Complex sentence: One of the accomplishments of the system was solid cast cannons bored to precise tolerances which allowed the walls to be thinner than cannons poured with hollow cores, but because cores were often off center the wall thickness determined the size of the bore.
Split into simpler sentences: One of the accomplishments of the system was solid cast cannons were bored to precise tolerances, which allowed the walls to be thinner than cannons poured with hollow cores. However, because cores were often off center, the wall thickness determined the size of the bore.