Way back in like... 2016[?] I tossed this together.
View attachment 271740
Weapon: BR55HB SR
Game: Halo 3
Linked file is scaled to be the in universe length of 29" for the main body and 35.4" including barrel.
Blue sections are final constructed pieces to show the overall finished prop.
Light grey sections are 0.125" Hardboard.
Dark grey sections are 0.5" MDF.
Dashed sections routed 0.125" internally for fittings. The top-most dark grey section is to be cut out twice with mirrored routed sections.
Barrel and gas piston housing [I'm assuming that's what they designed it as] are 0.5" diameter doweling.
Crossed sections are points to pre-drill for extra detail fittings.