According to the Payday Wiki:

The Shield's only weapon is the B9-S, though it is fully automatic and seems to have the properties of the STRYK pistol.

Seeing mods like M308 Rifle for Sniper had me wondering if I could similarly replace the Shield's B9-S with a STRYK. Turns out I could!

Known Issues:

• The Shield holds the B9-S weirdly in vanilla, and that applies to the STRYK, too. His trigger finger rests underneath the trigger guard, and the barrel is so high that it doesn't poke out the hole in his shield (purely a visual issue, doesn't prevent the Shield from hitting you). Not something I could fix myself at the moment.
• The preview images for this mod aren't very clear. I may go back and replace them if I can get better shots later.

This is my first real mod for a Diesel game, so if you encounter any issues or have any advice for improvement, please let me know in the comments. Thanks.