Ho Boy! I own 1 Mosin and 8 K31s, so you know where my loyalties lie. Nothing, absolutely nothing, beats the K31 on the milsurp market in terms of value, accuracy, and workmanship. The mosin may be cheaper to shoot (with junky surplus ammo), but it is engineered and manufactured by drunk monkeys compared to the engineering and craftsmanship of the K31. If the K31 was manufactured today, it would sell for upwards of $1000. Simply looking at the bolt design and metal work of the two is enough to convince me to never buy another Mosin as long as there are Swiss rifles for sale.
The ammo may be harder to find, but it's not HARD to find, and the surplus ammo available is MATCH ammo because that's all the swiss shoot! Currently it's running about $7/20
at AIM which might look a bit steep compared to milsurp 7.62x54, but it's brass cased match ammo capable of 1 MOA, and it's cheap by those standards. The K31 barrels are all pristine because the swiss ONLY shot this non corrosive match ammo in them
The Mosin is a beast to scope, requiring drilling and tapping the receiver and replacing the bolt handle, or using a scout mount that replaces the rear sight, but to what end? The rifle is still a crude bolt gun and the ammo is usually machine gun ammo, neither of which make for an accurate combination. The Finn Mosins are accurate with expensive ammo, but pricey and I doubt you'd want to butcher a Finn to scope it.
The K31 on the other hand is easily scoped with a StMarie clamp on mount. This mount is rock solid and does not require modification to the gun. There are also scout mounts available if you like that style, but a high powered scope really brings out the accuracy of the K31.
The bottom line is that if you buy a Mosin, you are almost at the top of the Mosin's game. The rifle can not do much better than it already does. If you buy a K31, you are well ahead of the Mosin's top performance already, and you have lots of potential ahead of you if you want to improve. Guys routinely shoot K31s at highpower matches and win. Ever see any Mosin's? You can reload for the K31 and wring out more accuracy than you get with the surplus match ammo. you can get diopter sets that attach to the StMarie mount to really fine tune your open sight shooting. And if you want, there is a left hand adapter that allows you to operate the bolt from the south side, but if not, the spent cases eject straight up, so there's no worry about brass in the face. The K31 bolt also operates straight back and forth, no turning, so it is exceptionally well suited for lefties as is. I just tried operating mine lefty style and can cycle it just as fast as I can right handed.
A mosin is just a here and now rifle, short sighted in my opinion. Cheap, yes. But boring and unfulfilling after a short while. The K31 will grow with you over time. It is a purchase with a vision.
Both are about the same price for the basic gun.
If you want to do some more research, go over to
swissrifles.com and check out their "library" and discussion forums (the Straight-Pull forum)
Now if you like, I have an excellent K31 for sale in the EE... I have too many and my CC is screaming from Christmas. But the above sentiments are true, regardless if you buy my rifle or not.
If you don't buy a K31 from me, please buy a K31 somewhere.