I apologize if this has been discussed before in here, but nothing came up in a search.

My AR-9 has been complete for a couple of years now, and twice I have had to tighten the barrel nut, as it comes loose enoug that the barrel wobbles slightly in the receiver.  The nut has been torqued to spec three times now (about 500 rounds between tightenings), and I assume the extra force caused by the heavy buffer/bolt/spring combo is the culprit, plus there's no gas tube to keep the nut in place.

Has this happened to anyone else, and what have you done to remedy the problem?

Edit: I'm using a Wolf extra power spring, Spikes 9mm buffer, MAS bolt, and JSE barrel; all in a Aero Precision M4-E1 upper.

Mine did the same after about 1400 rounds. I need to get it to a 'Smith to have it tightened up. I was thinking of having them use some blu or red Loctite on the threads, but with them being aluminum it has me a little worried.

What are you torquing it to?
You might consider using a soft set thread sealer

The extra power spring is probably doing you no good. Does it really take a lot of energy to strip a 9mm from the magazine? Loading energy in a 9mm is significantly lower and there is no bolt locking to accomplish so you really need a lower power spring in a 9mm.

Does the Spikes buffer limit bolt overtravel? Limiting overtravel is also helpful to reduce the wear and tear in a 9mm.

It seems like at one point people were having all sorts of problems breaking bolt catches, or guns just not operating properly, and I think the heavy buffer/spring was looked at as the way to go.

If I could get away with a regular carbine spring that may help fix some of the problem.
I'm just a little hesitant to make any changes, as this setup is problem free (besides the barrel nut of course).

I just tightened it again, this time to 50lbs.
Hopefully that will work.

Heavy buffer is one thing, changing the spring does something else entirely.

Despite the common perception, changing spring rates does almost nothing to slow the opening speed of the bolt. All it does is accelerate the bolt closing. That puts greater force on the bolt catch, as well as impact force against the barrel with every cycle. A 9mm setup needs much less spring force than even a standard carbine spring.

What slows opening in a blowback system is mass. The weight of the bolt and buffer together.

Lots of mass PLUS a stronger spring equals quite a bit of extra force slamming forward every time. That may be why the barrel nut is coming loose.

What will save bolt catches is reducing the overtravel. When the bolt gets a 3/4 inch running start at the bolt catch, that is why they break. Normal overtravel is like 3/16".

I think I'll try a regular carbine spring, thanks for the advice.

I torqued mine long ago.. I have not had any issues,, though I did gain a well used - blown out carbine gas tube to use as a "pin" for my plain AR-15 barrel nut.. pin it as normal into sight base, but where it go into upper, figure length and cut end off so it stop right at edge of entering into the operating area inside upper

I would not use Blue or Red,, I think you be better served to use Permatex HIGH Temp thread sealant (#59235) in the form of what I refer to as "teflon toothpaste"... it is a teflon based pipe seal, it will set up and hold, but still come apart too... I used it for thread lock on PWC hardware.

I've never had a barrel nut come loose either.

If properly torqued, a light or heavy action spring shouldn't be the difference between the barrel nut staying secure or loosening up.

Use the proper grease for the barrel nut and torque to the higher end of specs if necessary.  I personally wouldn't use any kind of thread locker or sealer.

This, if you had a spring strong enough to slow the opening of the bolt you wouldn't be able to manually cycle the bolt. 

I did the same.  My barrel nut hadn't loosened, but when I tried to install a rail, it wouldn't line up because the nut wasn't indexed.  So "what the heck," I pinned it with a chunk of old gas tube and a dab of epoxy (between the tube and the barrel nut) to hold it in place.

My thoughts on a "Properly Torqued Barrel Nut"

Use a steel washer between the barrel shoulder and the barrel nut.  This is especially important if the barrel nut is aluminum.

Use anti-seize on the threads and the contact surface of the barrel nut.

Torque the barrel nut and back it off three or four times,  Then torque it one last time.