Since these are on the T20 CD they should be legit to post. If not, please have the Moderators feel free to remove them.
[SPOILER="Gauss Pistol (Imperial Army & Navy versions (ca IY 1000) from T20 Weapons of Charted Space"]
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Imperial Army (& Imperial Marine) / Imperial Navy (ca I.Y. 1000)
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The back of the navy model looks like it has a plate for attaching a stock Mauser style.
I can imagine a navy or airforce (or any military not expecting ground combat) buying a bunch of sidearms and then carbinising them in the few times they might need longarms as a cost saving measure.
Ofc a long pistol/short carbine hybrid would probably be suboptimal at either.