01 - Obtain a Mosin in pristine condition.
02 - Chamber 7.62x54R rifle rounds in the Mosin.
03 - Shoot between 20 and 40 rounds.
04 - Note the rifle's condition - in my experience, it is almost guaranteed to either be ruined or badly damaged.

• NOTE ***

This behavior is also observable on an FN FAL, while single chambering rounds.

01 - Obtain an FN FAL in pristine condition.
02 - Chamber a single 7.62x51mm rifle round in the weapon - with no magazine attached.
03 - Shoot between 20 and 40 rounds using this loading mechanism.
04 - Note the rifle's condition - in my experience, it is almost guaranteed to either be ruined or badly damaged.

Inherent to all weapons, chambering single rounds causes a pronounced effect on weapon condition degradation.
For example, if you do not have an FN FAL magazine in your possession, chambering single .308 (7.62x51mm) rounds will quickly wear the weapon out.

Of curious note, I nearly destroyed my first FN FAL (got to 'Badly Damaged') in a surprisingly low number of relative shots fired.
I was target practicing with the new rifle shooting zombies at the Myshkino camp and my friend asked me to check my rifle's condition out of curiosity.
To my dismay, the rifle was 'Badly Damaged' after chambering approximately 20-40 rounds.

We restored the weapon's condition to worn using a weapon cleaning kit and agreed not to fire it again (as it is a rarer spawn) until we located a magazine for it.
After locating a 20-round FN FAL magazine, we were able to fire between one-hundred to two-hundred .308 rifle rounds before the weapon's condition deteriorated below worn.

It is my belief, based on this behavior and that of the behavior mentioned in the description, that the rifle wear condition is tied to the physical act of reloading of the rifle.
Whether loading one round (sans magazine), or loading entire magazines, the act of reloading itself seems to degrade the weapon.

I believe in the case of the Mosin, a single instance of manually chambering 5 rounds to fill the integral magazine is acting as 5 separate reloads, accelerating the rifle's decay.
This is obviously not an issue for other weapons with internal magazines, such as the Red-9, Trumpet, and/or the SKS,

It is my belief (again, this is conjecture) that the Red 9, Trumpet, SKS, and other "integral magazine" weapons have a flag or some setting that disregards the individual round load count, causing these weapons to deteriorate at a "natural" rate. This same "exemption" - if it exists - could/should be applied to the Mosin.