Note: You CAN use a carbine buffer and spring (in an A2 length stock).....with the addition of a special spacer. The spacer is nothing more than a machined aluminum "slug", which you insert into the rifle length tube, before installing the spring and buffer. It simply takes up the extra space in the longer buffer tube.
Spikes Tactical makes such a spacer. There may be others, but Spikes is the only one that I know of specifically.
If you have a carbine AR, this would be the preferred way to use an A2 stock. The A2 rifle buffer (that normally comes with an A2 stock kit) is generally too heavy for the carbine gas system. They will work, if the gas system is up to par, but only by a small margin. So, it is definitely better to run a carbine buffer with a carbine rifle....and only use a rifle buffer with a rifle-length gas system.
There are 5 different weights of CARBINE buffers, to cover the range of different length gas systems on various AR carbines and "pistols". There is only one RIFLE buffer, which weighs about 5 oz. CARBINE buffers weigh as follows: Standard (3 oz.), H (approx. 3.5 oz), H1 (roughly 4.5 oz), H2 (a bit over 5 oz), H3 (about 6.5 oz). For a standard CARBINE, a "standard" or "H" buffer is usually best.