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Reanimate the animation ?

Can i example a running animation and add a gun and repose it the arm like running with that weapon without use 3part softwares to do that or i have to use animation software

Yes, as long you have such an animation, and there are actually free animation in this regard found under the market free panel. Free - UE Marketplace The way you do this in Unreal Engine likely explained in these tutorials https://www.youtube.com/results?search_query=unreal+engine+tutorial+character+weapon