Head space question

[Bigdad]

I just lapped the receiver face on my 308 AR.  When researching this process several people said not to remove too much material from the receiver or you could cause a head space problem.  This confuses me because I thought the head space on an AR is determined by the distance from the bolt face to chamber shoulder.  Since the bolt engages the barrel extension not the receiver like a bolt action does I don't understand how removing material from the receiver face can effect head space. Please school me on this issue.

[sketch]

good question for the more experienced.. I would extend my experience as get some go no go gages.. just to be sure and future bolts and builds..  measure twice cut once. you get it.

What was your procedure and tools used ?

[Boot_Scraper]

I doubt the receiver has any affect on head space since HS can be checked with the barrel removed from the upper. Just my 2 pennies. 

Edited November 27, 2018 by Boot_Scraper

[Bigdad]

18 minutes ago, sketch said:

good question for the more experienced.. I would extend my experience as get some go no go gages.. just to be sure and future bolts and builds..  measure twice cut once. you get it.

What was your procedure and tools used ?

I used a Wheeler lapping tool and 220 grit lapping compound.  It didn't require removing much material to true it up.  Actually it probably didn't need truing at all.  Next I plan to bed the extension to the receiver as the barrel does wobble a bit.

[98Z5V]

6 minutes ago, Bigdad said:

  Next I plan to bed the extension to the receiver as the barrel does wobble a bit.

Use Loc-TIte 609 for that.

[sketch]

keep us posted as you proceed! 609 loctite... ^^

[308kiwi]

1 hour ago, Bigdad said:

I just lapped the receiver face on my 308 AR.  When researching this process several people said not to remove too much material from the receiver or you could cause a head space problem.  This confuses me because I thought the head space on an AR is determined by the distance from the bolt face to chamber shoulder.  Since the bolt engages the barrel extension not the receiver like a bolt action does I don't understand how removing material from the receiver face can effect head space. Please school me on this issue.

Removing material from the front of the upper receiver will not affect head space, all it will do is move the barrel/barrel extension assembly rearward in the upper receiver, as long as you are only removing enough material, like a few thou, to true the face of the upper then it will not cause an issue.

There seems to be a common belief that this is not so and that you can alter head space and/or 'set back' the chamber by doing what you are asking but I can assure you that it is nothing more than myth.

 

 

[Bigdad]

Thanks for the feedback everyone.  Here's another question.  I need to to shim the the barrel nut to get my hand guard picatinny rail to align with the receiver rail.  I ordered a shim kit from Bison Armory and their installation video shows that the shims should be cut and installed between the barrel extension and the receiver face. 

Why can't I just not cut them and install them between the barrel nut and extension?

[308kiwi]

4 hours ago, Bigdad said:

Thanks for the feedback everyone.  Here's another question.  I need to to shim the the barrel nut to get my hand guard picatinny rail to align with the receiver rail.  I ordered a shim kit from Bison Armory and their installation video shows that the shims should be cut and installed between the barrel extension and the receiver face. 

Why can't I just not cut them and install them between the barrel nut and extension?

The only reason you would fit the shims between the barrel extension shoulder and the receiver face is to stop the shim/s galling and becoming deformed between the barrel nut and the barrel extension shoulder, (should you fit them there) when you tighten the barrel nut. Common sense would dictate that you fit them between the barrel nut and the barrel extension as this way would not alter the relationship between the barrel extension and the receiver.

If you were to fit them between the extension and the receiver you would need to cut a segment from them to allow for the extension timing pin, just another bad idea.

If I was to shim a barrel nut to align a hand guard and gas tube I would fit them between the barrel nut and the extension and use a high quality anti gall lube to prevent them moving/deforming, but the best solution is to buy a barrel nut/hand guard system that doesn't require shims, there are plenty out there.

[Bigdad]

Thanks again Mr Kiwi.

[98Z5V]

Barrel nut shims, strictly for timing a barrel nut and handguard = put the barrel in the upper, put the proper amount/size of shims on there, then put on the barrel nut and torque it.  With the broad torque range of 30~80lb/ft or torque, they're often not even needed - unless you need to hit some lower torque spec from a specific handguard manufacturer.

All this "barrel nut timing for proper torque" bullshiit came about, because so damn many people put these things together, and don't own a decent torque wrench, or the proper tools.  There's so many "AR builders" that couldn't even identify a torque wrench for what it is, if you showed them a decent one...  My $0.02 on it.

Edited November 28, 2018 by 98Z5V

[survivalshop]

I have several HG's that use shims to time the the Barrel Nut & had no issues with them ALG HG's are one of them . The Shims fit between the Barrel Nut & the Barrel Extension . Here is a installation guide for an ALG HG , it looks difficult to assemble , but its real easy , I think some just over think it .

https://d1kuq0t26vxee8.cloudfront.net/media/files/ALG_-_V1_Instructions.pdf

[98Z5V]

^^^  Done 3 of those now, and they time up just right with that system.  No torque wrench needed, just follow the instructions - which comes back to not even knowing what torque you're setting it to.  I trust Bill Geissele, though, completely, and what he says will be right, certainly will be right. 

[Bigdad]

The hand guard on my rifle is similar to the one depicted in Survivalshop's post except it take a crow foot wrench.  It came with one shim installed between the nut and the extension.  I removed the shim and applied 80 lbs of torque to the nut and was able to get the picatinny rails to line up with out any shims.  

[308kiwi]

2 hours ago, 98Z5V said:

Barrel nut shims, strictly for timing a barrel nut and handguard = put the barrel in the upper, put the proper amount/size of shims on there, then put on the barrel nut and torque it.  With the broad torque range of 30~80lb/ft or torque, they're often not even needed - unless you need to hit some lower torque spec from a specific handguard manufacturer.

All this "barrel nut timing for proper torque" bullshiit came about, because so damn many people put these things together, and don't own a decent torque wrench, or the proper tools.  There's so many "AR builders" that couldn't even identify a torque wrench for what it is, if you showed them a decent one...  My $0.02 on it.

Exactly, I've never had to use 'shims', and god only knows how many AR's and M16's have been assembled over the last 5 or 6 decades without the need for any shims. It's just another solution looking for a problem IMO.

And I particularly agree with 98Z5V, "There's so many "AR builders" that couldn't even identify a torque wrench for what it is, if you showed them a decent one"

AND as a side note, when you add an extension, (barrel nut 'spanner' to a torque wrench and shift the axis of the head of the torque wrench away from the axis of the nut you MUST apply a formula to compensate for the differential between the two axis's otherwise your torque value may as well be a guess, you also MUST add the extension in the same plane as the wrench, ie not at 90 deg or any other angle,  the following formula works form this problem..........

T1 = T2 x L1 / L2

T1 is the torque setting, T2 is the actual torque that will be applied, L1 is the length of the torque wrench, L2 is the length of the torque wrench WITH your extension added.

Edited November 28, 2018 by 308kiwi

[Bigdad]

I used a crow foot 90 degrees from the torque wrench.  Its my understanding that does not increase the actual torque applied, correct?

[Boot_Scraper]

That's correct. 

[survivalshop]

    The Shim pak that comes with these types of HG's are for just the proper alignment of the Upper Rail & to also reduce Torque # on the Barrel Nut . There is a reason for a Torque spec range on AR type Firearms & its mostly to align the Gas Tube passage into the Upper Receiver , because the Gas Tube needs to be free floating & properly aligned with the Carrier's Key for proper function.

80# Torque is at the deep end & that's if your Torque Wrench is calibrated , you could be 60# or 100# unless Wrench is certified . This may come to a surprise to some , but I haven't used a Torque Wrench on AR's in many many years with no Issues , its all about the feel & I've been doing these for a long time . I also believe a high torque setting only stresses the Receiver Threads , its more important to have a good flush & secure fitment of the Barrel to the Upper Receiver . 

[308kiwi]

4 hours ago, Bigdad said:

I used a crow foot 90 degrees from the torque wrench.  Its my understanding that does not increase the actual torque applied, correct?

Yes that is correct, if you use the extension @ 90 deg then DON'T apply the formula that I mentioned. Sorry, the context of what I was giving is that you should be aware that adding length to a torque wrench changes it's value and in many cases you simply cannot use a torque wrench with an extension @ 90 deg. So a failsafe is to use it in a straight line and do the simple maths.

But you have to bear in mind that 90 deg is the only angle that you can use without applying a correction.

[Bigdad]

Thanks again.  Very helpful group here on 308AR.com!!

[washguy]

All this "barrel nut timing for proper torque" bullshiit came about, because so damn many people put these things together, and don't own a decent torque wrench, or the proper tools.  There's so many "AR builders" that couldn't even identify a torque wrench for what it is, if you showed them a decent one...  My $0.02 on it.

 

EXACTLY !         and the word of the day is...don't complicate your $hit...with ?      Wash