JP doesn't really require any proprietary parts. Even the side charger PSC line will still operate as normal using a standard carrier. You just lose the advantage of the side charging mechanism if you decide to go that route.
Depending on what you specifically think would add value to your current collection, there are a number of options that would make sense. If it's light recoil, and you are comparing that recoil impulse with a .223 I would definitely not suggest a 6.5G. Maybe something like a .204 Ruger or 5.45. If something that can be used in more of a general small to medium game hunting rig, long(er) range marksmanship type of training tool with advantageous external ballistic properties, in a lighter weight platform than .308, then the 6.5 Grendel is probably what you want. If it's a short range truck gun/home defense round - and considering you already have a 300 BLK - then I'd take a look at a .458 SOCOM. Some of those, like the 458 for example, would really only make sense of you reload and recoil isn'ta huge detriment in the context of your uses.
The only thing I would strongly disagree with that has been mentioned thus far is investing in an AR platform rifle chambered in .22lr. I'm a huge proponent of training with what you shoot in any normal, everyday scenario. I've got a father in law that insists on saving $70 a month and shoots .22lr 75% of the time he trains and he absolutely sucks massive dick when he shoots anything else. The recoil impulse, trajectory differences etc. make it seem like its the first time he's ever shot the 5.56 he's had for 8 years. If you want to keep similar ergonomics and feel its beneficial to shoot a lower power load with your wife and kids, then I'd just load some subsonic .223 and practice the fundamentals of manipulating a rifle with that and progressively move up. A 22lr won't even give you a common set of parts that make up a gun. Very few benefits - if any - of putting money into something like that if you ask me.
My kids learn it all before they earn the privilege to merely attempt to put what they've earned into real life shooting. If they want to use a suppressor then they disassemble it, clean it, and put it back together. They clean their own brass, they detail strip their rigs and everything must function when it's put back together. If it isn't, then they get another opportunity at some other point in time after they earn it again. It facilitates an extremely detailed understanding of what they are handling and highlights the significance of the privelage they've earned. Anyway, a bit of a tangent there, but I wouldn't have my 6 or 8 yr old shoot anything less than a .223 just to ensure they are acclimated to a minimum level of recoil that will, in the least, always be present regardless.
This one is the motivated one that naturally gets pissed when someone says she can't do something. "Disassemble, clean and re-assemble 5 cans, 3 rifles and a pistol? Gee Dad, I thought this shit was gonna be a challenge." 3 hours later.....the 6 year old churns out perfection.
