Welcome to T22 Popcop45, I believe the slot in the sides of some .22lr magazines are necessary to accommodate thumb studs that make loading the magazine much faster. A feature important to many shooters interested in high capacity, fast discharge firearms. On the shooting bench you'll find the magazines with thumb studs, and therefore slots, the most popular style.
I own quite a few of these type pistols and one of my favorites is the CZ Kadet. It's every bit it's centerfire cousin. Matter of fact, they share a common frame. If you like the SIG you might look into their .22 Classics. These are not Mosquitos but rather German centerfire frames with .22 conversions factory installed. As a matter of fact you can convert them back to centerfire. They offer the .22 Classic in the P220, P226 and P229. Another of my favorites is the Kimber Rimfire Target 1911.
I ran those specs by Vic, our resident tournament fisherman, and he says, and I quote, "that's good s**t". He says you have good taste in fishing gear.
I've seen a few of the Raptors and they are all that and a bag of chips. This SUV version would be the ultimate grocery getter, with or without the armor.
You've hit the nail on the head. "Big F**king Revolver" was the project's pet name during development. Name stuck so they invented an alternate meaning.
I've been legally carrying concealed since 1988 and have toted just about every combat handgun configuration available. First, I don't like a concealed handgun that requires holster carry, such as a 1911, even my little EMP, because the loaded chamber requires the manual safety which in turn requires a secure holster. Over the years I've migrated toward light weight DA trigger handguns such as your 642. My current favorite is my Kahr PM9 which drops easily into any pocket and I find easy to live with, translation: I'll always have it with me. I can't remember how many times I've left a big honking .45 in the car due to the PITA factor. I feel comfortable with my 7 shots of 9mm+P always at hand. At work, different story, I carry big.