There used to be a 20mm anti-tank rifle. 50cal sniper rifles are routinely used. It really depends on how you want to use them and how mobile you need to be.
And what you are doing with them,
eg do you want the accuracy high ?
If you are not going to put your eye down to site along the barrell, and ARe going to have a nasty recoil banging on your shoulder, you are going to be giving up on fine accuracy…
That, I think, is the 9mm version of the TAR-21 Tavor. I’ve handled one, and it weighs about the same as an M-16A2 (although the weight is better distributed).
None of the guns in the your links are particularly large, or fire particularly high caliber ammo.
Of course there have been guns that fire gigantic rounds. And these are generally impractical/ridiculous to use as a standard soldier’s weapon. They are support weapons, like the anti-tank rifles mentioned above, or sniper rifles.
Or they’re just made as apparent practical jokes, like this .577 caliber rifle. You wouldn’t want this even if you were hunting elephants, unless it was on a bipod mount.
Those are all totally ordinary guns that any reasonably healthy person, including children, could shoot. They’re not as large as they look and most of the bulky black material is plastic.
Something like the High Impulse Weapon System might be up the OP’s alley. Video of it being demostrated. Blogposts with a bit more data. It was basically a hand held 76 mm mortar that wouldn’t take your shoulder off. Per the links, it could throw a 3.5 lb shell around 600 m. Never made it into production, AFAIK.
Alternately, you could be like this guy, firing a 60mm mortar from the hip…
As to the ‘big guns’ being exhibited at SHOT, muzzle brakes make life a lot easier for the shooter (and much more unpleasant for anyone around the shooter. Wear your earpro.)
Maybe you could explain what you meant by “guns like this”, as all your links apparently go to guns that are A) possible and B) have never been considered impossible.
