Bordering states provisions went away in 1986, federally. Some states still have them on their books, which might limit some transactions.
If federal law is more restrictive, federal law overrides; but if state law is more restrictive, state law overrides.
Colorado has no LIMIT to bordering states, so kinda depends on the purchaser's state, not Colorado law, and this only applies to federally licensed dealers.
Non-licensed sellers may only sell to residents of their own state.
Non-licensed buyers may only buy from residents of their own state, or rifles or shotguns from licensed dealers in any state, provided both state's laws allow it.
"The only real difference between the men and the boys, is the number and size, and cost of their toys."
NRA Life, GOA Life, SAF Life, CSSA Life, NRA Certified Instructor Circuits' Feedback
Circuits' answer is the most correct.
Sayonara
What of handguns?
9mm - because they don't make a 9.1mm