Blender is a great modelling tool, and it's open source and free. There are tons of tutorials for it out there.

https://www.youtube.com/watch?v=TPrnSACiTJ4 is a famous one that you follow along with while it teaches you. But it covers a ton of things you don't actually need to know, because luckily for us, if we're just modelling something to 3d print, there are tons of things we don't have to worry about (like texturing/UVs/rendering/etc). So we'd only have to learn part 1 of this 4 part series.

In the end I watched this other guy's tutorials, which were more focused on modelling shapes vertex-by-vertex. I think I only followed along with a couple of his episodes before I felt I was being productive: https://www.youtube.com/watch?v=Hf2esGA7vCc

As a bonus, the Pulse Rifle isn't actually that complex (compared to a human, for instance). With some good reference images as the background behind your modelling, it's a bit like tracing.

Spend a couple of hours on it, and see how you go. I wasn't expecting to enjoy it when I started, but in the end I loved it.