Can I use a silicon controlled rectifier to discharge a capacitor?
Yes you can. SCR's are really cool in that once triggered they stay on until the min. forward current is reached. That means: an SCR will discharge your cap. But only to the level that the SCR turns off.  (see spec. sheet) Now here is some of the stuff to consider when selecting an SCR to discharge a cap..   


SCR's are reaally tough so -- good choice. 
They are not really fast.  but fast enough for your application.. 
The SCR looks like a resistor to the cap once it's fired, so the min "on resistance" will tell how far down your cap will discharge before the SCR shuts off. 
You want to make sure the cap is not charging while it's being discharged. So whatever is charging it must be off.
I think you should play with SCR's in mockup mode before you try to install in your gun. If you do go to the trouble others can benefit from what you learn. 
Good luck with this.. That's some good thinking on your part..