Any idea if any of the other White models are legal for Washington State hunting? From what I can see, the Thunderbolt only uses 209 primers, so would not be legal. I see where they have some used/referbed rifles on whitemuzzleloading.com and sent an email about current inventory. Am anxious to hear back.
Basically the original G-series such as the Whitetail and Bison are #11 cap ignition and open breech so they would be legal. They would and should also be great shooters as well. The difference between the original G-series and there Model 97 & 98 was the addition of the second locking lug bolt through the stock.
Another more modern style would be the Super 91-I and Super 91-II series. I believe from my reading that both of them are also open breech design and can be used with a #11 or musket cap like the Ultra Mag. There might be an Ultra Mag left out there some where. Good luck trying to get a present owner to part with theirs. Once they shoot them, they hold on to them.
I've read that while the Thunderbolt is accurate, talking to White owners, few of them recommend that model because of the bolt. They always point people at the Super 91 model or the Ultra Mag.
As long as I am asking questions, does the barrel length make any practical difference? Is longer naturally more accurate and/or produce higher muzzle velocity?
When I see this kind of question I think of my Wolverine LK-II with a 22" barrel. As far as accuracy, this short barrel rifle would be hard to beat. White G- series owners claim some remarkable accuracy from their 22" models and with good reason. Excellent quality barrels and well made rifles. Many of the G-series owners have the second locking lug put on the rifle. I think Doc White can make this adjustment for you and tune the rifle as well. Most of Doc's rifles come custom bedded and tuned. They are also guaranteed shooters or else you send them back.
Granted the longer barrel will give you a longer site plain for the open sights, but more accurate? I'd hate to live on the difference.The longer barrel will of course give you a minor increase in muzzle velocity but not really enough to worry or concern your self with. Face it, you are throwing a moderate powder charge with a large conical or a fast moving sabot load.
Something you and your son might want to consider is Doc sells kits in the Whitetail G series and Bison models. They are very reasonable for the most part. You could work together on your own rifles and build and customize your rifles together as a project. It would save you money and give you something to do together. Doc might even make you a package deal. He's a real easy person to talk to in e-mails.