Pinning flash hider requirements?

[JFettig]

I have a 10.5" .300 whisper upper for my pistol lower. I'd like to use this upper on my rifle for deer hunting this year in rifle zone when I'm not using my .308.

That being said, What are the requirements for pinning?

I'll be hunting twice one week apart, first weekend will be rifle, second weekend will be pistol/shotgun in a different state.

I want to pin an extension to make it 16.1" for rifle hunting then for the next weekend I want to use it for pistol hunting.

What are the requirements? Can I cross pin it with a roll pin or do I have to weld the pin in?

Thanks,
Jon