Well, there are lots of ways to do it. Some more legal than others. If the wayward sons live in another state, and you've owned the guns longer than they've lived in the other state, I'll be real hard for anyone to prove much of anything. Well, besides what you both swear to.

Even worse, if you bought the guns over the table, then anything goes. Not legally of course. If the gun doesn't have paper, and you live in a free state, it may not be legal.
But a bunch of what the legal/illegal stuff is based on what the prosecutor can prove. An old family gun given between two free states again means anything goes. The common lie is "I gave that to him when he was 16". And now he's 50. No sir, he just stored it here while he lived in an apartment in a bad area of town or was moving every few years.
So if you plan on skirting the legal letter of the law, there are things you need to know. First and easiest, how old are the offspring or relatives. Then how old or when was the gun produced. Then when you want the lie to have taken place.

Just like pieces of a puzzle there often is a way to fit them together.
And I take issue with the advice that you need to go to the state of residence to transfer a long gun. FFLs can sell to or transfer to residents of other states . Sure, all the other things must be in place, like not being a felon and their home state must allow out of state purchases. Yes, there probably will be a "transfer fee" because the FFL needs funds to pay his rent. We make this a lot more difficult than it needs to be.
The cheapest and most legal way is to go to your (donors) FFL and have it sent through the mail (USPS) to the receiving FFL. Both need to be aware and happy with the arrangement before hand. The USPS is the only way to save any money. And it will probably cost 2 transfers along with the postal charges. Anytime you involve UPS or Fedex, the cost goes way up.
In all this you also need to know your state laws. Both states.