Part two is certainly the best of the series. The story sort of makes sense - Germans planning a secret nerve gas super weapon. But it seems way too important for the use of a convict squad, particularly when you consider that if they mess up they all go back to gaol, so presumably the nerve gas attack just goes ahead unstopped?
I think director Lee H. Katzin forgot that this was a TV movie and it looks great, with some big explosive action scenes. Lee Marvin is gone, but Telly Savalas steps in ably and seems far more interested than Marvin did in part 2. Bo Svenson plays the extra crazy guy really well, seems fitting that he appears here when he is best known for 'Inglorious Bastards' which was a brazen 'Dirty Dozen' knock-off.
It is on a perfectly decent DVD in UK and US, apparently on Blu-Ray in Germany although rather unnecessary.
Dirty Dozen Deadly Mission full review.