Here are a few good pictures of characters from the games holding the rifle in question:
https://vignette.wikia.nocookie.net/umineko/images/c/c9/Ougon-kyrie.png/revision/latest?cb=20120122175431
These examples give us something to compare against visually in the future.
Incidentally, I’m sure you’ve already seen this but… just in case you haven’t… here you go: http://www.imfdb.org/wiki/Umineko:_When_They_Cry
According to IMFDB, the guns used in the games are allegedly Winchester 1894’s, chambered in .30, which have been modified to look more like the Mare’s Leg (in their words: by cutting down their barrels and re-fitting them with enlarged lever loops.) That claim, however, sounds completely wrong to me. At first glance, you would think it would be much easier to convert a Mare’s Leg to shoot .30 than to modify an 1894 so extensively as to look like a Mare’s Leg. Besides that the lever loop and the barrel-size of the “Rokkenjima Rifle” are obviously different than that of the 1894, the gun’s stock, the side of the gun, the shape of the wooden foregrip, and the front of the gun are also distinctly different from those of the 1894.
Actually, there’s really no other way to say this: The “Rokkenjima Rifle” would be a near-exact duplicate of Steve McQueen’s Mare’s Leg, if not for the fact that it has a blued-steel finish, as opposed to a brass finish.
As you can clearly see by these pictures, merely cutting down the barrel of an 1894 (pictured here: http://images.gunsinternational.com/listings_sub/acc_13414/gi_100828375/ANTIQUE-WINCHESTER-1894-RIFLE_100828375_13414_6E7B579FF85E3872.JPG) and refitting it with an enlarged level loop would simply not do the trick. Anyone with eyes can tell that the 1894 is nothing like the “Rokkenjima Rifle,” which resembles the Mare’s Leg much more closely. The 1894 and the Mare’s Leg are simply two completely different firearms.
Regarding the sound of the gunshot: .22LR is just about as quiet of a caliber as you can get. Silenced, firing a .22LR pistol is usually not much louder than clapping your hands loudly. Done correctly, it sounds sort of like a loud puff of air, and certainly does resemble the kind of sound which is described in “Our Confession.” Incidentally, you can buy a Mare’s Leg chambered in .22LR, but I seriously doubt that .22LR was the caliber which THE CULPRIT employed. My reason for saying this, besides that this theory seems to directly contradict what is said in-game, is because .22LR is notorious for its lack of firepower. This ““feature”” occasionally causes bullets to become deflected off of bone and remain within the body. Therefore, .22LR isn’t always guaranteed to create an exit wound, depending on the distance and angle from which it is fired. While this increases the caliber’s overall killing potential, believe it or not, and could even explain a few of Rokkenjima’s mysteries… I think it is much more likely that that the sound of the gunshot was merely reduced due to environmental factors. The island contains a lot of surrounding forest to dampen the sound and mitigate echo any time the gun is fired outside (not to mention, the sounds of fierce wind and downpour which could also help smother out a gunshot.) And if the gun is being fired inside… well, there you go. It was fired inside, and usually inside an adjacent building at that.
PS: Sayo is not the culprit, but that’s a completely separate issue which deserves a thread of its own.