Materials:
1/2 inch cpvc barrel, NOT pushed into a stub of 1/2 pvc
~length varies with the power of your blaster as usual
1/2 cpvc T
1/2 inch cpvc hopper- personal decision on length
1/2 inch cpvc stub of desired length
1/2 pvc stub to put over the cpvc stub above as an adaptor to most blasters
Assembly:
Its pretty easy and straightforward. Follow the picture below. I dont have an adaptor for the blaster because the 1/2 cpvc fits into the big blast i am using. For springers, you probably want to add an ammo stop to the back of the T to keep from wasting the deliciousness on you blaster internals. A simple hole and nail and some glue should solve your problems. In this photo, the long side is the barrel, it happens to be about 7.25 inches long(it also happens to be the barrel i shot myself in the hand with).

Ammo:
I think this is a win-win all around. To really make this work, you have to sort out the marshmallows that will easily fit in 1/2 inch cpvc; simply take a piece a couple inches long and hold one end over a cup or bag. Start dropping your delicious ammo through it until you find one that doesn't fit. Eat it. Enjoy it. Savor it. And then repeat until the bag is empty. I don't see a downside to this. It also takes less time to "make" ammo because you just have to sort it, not stick washers to felt pads, and get out the glue gun, and cut the foam, blah blah blah. And instead of ordering ammo materials and costing money, you can go to your local grocery store and buy thousands of potential rounds for just a couple bucks. Also, ammo littering becomes a thing of the past- any ammo left behind just melts in the next rain or gives sugar highs to any local wildlife.
Quick Note: Don't be a fookin cheapskate. Go namebrand, namely Jet-Puffed. If you have a Giant near you, spend the extra dollar and pay a whole $2 and get a quality bag of ammo. What i have found is that Jet-Puffed adds an extra layer of sugar or something to their marshmallows, so they all come individual. Giant's bags are solid blocks of deliciousness that do not fit in barrels.
Test Results:
I do not have a scale small enough to measure the weight of mini-marshmallows that pass the Sorting Pipe, so I can't say exactly how much weight difference there is between them and normal, less delicious forms of ammo; I think its just a tad heavier, and seem to have a tad higher exit velocity.
Using my big blast and a singled barrel, I was able to achieve the same distance results as with my slugs. I have a mostly Ace Frost King foam that has a decent fit, and a handful of Donquixote's white foam slugs that are a little tighter. The differences in the marshmallows gives a varied range that matches up to the average between the two slugs. I will be testing with a pumpsnap soon.
Another side to this ammo type is age. The longer you keep ammo before you use it, the harder it becomes and it will shrink a little bit, which just makes them denser and a little more painful... But otherwise un-puffing them means you will be able to fit more of the marshmallows through the Sorting Pipe.
I will be putting this blaster to trial by fire in a life-or-death situation this weekend at OSU's Infection (see: HvZ) Invite, and will update with results.
And my final result (integration write up coming soon):

/Usual asking for comments and flames/