This was the first box for the mould
This was the result, worse than finecast.
This time I built the mould use the proper technique, covering a half of the arm in clay and pouring latex on top of the clay half. Then, I remove the clay and poured the other half of the mould. I followed this tutorial from War Frog's Hobby Blog where he does a much cleaner and more precise job, but nonetheless here are my results from the second try.
Here you can see that the mould wasn't quite right resulting in some barrels that are malformed.
The new resin arms covered in flash.
The final product is satisfactory but by no means perfect. They required a metric shit ton of sanding and greenstuff filling. All but one needed to have the muzzles rebuilt using plasticard tubing as well.
I definitely want to make another mould and try again having learned from my recent mistakes. I think the key is how the item is placed in the mould and making sure the vents are large enough.