Hello Beldinn! I hope your goat is doing well.
The Core Marksman Rifle requires 300 Engineering of any type. To my knowledge, none of the Core recipes require any particular specialization. They do, however, require high skill (usually max, 300), and the ability to consistently farm Molten Core for recipies and components to manufacture the items themselves. You will need a solid raiding force to accomplish this.
If you look at the link for the
Core Marksman Rifle, you'll see it lists the requirements (300 engineering) on the item pic, and to the right it even tells you what mobs in MC drop it and how often. If this recipe required an engineering specialization, it would also be listed on the item pic, as is the case with the new
World Enlarger (which requires 260 engineering and the Gnomish Engineering Specialization).