Given the similarities but minute variations of the superb Vltor
RE-A5 and RE10/A5SR, we have taken some comparison pictures to make it
easy to distinguish one from another, also to illustrate the
differences.
Vltor's A5 requires a longer A5
length receiver extension (RE) to accommodate the A5/rifle length spring
and longer A5 buffer. RE-A5 and RE10/A5SR are of the same length,
whereas RE-A5 has 7 stock positions, RE10/A5SR has 6.

The
extra position on the RE-A5 provides slightly more adjust ability, but
more importantly, it allows longer stocks like the Magpul STR (7" in length) to more
fully collapse than the RE10/A5SR, as shown below.

Note
that this is only applicable to longer stocks. The 7 position vs. 6
position difference is irrelevant on stocks with lengths that are close
to the Colt M4 stock, such as the SOPMOD stock, at 6.5".

Another difference is the RE-A5 has more threads for the castle nut than RE10/A5SR, but this has no impact on functionality.
It
is probably not news we're a huge proponent for the Vltor A5 system.
It is a well engineered, tested, and vetted system that brings real
benefits to the reliability and functionality of the AR platform. Its
elegant simplicity, reliability and benefits make the A5 one of the most
worthy and best innovations for the AR15/M16, it transforms a rather
violent carbine into a smooth shooting machine, which has direct
correlations to the longevity of a carbine's components the short
carbine gas systems beats on mercilessly.
For a more detailed look into the Vltor A5, please visit this page: https://://www.arbuildjunkie.com/vltor-a5-buffer-system-overview/