I may offer some of my findings, as they are purely that and that only.
With all my tinkering adventures, I realized with all the polishing, de-burring and kitting, the most crucial thing in the air rifle rebuild is the piston seal. It can make or break a project.
I like the concave cup faced seals best. They seem to get a little extra puff at the end, where as, the parachute style flair out, and slow down. But the parachute is recommend anytime you need to mask a poor tube condition, as they should seal better.
ARH seals, tough as nails and are good long life materials. The SGS seals, which are parachute style, use to run in two sizes, and the smaller size fit my rifles best. Yet need some slight sanding for peak velocity. The larger SGS may be needed if you have larger compression tube. I have tried using the large one in the past just to sand off the entire rear wiper to get them to fit. I would recommend getting these seals very close to correct size, as I have tried to "shoot them in", with no success of seal wear.
Yellow seals below is ARH SGS seal on a HW 97K piston.
The black moly seal above is the Vortek Seal. It is excellent as a replacement seal, and provides a soft cycle. I typically find, shove them and shoot them. As long as, the rear band is not larger than the leading edge. I have had a few of the Vortek seals with the i.d. cut relief for the button is too small. This causing the rear band to bulge out when snapped on the piston and also not allowing the seal to spin. Hard situation to remedy at home. I have returned these to Vortek and they replace them no questions ask. That is good customer service. All my personal rifles have the Vortek seals, even when I run the ARH kits. The Jury is still out on the longevity of the softer moly seal.
ARH hornet, I have tried using them in the past, but they tend to run real loose on the button, at least the ones I have used and pitched. There are different methods of tightening up the buttons if this is the case. Brown papers are recommended by some, I have used adhesive back foil tape on the button to tighten things up with success. I am sure other methods are there, but little is shared on this subject? Teflon tape does not last in my trials.
I have used the Custom Air Seals below in my 97's, and the fit was excellent, so was the shot cycle. Prices on the site are in AUD so you need to convert to currency needed.
They also have a good seal fit write up I will show below, and just to add, I like the rear wiper, once snapped on the piston, a few thousands less than the leading edge.
WEIHRAUCH SEAL FITTING Weihrauch piston seal fitments vary considerably. Some seals spin and are loose on the piston others are very tight and have to be forced on. Scratches created when removing the old seal should be filed off, the seal should be spun on the piston a few times. A tight fitting seal will bow out the rear lube band causing it to be bigger than required. This will in turn pull the sealing edge in making it smaller than required, not a desirable situation. In an effort to combat this the internal dimensions of the seal have some clearance. Measure your seal across the rear band and the sealing edge, record the measurements. Now fit the seal on the piston re-measure and record. Did the measurements stay the same? Or did they increase or decrease? If the measurements stayed the same and the seal fit is not excessively loose no further action is requited. An excessively tight seal that distorts the seal is a problem. The solution is to either remove some material from the inside of the seal or some material from the piston. A loose seal is generally preferred as it is easily rectified. Teflon tape can be wrapped around the seal fitment or a circle of brown paper can be placed on the piston and then the seal fitted. The paper should be big enough so it wraps around the whole fitment. By adding tape or paper ( I prefer brown paper) the seal can be made to fit nicely. Keep measuring the rear band and the sealing edge and note the difference the paper shims make. In this way the rear band can be increased until it is close to the cylinder diameter. Once the paper is the correct thickness spray it with hair spray and fit the seal. The sealing edge will sometimes decrease as the rear band increases. The piston should be a able to be easily pushed down the cylinder without excessive force. The seal will wear in over the next 250 shots. Chronograph testing should not be done until after 250 shots have been fired and the seal has worn in and any excessive lube burned up. The seal should be lubricated with a small amount of molybdenum disulfide grease on the rear band. Excessive lubrication or lube on the seal face will cause a dieseling condition. Moly Grease can also be applied to the piston, this will work its way on to the cylinder walls and provide on going lubrication.
Best of luck,
Jason W Garvin