Well the Macharian crusades, delved deep into unclaimed territory and fought against xenos and humans that had never been imperialised before. The crusade was literally MASSIVE so anything could've happened to countless guardsmen.
Cadia was a bloodbath, obviously mainly cadian regiments were involved but there were lots of other regiments there too. After a hard fight, Chaos controlled the ground at the end of the campaign, but both sides were massacred during the process
Rynn's world was the reclamation of the chapter planet of the Crimson Fists from the Orks. With the Crimson Fists down to about 160 guys or so, there wasn't much they could do, the imperial guard did a hell of a lot of work and I imagine the lifting of the siege of new Rynn's City would've been a brutal and close quartered thing.
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