In verbs ending with consonant + vowel + consonant, you should double the final consonant before adding -ing.
Examples:
- run - Hurry up! We're running out of time.
- hop - She's hopping on one leg.
- beg - I'm begging you to come home.
- swim - Let's go swimming tomorrow.
- ship - The marines are shipping out next week.
Note also the doubling in the verb put, even though the vowel is not short:
- put - Isn't that sweet? He's putting his arm around his mother.