20 SLRs? Sounds a bit overkill, and as you say, prone to a cock-up of titanic proportions.
Give one SLR and a mag to a RockApe Cpl or Scuffer (unless there are any real soldiers on the camp) and have at it. I just think it a bit embarrassing for the Armed Forces to have to call in civvy coppers to shoot a cow. Certainly on a full-size unit with plenty of small arms and ammunition.
Actually, scrub the Scuffer. He'd probably use his pistol and just get the cow very angry.