This one is quite simple to do and can use either the Cadian or the Catachan plastic lasgun.
Starting by cleaning down the weapons, I have marked out on the picture below the areas that we will need to remove.
On the catachan pattern lasgun (top), we need to remove most of the barrel and the stock. Then simply scrape or file off the imperial symbol. On the Cadian pattern lasgun (bottom) we simply scrape off the imperial symbol and remove the weapon casing. As you can see the stock has already been removed in this picture, and it also needs to be removed.
You will need to keep a hold of the muzzle at the end of the weapon, so be careful when removing this from the end of the barrel.
You should end up with something like this...
Once we have this cleaned up we can move onto further cutting down the weapon. Next we have to remove the middle section as indicated in the picture below, cutting along the red lines and removing the highlighted area.
We are then left with something like this...
At this point I simply glued the weapon together and replaced the standard lasgun clip with a boltgun clip. This isn't necessary, however I think it adds a bit of punch to the weapons looks.
Next up we need to add a little bit of detail to the stock plate. On the Kasr-kin models, this is done with two small disks, so that is what we will replicate here.
We will use a length of rod roughly 1.5mm in circumference and a length of tube about double that.
Cut a small disk from each, roughly 1.5mm each.
And then glue them on the end. First the small one...
At this point the weapon just needs some trimming and filing before adding a little green stuff into the gaps.
...And once that's complete you should be ready for painting.