The Noveske switchblock is a "switch block". The switch has three positions: normal (unsuppressed), suppressed, and off. The normal position sends a preset amount of gas back to work the action when you don't have a can on and it works beautifully. The suppressed position sends a modified amount of gas back to work the action because the can changes the dynamics and this modified amount needs to be tuned to work with your specific can. The timing and pressures are quite sensitive, so GAP needs your upper and your specific can to ensure proper operation while in the suppressed mode. Working off of data for different can models is not good enough. They tuned my switchblock for my Cyclone can and it works reliably. The off position doesn't send any gas back to the action, so you can use that for single shot testing.
If you think you might want a can for the rifle, absolutely get the Noveske switchblock - no brainer. The tougher question is whether you want a brake that will accept a can, or just have the barrel threaded to accept the can directly. I went with the threaded option because...it's only a .308 and the stock absorbs the recoil easily without a brake. I shot 275 rounds without a brake or can and it was comfortable. Also, once you have a can with a tuned switchblock, you'll probably use it almost all the time. Of course, the Quick Disconnect feature of the brake/can combo is nice because you don't have to spin the can on and off each time you case the rifle. There are pros and cons to most choices like this - I'm just sharing what is working quite well for me.
"Not a sermon, just a thought." - Lon Solomon, McLean Bible Church
BTW, yesterday I shot my first subsonic hand loads through the suppressor and it was just a bit louder than a dry fire. Fun stuff.
