Short cartridges in full length autoloaders

[9x23w]

The Browning BAR (not the Shorttrac/Longtrac), Benelli R1, and Winchester SXR all seem to use the same action for their short cartridges as their long cartridges.  I have not examined any of these and am just curious what they do with the unused magazine space.  For example, if a 300 WSM rifle, is the extra space available to load heavy bullets so they don't run back far into the case?

[Redtail1949]

From what I have read the mag length forces the handloader as well factory rounds to be set back into case loseing the full potential of the rounds.