I've got a MI G3 Keymod rail to install on an existing rifle.

I don't own a torque wrench. The few barrels I've installed in the last used traditional (M16A2-sytle) delta rings, and I just tightened them with a DPMS wrench and they worked fine.

How sensitive are the torque values? The MI instructions list 40 ft-lbs for the barrel nut and 55 in-lbs (4.83 ft-lbs) for the clamping screws. The FAQ also has a section that basically says "If that doesn't work try 30 ft-lbs...and if that doesn't work, try 40 ft-lbs...then 50 ft-lbs" all the way from 30 ft-lbs to 80 ft-lbs, which is a big enough range that it seems to imply these torque values aren't that precise.

The cheapest torque wrench I can find locally won't go below 20 ft-lbs, and the tools I'm seeing online that will go all the way down to 55 in-lbs cost more than the rail did.

Any suggestions?

That is accurate - its not a specific torque for the barrel nut, it's a wide range from 30 to 85 foot pounds, and the 85 is a maximum to keep from damaging the threads on the nose of the upper. What you attempt is to get it over 30 then aligned with the gas tube aperature to install it straight in.

Plenty of posters have all bought into the torque setting thing and in threads it's also pointed out that attaching the barrel nut wrench adapter also distorts the actual reading as it adds length, and therefore, leverage. The 85 pound upper limit as spelled out in the TM is a calculated figure for assembly purposes, a "DO NOT EXCEED" number. It's clearly spelled out but nope, "I built ten of these and I know better" comes along and off we go again.

An individual maker and the screws he puts on the handguard is different - that is his spec for his fastener and the attachment driving the fastener doesn't add length to the torque assembly. Stick to his number as it is his calculated figure to keep things from coming apart, or worse, getting stripped.

When I assembled the last two barrel nuts they weren't "torqued" to any specific number. I could tell by feel it was definitely over 30 pounds, that's a firm single handed grip on a 3/8 box wrench doing thermostat housing bolts - and pushing it. What was the final result - I don't know. The wrench didn't click before I got to lining up the teeth to pass the gas tube. It was, therefore, torqued to MILSPEC. It didn't exceed 85 foot pounds.

But guys who never work on their own car will post up about how hillbilly that is and take more time explaining it that it takes to do it. I always ask - do you torque the lug nuts on your car? If you have, and you have done it for a living, you KNOW what 85 foot pounds is and you have checked it. Milspec states DO NOT EXCEED, not "twist it until you get there."

I'm starting to think that since MI include a plain old 90-degree-bend hex key for the two clamping bolts, the torque value there should be fine with "hand tight".

it often is....with the disclaimer that ultimately one man's hand-tight is another man's cranked-down-upon.  

Just go to harbor freight and get an 1/4-in drive inch-lb torque wrench for small stuff, and a 1/2-in drive ft-lb for barrel nuts and muzzle devices.  They're under 20 bucks and as long as you back it off after use and store it at 0, they are surprisingly accurate and hold that accuracy quite impressively, better than many more expensive brands.

Link if you think I'm lying.

I didn't realize that harbor freight 1/4 inch went down to 20 in lbs. I'll go hit my local Harbor Freight at lunch.

Thank you.

I didn't realize that harbor freight 1/4 inch went down to 20 in lbs. I'll go hit my local Harbor Freight at lunch.

Thank you.

I care about torque specs. Certain things I wouldn't imagine ignoring the torque spec or putting something together without it. A cylinder head comes to mind. Virtually anything on a seawater system or anything nuclear on a nuclear powered submarine. Other things I would use it if I had the info and the tool. Something like a barrel nut and handguard fasteners I am not sure I would worry about it all that much. That said, I have a torque wrench and I use it. I expect anyone who has done some repair work over the years could put a barrel nut on and snug some handguard fasteners without a torque wrench. Someone that hasn't fixed or repaired much over the years might not have the experience to know when tight enough is enough or a little more is too tight.

I have three name brand inch pound torque wrenches in my gun room that I haven't touched in ten years. 

If you have turned wrenches for many years, you get the feel for proper torque.  It is all about the feel these days.

I should sell them on the EE to finance my Labradar need.   Note to self...................