You know the most restrictive possible application would apply, if this case is ruled in favor of the plaintiffs.
Actually, I think it works this way.
When you make a lawful purchase you are subject to the laws in the state in which the transaction is completed. If you purchase something in person, you can purchase what is legal in that state where you receive the goods.
By mail, you are subject to the laws of the address to where the item(s) are shipped. While sellers try to avoid shipping items to jurisdictions where they are illegal, purchasers are cautioned to check their State and Local laws so that they remain in compliance with those. Items illegal in the jurisdiction of origin are generally not offered for sale from that jurisdiction (IIRC why Magpul moved their AR magazine production out of Colorado).
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Under the case being pushed, and by their logic:
To use Colorado as an example, someone from Wyoming, staying in Colorado, could not go to a dispensary and purchase marijuana for use in Colorado, because marijuana is illegal in Wyoming.
Similarly, someone from Colorado, working in North Dakota, could not purchase a 30 round magazine for an AR-15 or even a magazine for a pistol that held more than 10 rounds, because it is illegal in their home State, even if they never remove the magazine from North Dakota (or bought it as a gift for someone in that State).
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Commerce doesn't work that way, but in either case, compliance with the laws of the jurisdiction a person is present in (including compliance with the laws 'back home' if they go home to different laws) is on the person, not the seller as long as the purchase is legal in that jurisdiction where the transaction takes place.
The pot buyer would not be able to legally take the weed legally purchased in Colorado across state lines into Wyoming, the magazine purchaser would not be able to legally take the magazine back to Colorado with them. But they would be able to legally possess them where the purchases were made.
Similarly, if you purchase something in a jurisdiction, in person, lawfully, you are subject to the rules and regulations of that jurisdiction: you pay their sales tax (if any), you comply with the local laws.
At that point it doesn't matter what the laws are where you came from, only where you are.
So, no, you wouldn't be able to purchase those 30 round mags, unless you had them shipped to Kentucky, where they are legal. In Colorado, they'd be a violation of the law. So despite retaining your KY residency, without a police or military exemption (and I am not sure how that might work for you, being active duty), you could not expect that a merchant in Colorado would be able to sell you a magazine they might not even be legally able to possess.
Needless to say, this presents problems for people who own rifles or handguns which have magazine options that exceed the legal capacity limitations of jurisdictions they travel in. While I may pass through several states which have reciprocity for my concealed weapons permit, Colorado included, it is on me to not take magazines into Colorado which exceed the 10 round limit, whether they are for my carry weapon or a rifle. If i need to replace a magazine, I'm stuck with the legal capacity limitations of the State in which I make the purchase.
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As for the case in question, the judge should dismiss.
It is sad that the firearm in question was misused, but the magazines above 15 rounds capacity are standard for the firearm in the jurisdiction in which the sale was made.
Because magazines are detachable and can be sold separately, they are often classified as "accessories", and not an integral part of the firearm, despite being important to the optimal use of the firearm. Most firearms can be fired without the magazine in place (some pistols excepted--some have magazine safeties which prevent firing without the magazine in place), although loading single rounds into the firearm and firing them is a relatively cumbersome and time consuming process.
Whether the magazines had 15 rounds or 30 rounds, or more is generally moot, because the purchase was made legally in the jurisdiction.
The seller had no way to ascertain that the firearm or the magazines would be used illegally. The purchaser passed the background check.
Reducing the capacity of the magazines would only require the person using the firearm (for whatever purpose) to change magazines more frequently, something which can take as little as a couple of seconds. All they need do in order to fire a given number of rounds is reload.
To hold the seller liable would require the seller to do something not considered humanly possible: to predict the future use of that firearm and the future actions of all who handled it.
As I said, it is sad that the firearm was misused, in violation of the law. It is sad that these people lost their loved ones. But once we start down this slippery slope, will car dealers be held liable for those killed in accidents by the cars they sold? Will those who sell hand tools not generally used for administering lethal blunt force trauma be held liable in those instances where those tools are thus used?
Such logic could spell the end of the sale of anything which could be misused, from kitchen knives to claw hammers to semi Trucks.
