A good seal is definitely important - taking particular note of caps and specs which might interfere with it. Gel cups (which are standard on some, and obtainable for others) in my experience improve both seal and comfort.
The key attribute of the defenders themselves is IMO the attenuation-rating, though - and you need more for fullbore rifle than for shotgun, and even more than that for fullbore rifle with muzzle-brakes and/or in covered firing-points.
I use a an SNR26 set for shotgun (and, I confess, the odd rifle-shot in the field - but that is at my own risk, of course), SNR32 for fullbore rifle, and SNR32 with ordinary passive plugs under for covered points/muzzle-brakes. With plugs under, I just turn the amplification up so it shouts through the plugs.
Additionally, I have a set of electronic plugs, which are handy when out in the field in the rain as they allow the wearing of a proper hat - and they're fine for shotgun and (as ever, at my own risk) the odd unmoderated rifle-shot. They're all 3M/Peltor.
Sordin Supreme Pro X seems to have SNR28 - so IMO not enough alone for fullbore rifle.
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