No such thing as a license, it is an NFA item controlled by the BATFE. You can buy one ready made from a Class 3 dealer, he will do a Form 4 for transfer and submit to the BATFE NFA branch. You will pay the federal government a $200 Tax. Or, you can build one yourself, engrave a receiver with name, city, state, file a Form 1, pay the same $200 tax. On either form you will either need to get Law Enforcement signoff, usually County Sheriff, or put them in a trust. This is the short version.