Wait... Cantilever mount? or did you want the AK style SIDE mount?
Cantilever is a total DIY job. Here's how:
1.Go to OST/Neit/NEA website.
2.Order NEA inline stock adapter (the big one with the chunk of 1913 rail on it) and NEA Cantilever mount. as well as an M4 buttstock & tube. Pay and they ship to you.
3.Remove beaver barf or folding stock. You'll need a big jeebus flathead screwdriver, or in my case, a saw and visegrips, because I broke all my big jeebus screwdrivers.
4.Using supplied hardware, and maybe a little BLUE loctite, install the adapter.
5.Install buttstock.
6.Mount cantilever rail, ensure it's snug. Mount optic of choice.
7.Zero and have fun.
And if you lack THAT much confidence in your mechanical skill, send it to me, return shipping paid, and I'll do the work for free.
