I find 1/144 Gundam guns are often too large for 120mm (1/16) scale figures. Some may work for 250mm (1/12) figures but often times that makes the guns too thin for 1/12 scale. It’s a real “hit-or-miss” when it comes to using 1/144 Gundam guns for figure armament.
An option would be to try ETSY for specific Sci-Fi weapons that can be custom-scaled by the seller to suit your mech.
Three barreled weapon parts. I didn’t show the length as it has more parts than the howitzer including metal parts to assemble it. I will check the length another time.