I posted same questions on UK Chinese Forum, response was to use EPDM. And, -113 is the correct size.
The Buna N seal did shrink back to nominal size over the next few hours.
Related topic..
I have been tuning and shooting a new-to-me QB78D. To test seals and function and accuracy I am shooting with 1 live CO2 cartridge installed first and one empty. Seals are good. As received, it was weak at 250 fps, so I didn't do an accuracy test, instead I went straight for a polish and debur. Speed is now up to the limit, but accuracy is less than my other QB78 family rifles which typically do ragged hole at worst at 10 yards. This one is within a quater, but definitely not ragged hole.
Some questions:
1. I have typically shot my other QBs rested and either bulk filled or tethered, and get ragged hole.s This is the first time I've attempted target shooting on cartridge and only one cartridge at that. Anyhow, I'm getting about 30 shots, but I haven't chronied each one. When I did chrony the gun before and after tunes, I shot 3 in fairly rapid succession and noticed a ES of less than 5 fps. So, with may one cartridge set-up, is it likely that target shooting 20-30s between shots would affect accuracy?

2. Scouring the Yellow shows Archer believes the single most useful accuracy enhancer to be a muzzle brake (weight) for the 78. My 79 .177 shoots ragged holes at 10 yd and it has no brake/weight. So, has anyone noticed an improvement in accuracy with a muzzle weight on a QB78?

3. I did a brass screw/compound re-work of the crown. Under a 10x loupe it looks smooth and even. No noticable improvement in groups (well maybe marginal). I do notice that pellets are stiff to seat in the breach. Wheras the bolt closed somooth as silk without a pellet, the last 1mm of travel has significant resistance with pellet loaded (some worse than others). I think the shirts maybe being damaged. Is there a remedy for this?

Of course I am going to do some more tests (tethered, examine pellet condition, and try a brake), and will report on my findings, but would appreciate hearing your experiences.

Thanks,