In reply to:

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The rifle from your link seems to be done as liners in the old shot tubes, judgeing from the picture of the muzzel end picture! I would be real worried about how this rifle was regulated, or if it was at all! The removal of the express sight, and the addition of an adjustable ghost ring sight indicates an attempt to fix a sighting/regulation problem to me!

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I dont think that gun is built on sleeved shotgun tubes.
The frame is 12 gauge and the external diameter at the muzzels looks too small to be 12 g.
Also the width of the rib is too wide at the muzzels for the barrels to be 12 g diameter.
If they were still shotgun tubes the rib and the space between the barrels at the muzzels would be narrower.

I think the muzzels have "undercut" crowns and this gives a similar appearance to sleeves.
I think the gun has "proper" barrels rather than just sleeves fitted inside the shotgun tubes.

Regarding the removal of the rear sight, this is common if a peep is used and if the rearsight cannot be lowered or folded down.
Trying to use a peep or ghost ring with a fixed standing rearsight still in place is a real pain!