So here's my write up for the FAST hop unit.
1. With your calipers measure the diameter of the FAST hop unit (tolerances could vary between guns and units) and the inner diameter of the airsoft pro housing. The FAST unit at 16.9mm and the housing 15.8mm, so we have to grind out 1.1mm.
2. Measure the depth of how far you must reem out to fit the FAST unit I measured around 22mm deep. In the picture below you can see the difference in size of the two units
View attachment 21744
3. Next is to reem out the barrel housing. I suggest ordering the airsoft pro upgraded on as the stock housing does not hold the outer barrel very well, after a day of playing you might find the outerbarrel is loose. I don't have access to a machine shop or the proper reaming tools to do a proper job, so I improvised and used a drill bit with some 180 grit sand paper taped onto it. I marked my 22mm depth with a peice of masking tape around the sandpaper so I didn't go to far into the housing. If you take this route then it's going to take some time and effort aswell attention to detail. Basically I but the bit in a drill press to have the grinding tool stationary and then holding the housing ground each side. I tried to get an even grind by constantly rotating the housing while pressing it against the sandpaper of course while the drill was turned on. Here is what I ended with in the picture below. It's not a great job by any stretch but it seemed to work out for me and my gun is shooting straight. (Left side is the ground airsoft pro housing vs a brand new one on the right)
View attachment 21745
4.After this it's basically just install, now this too will take some attention to detail. There are no aligning posts or notches to ensure you hop is perfectly vertical. What I've done is install the outerbarrel and hop unit to the housing, do not secure the hop unit though. Then install this into your receiver and using the platform below the hop unit to try to level it. You can see this better from the pictures below. Then once "leveled" tighten the crew to secure the unit.
View attachment 21747
View attachment 21746
*note: do not use the supplied srs bucking as it does not work with the svd nozzle. I've been using a maple leaf 60° bucking and it's been great.
5. From here it's just assemble the rifle and take it to a game. Hope this helped any other springer svd users.