I haven't had a 27mm before (plenty of 26's) but a gunsmith of precision engineer will be able to ream out a set of 25mm rings (as long as they have a little bit of beef) for very little money, its literally a 15 minute job with the right gear.
If you're using it on a normal recoiling rifle (up to things like the 30-06 etc) having a delrin bush made will work (but again you'll have to have them turned to fit properly). I had bushes made for a light 7x64 (nearly a 7mm Mag) and with a heavy scope they slipped.
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