"James rifles" is incredibly ambiguous unfortunately. That is the way it was then, and still is today. Hazlett, Olmstead, and Parks in "Field Artillery Weapons of the Civil War" probably do the most thorough job of trying to explain and organize the many pieces using James rifling systems. James really refers to the projectiles and rifling systems meant to fire them, and was therefore applied to the pieces. What distinguishes it primarily is that it was for bronze pieces rather than for iron rifles that would soon become standard.

There were two primary calibres used for field artillery: 3.67" and 3.8". The 3.67" was often referred to as "12-pounder James" while the 3.8" was often known as the "14-pounder James" although the 3.8 inch would often be lumped under 12 pounder James as well. The reason for calling 3.67" versions 12-pounders is simple and straightforward: they were an adaptation of 3.67" six pounder bronze smoothbores (and often rifling of existing pieces) and the practice was to refer to rifled versions of given calibre as twice the poundage of the corresponding smoothbore. (But that gets thrown out the window if you look at 2.9" and 3" rifles...confused yet?)

3.8" could either be originally made rifled to that caliber, or also be reaming and rifling of worn 3.67" smoothbores to 3.8". In the old smoothbore system 3.8" was an unusual caliber slightly less than the 7 pounder's 3.86" nominal. But that was still close enough to distinguish. Applying the traditional 2x to the smoothbore pounder designation provides the "14 pounder"

I don't know about Guibor's armament at the time. Hess states he had two 6 pounder smootbores and two 12 pounder howitzers at Pea Ridge. Perhaps when Van Dorn crossed the Mississippi they were re-equipped with James Rifles (perhaps captured from Shiloh?) The CSA also had a number of 3.3" rifles from their western foundries at this time.