why is that superior?
surely the best contact points for an overbarrel mod is the thread and the rear bush?
Does the mod contact all along the "spigotted" part?
The thread is not a contact point. It is just the means to apply pressure to the contact and alignment area, which is the shoulder, whether it is spigotted or not.
To some extent a slightly loose thread is superior, because it allows the assembly to align properly at the contact point i.e. the shoulder. The spigot is a tightly toleranced feature, and a mod. manufacturer who properly understands the feature will ensure that the mating hole is similarly precise.
The spigot is not a contact feature, it is for alignment. It assures concentricity, which neither the thread nor the shoulder do.
It also acts as a safeguard, a secondary alignment feature, in case the mod. comes loose.
The spigot also extends the distance between the thread, and the shoulder. The longer this distance the more precise the alignment. By tightly tolerancing the spigot and its alignment with the shoulder the precision of the thread becomes less important, indeed a looser thread is desirable.
A similar result might be achieved with a very long threaded portion, but this would be impractical and have a great risk of jamming, and not allow the slight float necessary for the mating with the shoulder.
Those who don't offer it perhaps are not capable of working to the necessary tolerances, or use unsuitable materials for the thread, e.g. aluminium alloy.
The rear bush on a reflex design is not an alignment feature, it is there for safety, in case the mod. comes loose, or is knocked. It should ideally have a slight clearance with the barrel to ensure it doesn't touch and alter barrel harmonics, hence it does nothing for alignment just safety.
It is an antiquated design feature, introduced with the original BR Tuote design e.g. T8, which was designed for military applications, automatic fire, cost effective loosely toleranced mass production (welded together from inexpensive materials). On e.g. the T8 the bit where the thread is attached is rather flimsy, attached only by a welded three-legged spider of thin sheet metal. The bush serves to protect this weak feature from being bent by a modest knock.
Ase Utra pointedly eliminated it in their Northstar reflex design, which they designed primarily for the UK market where they were missing out because of our peculiar obsession with reflex designs.
The large expansion chamber of the reflex mod. is important when designing for sustained rapid or automatic fire where large internal volume is important. For our purposes it is an unnecessary weighty irrelevance that merely serves to cook the barrel.
It continues to be incorporated by "designers" who either do not understand the basics, or continue to provide it cynically because that is what the market demands.
Now back to the point,
Here is a useful table of recommended thread sizes and lengths for non-spigotted barrels, depending on barrel diameter, to ensure a decent shoulder.
Maintenance