Grease guns are used to insert grease into:

Grease guns are specifically designed for delivering grease to lubricate various mechanical components. The primary use of grease guns is to apply grease to Zerk fittings, which are standardized grease fittings used on machinery to facilitate the easy and efficient lubrication of bearings and other moving parts. These fittings allow for the controlled entry of grease, ensuring that it is adequately distributed to reduce friction and wear in mechanical systems.

The design of a grease gun enables a mechanic or technician to inject grease directly into these fittings under pressure, which ensures that the lubricant reaches the necessary areas where it is needed most. This is crucial for maintaining equipment longevity and operational efficiency.

While it's true that grease is also used in bearings, the question specifically focuses on the application method. Zerk fittings are the most direct and common application point for grease guns, making this answer particularly relevant. Other choices, such as hydraulic lines or air filters, do not apply to the function of grease guns, as these systems require different types of maintenance and lubricants.