See Hard Ball's point B. The platic which the bottle is made of will likely be different than the plastic on the gun, and have different properties, such as resistance to X,Y, or Z chemical.
Jarhead's idea isn't bad, but there may be long-term effects on the plastic that won't show up on a spot-test (eventual structural weakening, etc...) You may want to contact the manufacturers of the chemical and the pistol, and see if you can get an answer from them (although I hear HK isn't much for cutomer service(?))