SCR's, as said, are only needed when repeatability, durability, and control are needed. Don't think solid state is superior, solid state has its disadvantages like ESR.
I am planning a triggered spark gap for my 12,000 joule cap bank, for both the railgun and a future ETG.
For 200 joules, look for a large relay or contactor, or a knife switch. You can also do spring loaded plates. If you want a handheld, I would do a mechanical solution, a copper block is attached to a lever attached to the trigger, which contacts a stationary copper piece inside. A spring will help release it, and provide tension to the gun doesn't trigger when you tilt it.
Consider a small magnet in the beginning of the barrel, so the projectile doesn't fall out.
knife switch:
http://www.smithae.com/wirthco_knife_switch.jpg
Start with the boost converter, and we can help from there.