Thanks for the additional info.
5.56mm
You’re welcome.
Pharmaceutical chemists look at drugs (molecules) and biologics (made from living matter) as small molecules, small because they are designed to interact with molecules like proteins, hormones, enzymes which are large molecules about 100 X bigger.
Small molecules are called antibiotics when they are designed to deactivate or neutralize a bacterial element.
If a small molecule is designed to interact with an arthritic condition or inflammation, they might be called an anti-inflammatory agent. So a small molecule is named by what it’s designed for.
The key and lock analogy is a good one and is used in the field. Some molecules are designed for dual uses, like a key that fits more than one lock. Conversely, some locks can be opened up by more than one key.