There are dozens of Pulse Rifle kits available and sections of the Replica Prop Forum dedicated to building them properly. I however, being a bit of a design masochist, am going to build it my own way. I'm not breaking any new ground, every conceivable permutation of Pulse Rifle has already been done, but this project is just for fun.
From my research, I know the original Pulse Rifles were made from World War 2 era Thompson submachine guns. If you look closely at the gun Ripley is holding in the screenshot, you can see the charging handle, ejection port, upper receiver, and barrel.
Or you can just trust me... Its in there.
I’m going to start my build from an Airsoft replica of a WWII M1A1 Thompson. Starting here will simplify my build by giving me something with known dimensions to work around. As a bonus, the final creation will fire Airsoft BB's. I'm also going to use this 3d model of a Pulse Rifle from Thingiverse as a guide. It’s the same 3d model Freeside used to print their rifle, and it will save me a lot of time working out the weapon’s proportions.