The 
Nuprol High Performance Premium Degreaser/cleaner, also known as NUPROL DC, is a cleaning spray specifically designed for maintaining Airsoft replicas. It is formulated to strip away dirt, grime, and old lubricants without damaging internal rubber O-rings or seals, which is a common issue with other harsh degreasers. 
Key Features
Powerful Cleaning: The formula effectively removes dirt, grime, carbon build-up, and old lubricants from hard-to-reach areas.
O-Ring Safe: Its unique formulation is specifically designed not to degrade or harm rubber O-rings, which is crucial for maintaining gas-powered (GBB) and electric (AEG) Airsoft guns.
Pre-Maintenance Preparation: It is ideal for use post-game-day or before a full strip-down, preparing internal components for the controlled re-application of fresh oils and greases.
Application: It often comes with a folding applicator nozzle to help target specific, intricate parts of the replica. 
Usage & Best Practices
For best results, the manufacturer and airsoft community recommend using the degreaser as part of a two-step maintenance process:
Clean/Degrease: Apply the NUPROL DC to remove all contaminants and old residue.
Re-lubricate: After the replica is clean and dry, use a product like NUPROL Silicone Spray or other specific greases/oils from the NUPROL Maintenance Kit to lubricate all moving parts, thus ensuring optimal performance and reducing long-term wear.