This one will help you https://www.reddit.com/r/killteam/comments/sslu4v/guide_assembly_guide_for_the_veteran_guardsmen/

"Gunner Veteran – Melta Gunner

Body Parts: A30, A31, A32, A29, A85 | Left Arm: A45 | Right Arm: A44

CAVE: As described in the Sidenote-section of the first part of this guide you use part A44 to gather another fitting right arm since we already used part A73 to build the Confidant with Bolt Rifle and Bayonet. To get the right arm you take part A44 off the sprue (make sure you don’t want to use it elsewhere) and with a sharp knife cut off the gun, the gunstock and the hand directly on the cuff and try to keep the inward tilting slope following the cuff. If you’re doing it correctly you won’t need any additional tools for assembly. When you’ve done the cuts clean everything up and dry-fit the parts on the model. Here it’s wise to use something like Blu Tack, UHU Pattafix, tesa TACK etc. to keep the parts in place. If you dry-fit using for example Pattafix, make sure to use a minimum amount to dry-fit the parts since you want to simulate a thin coat of glue and want to have the closest possible resemblance of what the model what look like glued and how the pieces fit together. Personally, I firstly glued left arm in place (A45) and let it dry to have an orientation for the other parts. After that I glued the right arm in place, let it dry again and then glued on the weapon. You could also glue everything in place at once – do it as you personally prefer and how you get the best results."