over-torquing the barrel nut.

I have broken loose Armalite AR-10 barrels nuts thay required a 2ft breaker bar.
Well into the 150ft-lbs range of torque.
This was so much that my workbench would lift off the ground.
I put the DPMS 308 claw in the Kurt vise that's mounted to my 2600lb milling machine and the nut popped right off.
My kurt vise can make over 6000lbs clamping pressure.
The DPMS block does not twist.
If you are seeing the block twist, I propose that it's your vise.