C77 was not allowed to be used -- there was a small supply of Mk262 Mod0 at the time for it.
Some genius had determined that C77 would damage the bore.
Most rack grade C7's shot better, and a much easier CT gun was simply adding a FF handguard to a C7A1/A2, scoping it and stealing the KAC trigger out a C7CT and shooting the C7 (not that any units in Edmonton or Petawawa every did that)