Need help on buffer size

[terrywick4]

You can use either carbine or rifle-length extension tubes with carbine or rifle buffers. It doesn't matter, as long as the recoil spring/buffer combo balances out with your gas system.

Do not use a rifle buffer in a carbine buffer tube. The rifle buffer is too long. this come from gun digest:

Do not, under any circumstances, exchange buffers between rifles and carbines. The rifle buffer is too long, and firing a carbine with a rifle buffer in it will cause the buffer tail to strike the end of the tube at higher than designed velocities.If you are lucky, the result will be that the carrier key screws shear off. If the screws do not shear off in time, the lower receiver will crack at the buffer tube threads. The only repair for that particular mistake is a new receiver. Install the proper buffer, and avoid the hassles of a new lower to correct the mistake.

http://www.gundigest.com/guns/rifle-revi...il-springs