Tight AR barrel/upper fit

[O2HeN2]

I have a Lothar Walther 300 BLK barrel and a YHM upper. The fit is extremely tight. Is there any issue with putting a thin film of lightweight oil on the barrel to assist in seating it?

O2

[ray1970]

I usually put a thin coat of molybdenum grease on mine. I'm sure oil would be just fine.

[BPTactical]

I'm gonna part ways with Ray on this one. That tight fit is conducive to consistency of grouping. You want absolutely no movement of upper/barrel assembly. You don't want a lubricant where you want zero movement.
National Match rifle barrels are frequently LocTite/JB Welded to the upper.
I would say dry fit and as long as you can seat it with a few taps of a soft faced mallet without undue force you are good to go. You run very little chance of it seizing with it being anodiized. If it is too tight for your comfort level lightly polish the barrel extension with 400 grit wetordry shoeshine fashion with soapy water or spit.
You want that tight fit and the Lothar barrels are to the large end of the tolerance range for that reason.
Moly lube your receiver threads after snugging them up and backing them off 3-4 times. That makes sure the barrel is seated well and the threads are slightly burnished so you get good torque values. 40 ft/lbs to start then align your gas tube notch/hole. Don't exceed 80 ft/lbs.

[O2HeN2]

So if I can't fully seat the barrel in the upper by hand, is it OK to use the barrel nut to "draw" the barrel down those last few mm?

O2

[BPTactical]

I would rather tap it in with a soft faced mallet, that way you can monitor the progress visually. If it takes ANYTHING more that a few light raps-polish the extension a bit as described above.
A tight slip fit is acceptable but you should not have to beat the thing in.

[O2HeN2]

Got it seated with a rubber mallet and a piece of wood, which brought me to my next problem, which I had wondered if it could happen, and promptly ran squarely into.

Tightened and loosened the barrel nut a couple times to seat everything and burnish the threads. "Mollyed" up the threads and went for the final torquing down... At 35 ft/lbs one hole was just barely too far. At 60 ft/lbs I was only 1/3 of the way to the next hole -- to get it to the next hole was going to be way over 80 ft/lbs. So I had to back off and only tighten to ~25 ft/lbs to align a hole for the gas tube.

Is there a fix for this, or is that light a torque OK?

As an aside: I wanted to learn how to build an AR (and have a good excuse to buy new tools ) but I didn't want it to turn into the adventure it has become:

• Defective flash hider
• Defective Spikes lower
• And now can't torque the barrel to the suggested range

If defective parts and strange situations come your way after you're familiar with a process that's one thing, but this has been an exercise in frustration.

O2

[Great-Kazoo]

Got it seated with a rubber mallet and a piece of wood, which brought me to my next problem, which I had wondered if it could happen, and promptly ran squarely into.

Tightened and loosened the barrel nut a couple times to seat everything and burnish the threads. "Mollyed" up the threads and went for the final torquing down... At 35 ft/lbs one hole was just barely too far. At 60 ft/lbs I was only 1/3 of the way to the next hole -- to get it to the next hole was going to be way over 80 ft/lbs. So I had to back off and only tighten to ~25 ft/lbs to align a hole for the gas tube.

Is there a fix for this, or is that light a torque OK?

As an aside: I wanted to learn how to build an AR (and have a good excuse to buy new tools ) but I didn't want it to turn into the adventure it has become:

• Defective flash hider
• Defective Spikes lower
• And now can't torque the barrel to the suggested range

If defective parts and strange situations come your way after you're familiar with a process that's one thing, but this has been an exercise in frustration.

O2

You would be surprised how many Factory rifles when removing the bbl for upgrades literally came off with a good swift tug on the wrench.

The more you do the more comfortable it becomes, especially after you buy some tools. Then you score another bbl you liked, to slap on the A3 upper as it's your new distance ar etc.

[BPTactical]

Typically if that issue arises a different barrel nut will cure it.
Let me throw you a curve ball-did you account for the offset of the wrench?
When tightening the barrel nut the axis of your torque wrench is not on the same axis as the center of the barrel nut. You needed an adapter to fit the barrel nut. If you measure from the center of wrench axis to the center of your barrel nut wrench it increases the effective length of the wrench.
This increases the effective torque you are applying to the barrel nut and needs to be kept in mind. There is a formula for calculating it and last time I did the math I needed to reduce my torque setting by about 7 pounds.
So your torque is probably closer to 35 ft/lbs or so and as Jim mentioned, some factory rifles are virtually loose when disassembled.
Keep in mind also that a lubricated fastener is more true to the torque value than a dry one. Due to friction a dry fastener that reaches it's torque at say, 25 ft/lbs may in fact only be torqued to 15 ft/lbs true torque.
My money says you will be just fine.

[O2HeN2]

Let me throw you a curve ball-did you account for the offset of the wrench?

Well that answers yet another question I had. All the instructions I've seen simply stated "NN ft/lbs" and I was left wondering "Is that ACTUAL ft/lbs, or is the spec rough enough that they simply ignore the offset's mechanical advantage?"

So no, I didn't include that, which will bring my ft/lbs up a bit!

Thank you everyone for the help!

O2

Added on edit: Well, not that much more -- about 27ft/lbs: (18" + 1.3")/18" * 25 ft/lbs = 26.8 ft/lbs

[SAnd]

Can anybody talk Official Mil-Speak?

The M16 / M4 maintenance manual (Army TM 9-1005-319-23&P) says "Torque is measured when both wrenches are used together"

The wrenches referred to are a torque wrench and a "Wrench, Combination" (part number 11010033).

I always took that to mean the 30 to 80 torque specs already factored in the Combination Wrench. In other words you read 30 pounds with the Combination Wrench installed.

Somewhere along the way I found a dimensioned drawing of the military Combination Wrench. The distance between the barrel nut center and the square drive torque wrench hole is the same on the common commercial combo wrench's and the military Combination Wrench, in my limited experience. I can't find the drawing right now or I would be more specific.

Just adding my 2 cents of confusion.