Let me start this thread by confessing that I am a 'Mosin Purist'. - I do not modify my Mosins much further than a little trigger and bedding work. That said, I have been a long time G&G member that has recognized several ingenious modifications that are worthy of mention.
ALL of these are easy to remove and the rifle can easily be brought back to the original configuration. 
This also means that the mods can esily be transferred to another rifle.
In other words, IF I, the 'Purist' were to sporterize or improve my Mosin, this is where I would start pretty much in this order.
#1 Front post extension.
http://www.crsurplusspring.com/frontpostext.html
Many Mosins are plagued with sights that make the bullet impact too high at short range. (I really don't know why the Soviet quality control was so slack in this important area)
Sure, you MIGHT find something around the house or the hardware store that will do the same thing, but Clyde has the perfect answer for a few bucks.
#2 Trigger slack spring
http://www.crsurplusspring.com/SlackSpring.html
Most Mosins have triggers that flop around- very annoying and potentially dangerous in a hunting rifle with a light trigger. Clyde improved on the Finn M28 'mousetrap' trigger spring design to bring us this very worthwhile mod. The slack spring not only keeps the trigger from flopping back and forth, but transforms the trigger to a classic 2-stage. Highly recommended mod.
#3 Trigger improvements
http://www.crsurplusspring.com/FinnSear.html
Anyone who has pulled the trigger on a variety of Mosins can relate to some having an incredibly hard trigger pull. In my mind, a trigger in the 4-5# range is achievable and safe. This can be accomplished by dressing up the sear and cocking piece. The sear is the most critical and Clyde is currently offering modified sears from Finland. Sure, you CAN do the work yourself, but for this price, why bother. I just hope he can continue to offer these after his Finn connection runs out.
#4 Improved front sights
Josh Smith-Sights for the Mosin-Nagant
If I was hunting with a Mosin with the original iron sights, a high visibilty front sight would be invaluable. Josh has done a great job offering a variety of front posts to fill the need. In fact, I would challenge anyone who is contemplating scoping a Mosin to try one of Josh's sights first.
For the target shooter, a narrower front post allows for finer sight alignment. Josh has that too.
#5 No drill and tap Scope Mounts
Jmeck Scope Mounts
Jmeck offers a variety of mounts with his ingeneous band mounting system. The Jmeck mounts do not require the side of the stock to be cut out, but may require a little relief to the stock on the sides of the action or a little bedding work on the bottom of the action- both would not detract too much from returning the rifle to the original configuration.
For those that MUST mount a scope on that pet Finn tackdriver, Jmeck is the only way to go.
#6 Bent bolt
The original straight bolt handle is awkward to operate and most scopes require that a bent handle be used. The classic sniper bolt handle is longer and easier to operate- even if you are using iron sights.
There are several people who offer bolt bending services.
Max Shepherd Jim the Bolt Man
Important-
If your rifle has a matching serial number, PLEASE buy a body that is already modified and keep your original. The bolt body is fairly easy to install if you have the tool that comes with most refurbished Mosins.
Since the Mosin headspace is dependent on the bolthead, you can swap the body without worry. This is contrary to almost all other bolt action rifles.
#7 Cheek pad
There are several people who offer the cheek pads.
Can somebody provide links to your favorites? Blackhawk Rifle Cheek Pad
If you have ever fired a Mosin sniper rifle, the first problem you have is getting a cheek weld to the stock. We sold our reproduction PU sniper just because of this issue. I wish that we had tried one of these first.
#8 Ring safety
There are several people who offer the ring safety.
Max Shepherd
The Mosin does indeed have a safety that works OK, but is less than ideal. The ring modification makes the safety work just like the Swiss rifles. Pull back with one finger and turn. If you hunt with your Mosin, the ring safety would be highly recommended. For the plinker or target shooter, I wouldn't bother.
#9 Firing pin springs
http://www.crsurplusspring.com/SSfiringpinspring.html
The original springs on Mosins are decades old and many are not as strong as they should be. When Mosins are stored, the bolts SHOULD be de-cocked to relieve the tension on the main (or firing pin) spring. This is seldom done, even in those crates of refurbed Mosins.
If your spring is weak and doesn't fire that old surplus ammo, first check the firing pin protrusion and second, replace the spring.
I see no reason to take a chance on an original spring when Clyde offers these new springs at a very reasonable price.
I hope this list helps those who are contemplating improving your Mosin.
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