The L7A2 GPMG (FN MAG/M240 to heathens from other nations) has sort of a selective rate of fire, in that unless you adjust the gas regulator to the correct position for that particular barrel on that particular gun (known as balancing), your rate of fire will be either too high or too low.
This is quite a good description of how and why we do this:
http://www.arrse.co.uk/wiki/Balancing_the_GPMG
Also, the .50 cal HMGs we use (which have a quick change barrel) can still be set to semi-automatic. It has been suggested that this is to be used for ranging shots, but a burst does the job just as well.