If your not comfortable or confused with using alternative software’s I would try starting small and see what type of gun your trying to create inside (Blender or Roblox studio).
For creating complex models this can be done in a software rather then studio, start small and slow learn the software first before attempting to create models basically learn the curve and ropes around blender, see which software your more comfortable in using.
If your just starting out in using 3D softwares I would watch a few beginner tutorials to get the hand and curve around blender, you may want to watch series to get an overall understanding of the (interface, tools, workflow and more). Different channels go into more details about blender it would be good to take notes, so you could use that later down the line.
Every model you create isn’t gonna turn out how you’re hoping it too, since you’re new to creating models it’s good to try to improve it and see what the end result brings you. Here is a related question: