Ah yes, this quite an interesting one lol.

Well, the model itself is a short H&K G3 using rifle round still, apparently it's not really a real gun, but rather just an airsoft model made by Tokyo Marui, which you can see it in action in the video.

But still, quite interesting to see it being reimagined as an actual weapon with rifle rounds instead.

You can find more details about it in the following Artstation page:
https://www.artstation.com/artwork/L3Gzwr

SFM version:
https://steamcommunity.com/sharedfiles/filedetails/?id=2749948311

Includes:
artstation\sylvainlebeau\artst_hk_g3_sas.mdl
artstation\sylvainlebeau\artst_hk_g3_sas_bullet.mdl
artstation\sylvainlebeau\artst_hk_g3_sas_magazine.mdl

Credits :
Tokyo Marui - Original creators of airsoft model H&K G3 SAS.
Sylvain Lebeau - Original creator of the model.
NeoLin - Porting to Source.
And PBR Converter by Fennecai ( https://linktr.ee/Fennecai )

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