What Torque Wrench Are You Using?

[Disciple]

30ft/lbs minimum with the tool in line with the torque wrench is still below where most would want a barrel nut

What actual torque do you recommend for a nut that doesn't require indexing?

[MistWolf]

The torque wrench specified in the manual (TM 9-1005-319-23 & P) comes in two flavors, one the torque wrench in "Shop Set, Small Arms: Field Maintenance, Basic, Less Power", P/N SC4933-95-A11 (NSN 4933-00-754-0664), and Torque Wrench, ft-lbs, P/N A-A-411 (NSN 5120-00-640-6365). Neither of these torque wrenches part numbers are a specific brand or model torque wrench but are a description of a commonly found commercial item, and mainly deals with the accuracy and repeatability requirements. The one thing that is specifically not stated is the overall dimensions of the wrench, so the length of the wrench is anywhere from 17-1/2" to 24".

The different lengths of the acceptable types of torque wrench in the manual gives a +/-5% variation on applied torque.

What you say is true. I've never found anywhere specifying TQ wrench length for TQing the barrel nut. Truth is, after getting a few assembled uppers under my belt, I don't use a TQ wrench anymore. I don't advocate others doing the same because I don't know their level of skill or experience. I also assemble one upper at a time and give the barrel nut my full attention. If I were working a line, I would definitely use a TQ wrench in case my attention wanders.

This is why when you make assembly drawings you put the actual torque down not the indicated torque for the collection of tooling assemblies. Because that one specified piece of tooling with break and somebody will have to figure out to do the job with non-specified tools, or somebody will change the design of the specified tool without telling you and for a year everything will be built wrong, until you find out.

For the most part, true. But I have run into prints with very specific TQ specs, such as using the TQ extender inline with the wrench, or TQ to so many degrees after snugging the fastener and so on.

30ft/lbs minimum with the tool in line with the torque wrench is still below where most would want a barrel nut, so being 90 degrees off or inline really doesn't mean anything, ESPECIALLY because that barrel nut needs to be advanced to get the gas tube through the teeth gaps on a TDP M4/M16, and 80ft/lbs even with the wrench inline is safely below the "actual" limit of a good receiver and barrel nut.
Also remember that the US Army mislabeled both the gas key screw torque and the "castle nut" torque at one point, so that manual should be considered "reference" more than anything else.

In aviation, I'm not allowed to consider an approved manual or any approve data a reference. It has the force of federal regulation.

But I do get your point. I also agree with you and Lysander that we're given wide latitude and the important thing is to apply enough TQ to the barrel nut that it doesn't come loose.

I asked for your input because I want to learn and avoid working in the vacuum of isolation.

Thanks to you, Lysander and GH41 for the discussion.

[lysander]

. . . . or TQ to so many degrees after snugging the fastener and so on.

That is a "torque-to-yield" (TTY) fastener, and that is another kettle of fish altogether.

TTY gives a very uniform clamping load under a very wide variation of lubrication conditions.

[Hart]

C4IGrant once mentioned 40 ft pounds as the sweet spot for barrel torque. Might give him a shout for clarification. Was in a thread where he built a blaster using a Bravo stainless barrel shooting really tight groups. I think he showed us something like a 1/2 inch group.