Boring and Trepanning Association Drilling (BTA Drilling) is a method of boring holes for diameters normally greater the 19 mm. High-pressure coolant is forced down the gap between the drilling tube and inside surface of the hole / bore.
This coolant is forced around the drilling head and cutting face (cooling it as it goes) and picks up and removes the chips / swarf. The coolant then escapes down the centre of the drill tube to exit at the back of the machine.
This method allows for a perfectly smooth bore as the swarf and coolant escape down the centre of the tube and do not require a groove in the bore wall in order to escape, as is required in gun drilling.
BTA and STS are essentially the same technology. BTA Drilling is also known as Single Tube System Drilling (STS Drilling). All Deep Hole Drilling machines manufactured by PTG Heavy Industries are STS systems using BTA or STS tooling.
There is a second system that uses two tubes where the swarf and coolant escape down the gap between the inner and outer tube. This is known as the Ejector Drilling and does not require a pressure head, but this is normally used as a system for converting a conventional lathe (for example) to be able to perform deep hole boring operations.