Thank you for that link, I was actually looking at that before. I just wanted to mess around with parts that I had on hand. If I use your idea, I will definitely have to find a toy gun with more room inside.
Finding a gun shell with enough room can be a challenge on it's own. I found that it's usually better to just find one that was already made to fit a recoil solenoid.
Those jolt guns made for the PS1 and Saturn are pretty good for a diy project. They are one of the few home guns to use a proper solenoid and they have far more space for electronics than the Time Crisis arcade guns. It saves a lot of time an money. With a little patience, they can be found for as little as $30.
I bought a pair of Terminator Salvation solenoids for projects. They aren't the best in terms of force but they are a good bit smaller than most arcade solenoids. They won't fit in an Aimtrak shell but they fit nicely in my House of the Dead 4 guns. They'd work well if you wanted to copy the awesome mini Uzi project thread on this site.