I have an A5 with only the standard buffer –– I have not timed it, but since switching from a milspec collapsible-stock receiver extension and an H2 buffer, it seems to run a tad faster –– an increase in the 25-50 rpm range –– for various barrel lengths in 5.56 and 9mm.

From my talks with Vltor folks, the two optional heavier buffers slow the ROF incrementally ...

I'd be interested in the results if anyone has timed an A5, but the results would have to be compared with a standard shorty tube and known buffer, using identical uppers and ammo, to get any sort of correlation...

Note that the A5 system uses a standard, milspec M16 rifle buffer spring. In addition, the difference in length between the rifle buffer and the A5 buffer is exactly the difference between the rifle receiver extension (buffer tube) and the A5 receiver extension.

Thus, the travel length of the A5 buffer, and the spring rate, are identical to the rifle cycling system. In addition, the A5 buffer weighs the same as a rifle buffer ... so, in effect, Vltor with the A5 system has been able to duplicate the rifle cycling system in a receiver extension (buffer tube) only 7/8" longer than the milspec carbine receiver extension, which has a shorter buffer travel, a different spring length and spring rate, and which (in turn) are not all that compatible with 20" barrelled uppers, the "unsupported configuration."

I've never heard of anyone who had issues using a rifle receiver extension on any length barrel, down to 4" ... but carbine receiver extensions are notoriously finicky with 18-20" barrels.

IMHO, the A5 solves some serious long-standing reliability issues.

YMMV.