Brandon at SDA2RC posts this definition of the mass as sacrifice from the 1962 Daily Roman Missal published by Angelus Press (the SSPX publishing house):
The Mass is thus the perpetual prolongation of the Sacrifice made on the cross. Consequently, every Mass is the one immolation of Christ repeated in the Act of Oblation.
That’s an interesting way of putting it, which I think makes the reasons for the Protestant criticism all the more clear. Christ sacrificed himself once for all; that sacrifice is neither prolonged nor repeated. For more, see my article, The Sacrifice of the Mass.