Complex sentence: Initially the production factory at Uralmash attempted to reject the design since the larger gun breech meant that the entire hull would need to be modified, but they were required to do so nevertheless, and the modified design was put into production.
Split into simpler sentences: Initially the production factory at Uralmash attempted to reject the design since this meant that the entire hull design would need to be modified. Nevertheless, they were required to do so, and the modified design was put into production.