I can't edit my post anymore, so I'll just post again...
I just finished modding my ACM UBR. It was really easy.
Thanks to Rookie Ab for helping me out with this and giving me some great ideas this week!
This guide works well for all UBR's made to work with AEG's (AEG type buffer tubes), if you have one thats meant for a gas blow back rifle (has threads on the buffer) read the extra step.
1.) Unscrew the grub screw from the bottom of your original stock tube, after removing the stock you had on the tube beforehand. Also, at the time remember to remove the buffer and buffer spring from your buffer tube.
2.) Unscrew the rear cap from the stock tube. If you're is stuck, try using some Vice Grips and then turn them while they're clamped on to the cap. This should give you the friction you need and is way better than using some ridiculously big flat head screwdriver on that enormous cap. Once you've gotten that off, slide off your old stock tube which should expose the brass inner buffer tube unique to only KJW's.
GBBR Threaded Buffer Tube EXTRA STEP: If you've got a GBBR UBR, you're going to need to wrap the brass buffer tube in some kind of heavy duty tape in order to beef up outer diameter. Since you can't use the UBR buffer tube, you're going to need to translate the UBR parts onto the original KJW buffer. I have not done this so I don't know how, but you can PM "Rookie Ab", as he has utilized this method.
3.) Now that you're ready to slide on the UBR buffer tube, you'll realize if you do this, there is a gap between the buffer and the receiver. This can be covered up in some electrical tape, or any other black tape. I used Gorilla Black Duct Tape, and it worked just fine. Keep test fitting the tube to make sure the tape doesn't stop the tube from coming all the way forward, and to see if you've covered up the gap enough.
4.) This is the fun part. Take a #8 or 5mm drill bit. Screw the end cap onto your brass insert WITHOUT THE UBR TUBE ON IT (duh, you can't!). Once its on nice and tight (its not coming off again baby, better make sure its tight) place the UBR buffer tube over the end cap and and brass buffer tube. Put the #8/5mm drill bit onto your favorite powerdrill and use the hole from the UBR buffer as a guide and drill through the end cap.
5.) After you've drilled the hole, remove the UBR buffer tube, exposing the brass buffer tube, and take your handy dandy tapping tool, and tap an M6 x 1 thread into the hole you've made. You have now made the correct threading for the screw you need to insert into the back of the UBR to hold it in place.
6.) Take your M6 x 0.75" long black screw (with one of those angled heads.. forgot the name of them right now) and screw it through the UBR buffer tube, and into the now tapped end cap that is sitting on your brass buffer tube. It should be super sturdy now, and even sturdier after you screw in the screws that hold the cheek piece over the UBR buffer tube.
7.) Slide the UBR stock onto the UBR buffer tube system, and bam, you're done!
Took me about 35 minutes!