If the instructor was referring to Modern Rifle season, he is WRONG!! It is perfectly legal.

Muzzle Loader season is a different story. Even blackpowder revolvers are technically not legal.

I have hunted deer during Modern Rifle season with a 44 Mag for years with no issues.

Instructors that don't know what they talking about are a disservice to the people they "teach"

I would call the WDFW and let them know what this guy is "teaching"