With all due respect, there is quite a bit of misinformation here...
First, about silicate inclusions being beneficial for the performance of skelp welded barrels. The answer is no. They may contribute to slightly better corrosion resistance, but that point is minor. They do not enhance the mechanical properties of a skelp welded barrel. With all this said, wrought iron of sufficient quality has been thoroughly proven satisfactory for the application. You must understand a couple of points. First, silicate inclusions offer little mechanical strength in comparison to the iron matrix. Since they are long and stretched out, they in some ways mimic a crack. The second point to consider is how the load is applied to a gun barrel. A barrel with wrought iron grain, or inclusions, running longitudinally is actually the worst oriention relative to the load a barrel experiences. The stress is such that it is trying to make the barrel bigger, or I beleive what is refered to as a hoop stress. So the mechanical properties of wrought iron are quite different in different orientions (anisotropy). This is why one can start a cut and then tear wrough iron along the grain wih a hammer. Again, if the quantity and size of inclusions is small enough wrought iron will work given a barrel of suficient size.
About the idea of a crystaline structure being prone to cracking. This is not necessarily the case. There are many structures with very low inclusion content that are very ductile. As was mentined, inclusions are generally viewed as a negative attribute in terms of mechanical performance. Cracks can run along crystaline boundaries, but don't have to. This is the idea of ductile versus brittle fracture. An example is low carbon ferritic steel or iron. This will typically break in a very ductile manner. Now, to be clear, the inclusions in wrought iron MAY (though I'm not sure about this) slow down crack growth that is perpendicular to their longitudinal axis, but this is not the direction one would encounter a crack in a skelp welded barrel. A The primary reason flake iron or gray cast iron is brittle is the lack of mechanical strength of all the free graphite and it's pointy thin nature (much like a crack). The example shown is particuarly bad in that it has a lot of small flakes refered to as spiky graphite as well. Not a desirable attribute.
There may be more I've missed, but this is what comes to mind based on what has been said. This sort of stuff was once a big part of my life, but hasn't been for a while. Guns are more fun!