Overbarrel is louder than muzzle can for a can of the same weight. It's less volume efficient and the gas is flowing forwards not backwards. They can be shorter though (but probably louder). They prevent use of a Belleville washer or similar as a locking washer to prevent the suppressor coming loose. They can also "spring" the barrel if the bore is non-concentric to the OD of the barrel and they have a tight bushing. They also trap heat.
They're less likely to be damaged by a baffle strike if they come loose but the lack of ability to use a lock washer = more likely to come loose than a prudently installed muzzle can
All that said I've got a shiny new Roedale Delta Pro-X over barrel to try reduce OAL at cost of weight, supposedly same suppression
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