This comes with a modified bcg, it will only work with this setup, not a standard AR. The buffer and springs are included. This system replaces the standard bcg, receiver extension (buffer tube), buffer spring, buffer, and stock/brace.
Extension length is 9.5 inches off the back of the lower receiver and 12.5 inches LOP and has six positions. It will work with binary triggers. The kit comes with multiple springs of various pressures which may have to be swapped to fine tune the weapon and trigger.
Yes. Comes with two springs to adjust for more or less stiffness on the buffer.
If “buttstock” means receiver extension (buffer tube), then kind of... the included bcg is shortened for the system, it will not work with standard AR. The shortened bcg and modified buffer use a second spring, one between buffer and receiver extension, and one between buffer and bcg. But yes if you take out your standard bcg, receiver extension, buffer spring, buffer, buffer retainer and spring, end plate, and castle nut, then you can just install this setup and you’re good to go.