The gun is pushed aft when it launches the piston fwd, which is virtually identicle to the gentle recoil of a firearm. Then when the piston hits the breech end it's a sharp and violent high-G impact that knocks the gun fwd as if hit with a hammer, because basically that's what's happening. This is where the term double recoil comes from. Many/most think it's the "double" part that breaks stuff, as if parts being shook in two directions is the problem but it's only the piston impact that breaks stuff. The initial one is harmless and mild, the second is basically the same amt of energy but all at once, which is why it cause scopes or mounts to slide aft, or break lenses free and break reticle wires. You may already know this but just in case...
So if your eye is touching the scope then yes, it'll push into you, but I've never seen a scope with zero eye relief. Seems odd they'd make such a scope, I mean why would they because most guns would not be able to use it, and even if they could, why would they want to? I assume it also means you have to mount the scope further aft, which may require a special mount? If so I spoze I'd use this one;
https://tinyurl.com/yajzzpyc I'd drill/tap for a couple extra screws so it won't slide. Speaking of that, many mounts come with a stop pin, which is fine for a cheap gun but I refuse to use one on a nice gun because they damage the hole they go into. This is why I'd put extra screws on it, but another drawback to using a pin is you have to put the mount in that one position unless you drill a new hole. So instead I use stronger mounts, and if needed I add screws to make them even more so.
Regarding the tuning, you can do that yourself and I can send you the how-to info if you'd like.
As for the Xfer port, I do NOT suggest enlarging it on purpose. I enlarge many of them but the reason is to clean them up because they're rough inside. So if the port is not that bad, like I'm guessing yours are, you can simply sand them. The chinese stuff is nasty so I have to drill out a size or two larger (sometimes three larger) to clean up the mess, then sand it. For me three sizes, in # drills, is taking the oem B18 port of .125" to .140", so imagine how nasty a hole has to be that even .136" wasn't enough.
The port size is part of a balanced system just like the intake manifold on an engine, so if you go bigger w/o having more displacement you're hurting power. In my case it's a give/take, I get better flow since it's smooth, but lose compression and increase piston impact. Since I tune the gun, at least the way I do it, I get the compression back and some. Ideally, if I bothered, I'd hog out the port an put a nice new insert in of the oem size. Fyi the oem size is not always ideal, even the factory sometimes uses too big a port.
The 98 should have a 3mm port (~.118"), and I'd imagine you could go to .125" without any loss in power but any increase in size will increase the G's of piston impact.
The port on the 97, if I recall correctly, is too big so you're just making bad into worse. My memory on that port is fuzzy at best, but at the very least I don't believe it's too small. Depends on how much bigger you go too, so hopefully this guy is just cleaning it up like I do, but I'm kinda worried.
People have asked me to tune their guns but I tellem shipping would be the deal breaker. I shipped one gun a few states over some 5+yrs ago and I was shocked how much it cost. I can't imagine to the UK, plus I've heard CA rates are higher than US.
chevota at hotmail and I'll send that, which explains tons of stuff, including how to make the tool to sand the port. The buttons you can do yourself as well, and I like to think my way is better than machined in versions. At the very least look at the info and decide for yourself.
On the 98 you may want to dry lube the cocking shoe and the parts it slides on. When cocking the gun and you're mid stroke or so, look at the cocking linkage angle. Then calculate the force on it which is easily >100lbs. All that force is on the shoe pushing on the thin contact patch of the cocking slot where it rides. So I prep the surfaces, impregnate with dry lube, then make sure it always has grease. I got some special grease for parts like that which is extra sticky and extra thick, so it'll work better and last longer. That spot is the highest friction point in the gun, so at the very least I'd grease it often. The 97 has it even worse, but hopefully your guy fixes it?
Fyi I'll share my story fo why I'll never shoot a side/under lever gun again. Aside from the cost, weight, pita loading and power loss, one almost disfigured my fingers when the breech slammed closed a micro-second before my fingers were in there. It was a Diana 52 btw, and I had not modded it yet, it was new with just a few shots thru it. I then put a pencil in the breech and pulled the trigger to see what would happen, and it basically it was as if it had been smashed under a large rock. It was splinter city with more splinters flying and the edge of the comp tube acts like a shear to ensure no bones survive. It's as if it was built to break and disfigure fingers. Yes it has an anti-bear trap device, not it doesn't work. It may have on paper, but the rounded teeth of the catch allowed the assy to skip right over them. Hard to explain but if you saw the part it would make sense. So aside from all the non-finger mangling issues, I see no point to them, but add that and forget it! I'm saying it for you to be wary, and like any safety; do not expect it to work! Basically I'd hold the cocking lever with one hand, or arm, or however so it can't do that.