Ah yes, one of the so-called "Waifu Guns", fully automatic pistol, designed by Silent Abyss, and featuring Emilia, one of the main characters in the Re:Zero − Starting Life in Another World light novel/manga/anime series.

SFM version:
https://steamcommunity.com/sharedfiles/filedetails/?id=2788842688

The model was ported from a L4D2 mod with permission granted:
https://steamcommunity.com/sharedfiles/filedetails/?id=2640892938

Includes:
l4d2_mod\silent_abyss\emilia_pistol_retrofit_kit\l4d2_mod_emilia_pistol_retrofit_kit.mdl
l4d2_mod\silent_abyss\emilia_pistol_retrofit_kit\l4d2_mod_emilia_pistol_retrofit_kit_bulet_case.mdl
l4d2_mod\silent_abyss\emilia_pistol_retrofit_kit\l4d2_mod_emilia_pistol_retrofit_kit_magazine.mdl

Credits :
Silent Abyss - Original creator of the model and the L4D2 mod.
NeoLin - Porting to Gmod and SFM.

Feel free to support the Lambda Group for more addons.👌
https://ko-fi.com/neolin7812
https://www.paypal.com/paypalme/NeoLin7812