just spent a little bit farming the one sword/shield knight next to the Lothric Castle bonfire. Easy farm: free backstab with slumbering dragoncrest ring, about 3 kills per minute.
122 kills for 20 chunks at 357 ID for 16.39% drop rate (counted 102 large shards, he always dropped either a large shard or a chunk)
divide by 3.57 and get 4.59%, so around 4-5% base drop rate?
I know that's hardly an exhaustive sample size, but just to give a general idea.
Also, if anyone cares: 9 swords, 17 knight's crossbows, 12 shields, 14 helmets, 18 armors, 13 gloves, 11 legs, and I think 8-9 sunlight medals (forgot to check my count on the sunlight medals before hand)