I bought some Bushmaster kits from CDNN because they looked neat and were kind of a good deal but am trying to figure out how to assemble them.
Most rails these days seem pretty straight forward and easy but these Bushmasters have been a puzzle.
With a barrel installed and the barrel nut screwed on to a proper torque the bi-pod screw holes of the also threaded on handguard were on top.
So I added a shim which aligned the barrel nut for the gas tube but the holes for the bi-pod are now about 10 degrees too much past 180 on the bottom.
I can get those holes properly at the bottom by backing off the screwed on handguard but the torque is too little. If I back off the barrel nut the gas tube wont go in.
Its kind of a Mickey mouse set up IMO.

I'm thinking of drilling a hole through the bottom of the handguards up into the barrel nut and then tapping and inserting a black button head screw to keep it from rotating.
In a perfect world there would be shims for the handguard to barrel nut. These would have to be custom machined.

All the instructional vids on youtube are for straight forward idiot proof adjustable handguards.
I cant seem to post a pic but here is a link to the kit.

https://gun.deals/product/bushmaster-xm15-build-kit-burnt-bronze-cerakote-39999

What surface does the handguard torque down against when it is threaded on?  The forward face of the upper or something on the barrel nut?

Oh, and what thickness shim did you use?

Isn't that a knurled nut between the handguard and the receiver? I recall my Bushmaster Varminter had the same kind of handguard and it is indexed using that nut. Shims not required. Use blue Loctite. I ended up replacing that handguard.

The threaded handguard butts up against the barrel nut itself.
Shim is .07" which is just right.

Thinner or thicker and the gas tube hole wont line up with the upper.

Wow you may be right !

But I tried to back off by hand that knurled nut and it is frozen onto the bare aluminum treads of the barrel nut.
I can of course hold the barrel nut itself with the proper wrench but how to break the knurled nut loose without buggering up the cerracoating ?

Not considering any threadlocker release procedures, this may work, Use cord that stretches the least.


Edit to add: I would wrap everything else with blue painters tape in case you slip.

Awesome ! Will try that thanks.

I highly recommend heating it up first using a small propane torch or a heat gun if you have one. I prefer a torch. Watch for smoke then you know the thread locker is cooked. It should come off pretty easy then. Even if there is no thread locker ... it won't ever hurt to use the torch first. I suspect more people gall up the surface with repeated attempts to remove it before using the torch and getting it off in a single attempt.

The torch will NOT harm the coating or the parts.

When reinstalling the handguard, use blue thread locker on both the handguard and the nut. I wouldn't get carried away with tightening the knurled lock nut.