Pressure threshold for 750ml belgian bottles

My fellow brotherinlaw and myself just bottled his braggot mead and we wanted to carb heavy in champagne bottles but the local did not have any. So we got Belgian 750's and decided to go with 4 volumes...well its been aging a while and we kinda added priming sugar (before racking) for 5 gallons but ended up with 4. So now the 8.6 ounces of corn sugar is predicted to add 4.8 volumes more or less for this amount of mead.
Has anyone put this much pressure into these types of bottles before? the reason im hoping it will be okay is that we corked and capped them so it would be a pain to vent them all at some point. also there is a chance the carb is weak or none cause we did not add any new yeast to the 9month old batch.
so help if anyone can and thank you guys.

Update:

The mead had aged for so long that there was no viable yeast left and it has not carbonated. Never thought about that. So we effectively back sweetened it.