Jmart: Do not use a carbine buffer in a full size tube! The bolt assembly will have the capability to come back and contact the rear of the lower receiver possibly causing damge in the long run. If needed to correct the timing/function due to the carbine, use a light weight full size buffer.
Tman: Could you elaborate on the timing issue? Is there a problem with mating an A2 rifle buttstock with a 16" bbl carbine upper?
No, it is not a timing issue at all, it is simply a matter of length of the parts. The rifle tube, buffer and spring, are all longer. The shorter CAR buffer is made to fit the shorter CAR tube. The shorter CAR spring is made to give the proper spring rate in the shorter tube.
If you put the short spring and buffer in the long tube, (1) the spring does not have enough compression, so gives less resistance to the carrier, and (2) it allows too much bolt carrier travel such as Jmart described above, because the shorter buffer is not stopped by the end of the longer tube. What is supposed to happen is that the rubber bumper on the back of the buffer is supposed to limit excessive rearward travel to prevent damage to the various parts.