Last night I measured what the length of pull would be if the new stock was on the gun. It had a LOP of 15" MINIMUM, and could extend a couple more inches from there. That was unacceptable, so here's what I did.
1. I took the M4 stock off my AR-15 and put the new stock on instead. And I put the M4 carbine stock on my MechTech CCU. That's one of the heaviest guns I own BTW, because it's blowback operated and the bolt is huge. Neither gun kicks bad enough to need a recoil pad, but the rubber pad stays planted on my shoulder better than solid plastic. Since the the buttplate of the CAR-15 is even smaller, a slip-on pad makes it more comfortable for me to shoulder it. Both sizes are available online if anyone wants one. If I had another CAR-15 stock I'd get a pad for it.
2. I took the CAR-15 stock that was on the CCU and...
3. cut the front end with a coping saw just a hair in front of the 90 degree corner, and used a hacksaw to get through the steel insert. I didn't even know it was there until I took the stock off.
4. I also cut the back off of the "buffer tube" stock adaptor. I cut less off of it than the stock and the with the stock collapsed, the 2 parts are so perfectly flush at the rear you'd think I knew what I was doing.
.5 Here are the 2 pieces I cut off nested together. The new stock and tube are supposed to be commercial spec and everything else is all Mil-Spec, but it's all interchangeable with no looseness or binding. That seems to never happen to anyone else, but I got lucky.
.6 This is when the "fun" really started. There was nothing left to keep the stock from coming off when you extend it. So I found a bolt that fit in the hole and stuck it in with green Loctite. I turned it with an Allen wrench and it felt like the threads grabbed onto the plastic in the bottom of the hole. After I cut the bolt head off, ground down the end, and filed it, I painted it black with a touch up pen. The 5 position stock was now down to 3 positions and it was still too long. I drilled a hole where the front hole should be, and not having access to a mill or any other proper tools, I had to extend the slot forward quite a bit. It already had a narrow slot in it to lighten it, so I drilled down into it, then as close to horizontal as I could with a cordless drill. That left a LOT of material that that I picked away with a utility knife until the slot was long enough, and wide enough to slide back and forth. I rounded it off at the end just for S&G and called it good.