As Cal says, all models have a 15mm (nominal) outside diameter, except for the QB78 basic (ie non Deluxe) model, which is 14mm nominal.
When I've measured large numbers of these barrels, the averages were that the 15mm barrels are actually at - or just below - the 15mm nominal diameter. The 14mm barrels were slightly larger than moninal - more towards 14.3 mm on average. Though - of course - your mileage will vary slightly on any individual barrel.
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