Well with the SPR having about the same length or the same length outer barrel as the m16 I don't see why he couldn't run the 12". I'm not sure how it would look but it would save him some money instead of having to buy another outer barrel.
EDIT:
RS mk12 spr's run a rifle length gas system so you should be using the rifle length handguard. This is assuming you wanted to stay as close as possible to the real steel version.
EDIT:
RS mk12 spr's run a rifle length gas system so you should be using the rifle length handguard. This is assuming you wanted to stay as close as possible to the real steel version.