Seems store_in doesn't work or I do something wrong
mongoid (2.4.6)

class ProcessedRecord
include Mongoid::Document
store_in "processed_records", capped: true, size: 6_400_000_000, max: 100_000_000
end

1.9.3p0 :028 > ProcessedRecord.collection.drop
=> true
1.9.3p0 :030 > ProcessedRecord.create!
=> #<ProcessedRecord _id: 4f89ac39eeef20709c000002, _type: nil, md5: nil>
1.9.3p0 :031 > ProcessedRecord.db.collection("processed_records").capped?
=> false

It works great if I create it in mongo shell or using following code:

def self.create_collection
db.create_collection("processed_records", capped: true,
size: 6_400_000_000, max: 100_000_000)
end