ER11 - 5/16

I knew ER16 had an out of spec oversize collet for 7/16 available. I recently discovered that ER11 has one that is 5/16. I've got one small CNC
mill with a hgih speed spindle that has an ER11 spindle nose. Occassionally it would be handy to slot with a 5/16 ball mill. Strictly light fast cuts. No heavy HP stuff. I've got my FS calculator set to limit loads for that machine to .25 HP or less already anyway.
Anyway, since I've been told that size collet is out of spec for ER11 what are the risks of running it? If its just breaking the collet I don't think that's a huge issue given my care to limit cutting loads.
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