The sturmtiger is a rocket artillery piece added with patch 1.11.3, a video can be found here...
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appears in editor as "sturmtiger" without the quotes here...
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It's hugely powerful, with a range of 170m in game and a massive blast radius, but is slightly smaller than a panzer 1 so doesn't suffer the mobility problems that the Karl mortars have, nor is as large as the sevastapol 305mm fixed gun, all of which can be seen here (including a weird chequered 155mm cannon)...
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it would be great to know how to get the sturmtiger into the game on the soviet side to play with
you could make a mission or edit a mission so that your units have the sturm. But i think there is a def file not in the entities file but in a properties one that chooses what goes where. Also making a def in that folder and using a different variant and skin (dds) would be something a little tricky i suppose.