Thats a general issue with attaching items to each other. Guns are always the same size regardless of having a buttstock/scope/suppressor or not.

Combining a gas stove, gas canister and cooking pot/frying pan will always result in a 1x2 item even though the parts are 1x2, 1x/2 and 2x2. You can even store items inside the pot/pan.

Hello Darcion and thank you for the report.
This is an issue we are aware of (#0022523) and it has been scheduled for a fix.
Regards,
Geez