Squaring the face of an AR upper

[robertc1024]

I'm slowly setting out to build a precision AR. Before I start hanging parts on it, I want to be sure the face of my upper is square with its bore. I know you can buy a tool from Brownells to lap the face, but I've also got access to a lathe. I could pretty easily make a tool like the Brownells one.

From what I've read to cut it on a lathe, you have to have a very tight fitting "rod" that goes through the upper and the two are somehow held in place (I've got some ideas on that already.) From there, just chuck up the rod and touch the front of the upper with a cutting tool.

I realize that jacking around with this requires a very light touch to not booger up the headspacing.

Any experience or suggestions would be greatly appreciated.

 

[ed308]

Buy a kit. They're cheap and squared. I use this one, $24.99 via Midway. I done 5 or 6 AR with it. The part that attaches to the drill is flexible so you won't mess up if you don't hold the drill perfectly squared to the upper receiver.

http://www.midwayusa.com/product/82...elta-series-upper-receiver-lapping-tool-ar-15

 

[tmd11111]

Buy a kit. They're cheap and squared. I use this one, $24.99 via Midway. I done 5 or 6 AR with it. The part that attaches to the drill is flexible so you won't mess up if you don't hold the drill perfectly squared to the upper receiver.

http://www.midwayusa.com/product/82...elta-series-upper-receiver-lapping-tool-ar-15

^^^This^^^ About 30 seconds with some lapping compound is all it takes.
Does it make it more accurate? Depends. If you're just firing a few sshots at a time then not really. If your going to do a couple of mag dumps then yes.

 

[Mexican_Hippie]

I've never heard of squaring an AR. Didn't know that existed. Sub'ed.

 

[robertc1024]

I've never heard of squaring an AR. Didn't know that existed. Sub'ed.

From what I've read, it's not that big of a deal. It basically aligns the barrel more closely with the axis of the bolt - which is easier on the bolt. It also helps true up the upper rail to the barrel which makes the scope's zero closer to zero. I've read anecdotal evidence about it helping accuracy, so I figured it wouldn't hurt anything.

 

[ed308]

Biggest difference I've noticed is with scopes. You won't need to crank the knob as far to the left or right. So its worth taking the time to lap the upper receiver IMO.

 

[hoghunting]

I use the Brownells lapping tool:

http://www.brownells.com/gunsmith-t...16-upper-receiver-lapping-tool-prod20220.aspx

It fits the receiver perfectly and is worth the extra step when building an upper.

 

[robertc1024]

Thanks gents. I guess I'll go with a lapping tool.

 

[bptactical]

I'm slowly setting out to build a precision AR. Before I start hanging parts on it, I want to be sure the face of my upper is square with its bore. I know you can buy a tool from Brownells to lap the face, but I've also got access to a lathe. I could pretty easily make a tool like the Brownells one.

From what I've read to cut it on a lathe, you have to have a very tight fitting "rod" that goes through the upper and the two are somehow held in place (I've got some ideas on that already.) From there, just chuck up the rod and touch the front of the upper with a cutting tool.

I realize that jacking around with this requires a very light touch to not booger up the headspacing.

Any experience or suggestions would be greatly appreciated.

Older post and not trying to drag up the dead but a couple things- I have trued a pile of uppers, some for match rifles, some for precision rifles and others because the owner insisted on it.
As expected, higher end uppers such as Seekins, Noveske, older Colt, Black Rain etc generally don't need it and if they do, just a tiny face cut, say .0015 to .003 cleaned them up. Lower budget items like Palmetto, DPMS typically need heavier facing. The worst I saw was a CORE upper, it took nearly .015 to square it. The barrel was visibly skewed to the left, about 1/8" clearance on the left and 3/16" on the right relative to the forend, it was really bad.
The Brownells tool does not ensure the face of the receiver ring is perfectly perpendicular with the axis of the receiver, all it does is make the face flat. If it does not fit the receiver bore perfectly with zero axial runout and then the pressure is not applied exactly in line with the axis you are accomplishing nothing.
You don't want to chance any lapping compound whatsoever getting into the interior of the receiver ring, you want the interface of the barrel extension and receiver as snug as possible.
On match and precision rifles I have gone as far as red Loctiting or Devcon 10110 "bedding" the barrel extension into the upper after truing the upper. You want absolutely no movement between the barrel and receiver and these guys consider the upper/barrel a disposable commodity.
I have one customer that special orders his upper receivers unthreaded and I thread it in the same setup as facing it. Not a bad way to build it, we know the face and the threads are exactly 90 degrees to each other. The exact same thing is done when truing up a bolt gun receiver, only the threads are internal.
The only way to be absolutely sure the face of the receiver ring is perpendicular to the axis of the receiver is to true it on a lathe.
I made a fixture that registers on the bore of the receiver and the receiver ring. The receiver is locked into place via the slot in the bottom of the receiver and the fixture is held by a collet at the headstock and live center in the tailstock.
I run an indicator in the ID of the receiver ring and adjust the fixture accordingly. Only then is the truing done to the face.

Removal of material from the face of the receiver ring has no effect whatsoever on headspace. Headspace on the AR platform is determined by the barrel extension/bolt relationship.

Here you can see 1st, 2cnd and final cut, each cut was about .0015

 

[robertc1024]

Not dead at all. Thanks for the info - makes sense. I'll have to study it a bit tomorrow.


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[robertc1024]

OK - it seems to me we've got at least three things going on here (I realize there are a lot more):
1. The perpendicularity of the face of the receiver to the bore axis.
2. The perpendicularity of the face of the receiver to the thread axis.
3. The alignment of the bore and thread axis.

For #1, I don't see how the Brownells tool wouldn't work - unless I am mis-understanding it completely. I was planning on turning a chunk of metal with an OD just under the ID of the bore and a flange on one end that would serve as the lapping surface for the face. It would at least approximate the concerns with #1. I understand your setup on the lathe and might still go that way if I could figure out a good way to make the jigs to hold it in place. I put my upper on a granite flat on its face and I was only measuring ~0.001" of difference on a 5-6" section of the bore of the upper so it seems pretty close.

I don't have the capability of measuring #2 or #3 - doing your own threading like you did seems the only way go guarantee that, I guess I'll just have to trust my luck on that.

I gotcha on the fit between the barrel extension and the upper. I hope the fit is so tight on mine I get to heat up the receiver and cool the barrel to get them together for a shrink fit - if not, I'll definitely use some gap filling loctite in there.

Again - thanks for the information.

 

[bptactical]

OK - it seems to me we've got at least three things going on here (I realize there are a lot more):
1. The perpendicularity of the face of the receiver to the bore axis.
2. The perpendicularity of the face of the receiver to the thread axis.
3. The alignment of the bore and thread axis.

For #1, I don't see how the Brownells tool wouldn't work - unless I am mis-understanding it completely. I was planning on turning a chunk of metal with an OD just under the ID of the bore and a flange on one end that would serve as the lapping surface for the face. It would at least approximate the concerns with #1. I understand your setup on the lathe and might still go that way if I could figure out a good way to make the jigs to hold it in place. I put my upper on a granite flat on its face and I was only measuring ~0.001" of difference on a 5-6" section of the bore of the upper so it seems pretty close.

I don't have the capability of measuring #2 or #3 - doing your own threading like you did seems the only way go guarantee that, I guess I'll just have to trust my luck on that.

I gotcha on the fit between the barrel extension and the upper. I hope the fit is so tight on mine I get to heat up the receiver and cool the barrel to get them together for a shrink fit - if not, I'll definitely use some gap filling loctite in there.

Again - thanks for the information.

If you are going to go through the work to turn a lapping tool, you are 90% of the way to making the arbor to face it in the lathe. Think in the reverse of what you are doing, use the flange end to butt the rear of the receiver against.
See the long slot in the bottom of the upper? There is your method of locking it to the arbor you just made.
Centerdrill the other end of the arbor so your live center can register in.
There is your fixture.
Now think about the .001 deviance you just mentioned. If you have a .001 deviance at 5" what do you have at 10"? .002. It's an exponential equation.
Now calculate that deviance at 100', 300'.
It adds up and can be a substantial number at a distance.
Make your fixture and square up the receiver in the same setup if possible. Once you pull it out of your lathe it is no longer dead bang aligned as it was. If you do remove it mark the fixture with what lathe jaw was where, that way you are minimizing runout errors.
Don't be afraid to indicate the fixture in if need be.
I run mine in a collet closed so runout is not an issue.

 

[ROGER4314]

Hi Robert!

Just offering information from experience and observation here........

I've had a cotton pickin' pile of those rifles including several that were custom built for me. I've never had any of my rifles machined like that nor have I known any competition shooters who did.

There are so many inexpensive and easy ways to improve accuracy of an AR-15 that I'd recommend that you start here.
High quality barrel.
Good trigger- smooth and predictable.
Use of "sled" magazine for single round firing.
Float tube for sling attachment.
Proper use of a sling.
High quality projectiles.
Proper match of the projectile weight/length to barrel twist.

These things give sure fire gains in accuracy and none of them are very expensive!

Flash

 

[TheDan]

I made a fixture that registers on the bore of the receiver and the receiver ring. The receiver is locked into place via the slot in the bottom of the receiver and the fixture is held by a collet at the headstock and live center in the tailstock.

View attachment 36847 View attachment 36848 View attachment 36846

Very cool. Thanks for sharing.

 

[Hogger]

Squaring the receiver will not change headspace. The barrel, extension and bolt are the 3 things that effect headspace. Squaring the receiver also make sure all of the bolt lugs bear evenly on the extension lugs. Most broken bolt lugs I have seen were due to the receiver not being square.

 

[Texas42]

Hi Robert!

Just offering information from experience and observation here........

I've had a cotton pickin' pile of those rifles including several that were custom built for me. I've never had any of my rifles machined like that nor have I known any competition shooters who did.

There are so many inexpensive and easy ways to improve accuracy of an AR-15 that I'd recommend that you start here.
High quality barrel.
Good trigger- smooth and predictable.
Use of "sled" magazine for single round firing.
Float tube for sling attachment.
Proper use of a sling.
High quality projectiles.
Proper match of the projectile weight/length to barrel twist.

These things give sure fire gains in accuracy and none of them are very expensive!

Flash

Precision barrels not expensive?

 

[Dawico]

Precision barrels not expensive?

Guess it depends on what you call expensive. DIY for a few hundred dollars, if that.

Much cheaper than a bolt action replacement.

 

[ROGER4314]

There are lots of ways to improve AR-15 accuracy and some cost absolutely nothing! Selecting the correct projectile for the barrel twist is the single most effective plan to get the most out of your AR. It requires no precision, just good choices.

1:7 twist 77+- grain according to my match shooter friends. I have no personal experience with that twist.

1:8 Personal experience, 69 grain Sierra Match Kings are the sweet spot.

1:9 55 grain projectiles are the best from this twist, and I won prize money at 600 yards with this combination!

Your experience may differ.

Within 200 yards, bullet quality matters little. At 600 yards, bullet quality becomes a major factor. I found the Sierra Match Kings to be top of the heap at 600 yards.

I only had a few black rifles with "match" barrels. Most were decent quality 20" heavy barrels and a few 16" barrels. Never noticed much difference.

Float tubes are the same. Never noted a lot of difference in floated or non floated front ends.

You can fuss with the barrels if you want to but I never saw drastic improvement by using a fancy barrel at 200 or 600 yards.

Flash

 

[Hogger]

Our opinions seem to differ, no offense. In Highpower the game is much more about the Indian than the bow. John Holliger himself told me any rifle shooting MOA can win a match, said he stops load development when he gets to 3/4 MOA. Derrick Martin had the same opinion 20+ years ago when he was in the game. My standup game was never good enough. Shooting little groups off a bench and prairie dogs at 300yds requires a little better accuracy than 3/4 MOA required in highpower. A freefloat tube or rail definitely makes a difference in anything other than service rifle at the local level. The 80 Amax or Berger VLD will shoot fine out of a true 8 twist as will anything lighter than 80. Kriegers 1:7.7 twist was optimized for the 77 and 80gr bullets. I've been shooting them for about 25 years and have too many groups in the .2s to count. The 75 Amax loaded long was unreal at 600. The new 90gr VLDs need a 6.5 or 7 twist to shoot them well but I would never buy a mil spec 7 twist 6 groove barrel. There are too many good barrels for under $200 to buy a junk barrel that wholesales for $40 and only military requirement is 3MOA . OP, You're on the right track, square the receiver it is cheap insurance.

 

[robertc1024]

I didn't expect this thread resurrection, but for what it's worth - I did make a tool to check it and with zero lapping, it was already perfectly square - or so close I couldn't tell. I figured it wouldn't hurt anything and I'm happy with the rifle's performance.