The Sharpshooter 2 is an older gun with (for those unfamiliar with the gun) the somewhat unique characteristic of having two barrels, one of which can be selected for firing at any one time. This is the only unique aspect about it; its spring is average at best and stock ranges are not awe-inspiring. I realize of course, that there are other modifications out there for this gun, including a very similar mod using CPVC instead of markers, but I could find no other Crayola mods. Nonetheless, my friend had one in his closet and said that I could modify it, since it shot mega darts, and he had lost all of his megas (and indeed all his darts except two sorry-looking Airtech darts). After modification, it is more or less indistinguishable externally from a stock Sharpshooter 2 (save for the head-on view, but we'll get to that), but can shoot all stock micros and achieves much better ranges than the mega-shooting incarnation ever did.
Here is how the final product looks.

On to the how.
First, open the gun. Note as a point of interest that the screws are bigger and less numerous than the screws Nerf currently uses. The internals are mostly straightforwards. Plunger, tube, and barrels. The only thing of note here is the green barrel selector mechanism.
An after shot

Note: the top barrel has e-tape around it because there is an air-restrictor hole under it. This is unnecessary because you are putting a new barrel in it. Do it if it makes you feel better.
Take the barrels and tube off the green mechanism and use electrical tape or equivalent to improve the seal the mechanism makes with the barrels and tube. Some air will still be lost in the mechanism, but this cannot be helped. I also chose to wrap some more tape along the plunger head. As this didn't seem to help, I would say this step can be omitted at your discretion.

Here is a picture of tape around the junctions of the plunger tube with the green mechanism and the green mechanism with the barrels.

Make sure that putting the pieces back together does not displace the tape as happened in this picture.
Next take your marker barrels and cut them to length. Wrap as much e-tape around them as you can while still having them fit and cram them in the existing barrels. Here is one of mine out of the existing barrel.

And that's it. Close it up real good.
Ranges: Ranges were a little tricky to measure so don't take what I write here as gospel. The top barrel fit more darts better and is more accurate due to being straighter;

but the bottom barrel seems to have a tiny bit more power. Best results are achieved with stock taggers.
Taggers: Top barrel: about 40 feet. I can only measure with true precision up to the 30-foot line, but the dart had a fair bit of oomph left at that point (about waist level but with excellent velocity.)
Bottom barrel: about 35-40 feet. Ever-so-slightly less powerful than the top.
Sonics: As high as 50 feet or as low as 30. This is too inconsistent for me to count Sonics as the best ammo type for the gun, and accuracy is rather poor to boot.
Micros: Bottom barrel: Barely hitting 25. Most micros are too loose.
Top barrel: about 30. They fit the top barrel better.
I refuse to dignify streamlines as an ammo type by including the results. No accuracy. That's all I will say.
I am quite satisfied by these results. The resultant pistol retains the usefulness of the two functioning barrels and gets better ranges than my admittedly poorly modded NightFinder (which has the stock barrel, probably the reason for the suck) and shoots about as far as my stock Hornet. My friend can also undo all that I have done, if he choses, and this is good because I would feel bad if I did anything irreversible to someone else's gun and they didn't like the results. The gun is also more than accurate enough to make a worthy secondary. Some cosmetic work might hypothetically improve the gun, but I have not the skill and this would also be irreversible. Besides, the gun has a kind of retro-chic to it and I like its colours and lines as they are. May all you with SS2s enjoy them. 'Tis a good gun.