Pay special attention to the condition of the muzzle. Mosin Nagants are my special poison, and I've ran into them with nearly pristine bores and all matching (not force-matched) components, yet their accuracy was garbage due to a munged crown.
A lot of Mosins have had their muzzle's counterbored. Some collectors dislike this, but if you're going to own a shooter, having a counterbored rifle isn't a bad thing.
As available excellent quality Mosin Nagants dwindle, having one that's CB'd but with matching parts, furniture in near-perfect shape (even if an rearsenal job), and a shiny & sharp bore won't subtract as much from it's collector value.
Also with MNs, many have been rearsenaled (refurbished) as part of Soviet make-work programs. These rifles often have components which are force-matched, with the original serial-#'s lined-through & matching #'s electro-pencilled or stamped on. Most often these force-matched bits are buttplates and magazine/trigger assemblies, but I have seen some with force-matched bolts. While that may be an issue with Mausers or Lee-Enfields, Mosin Nagants seem to be very tolerant of miss-matched bolts. I've not yet heard of a single MN with a headspacing so bad from a non-matching bolt, that it caused the rifle to be unsafe.
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7.62x54R.net for all the information you could ever hope for on Mosin Nagants.
I like them for collecting because there are just so many variations to look for, and some are very attractive as collection centerpieces (the tiger-stripe stocked MNs are one). M91/30s can be had as Kurts Armory found his, and they make fun & accurate shooters. Finnish Mosin Nagants are expecially desirable, and are known as being the most accurate of all MNs. I like my M91/30, which was also in un-issued condition, but my M38 carbine is what I grab for most often when heading out camping or hunting boar.