I have a new Merkel 240 cape gun and some Talley bases (flat bottom for a Tikka T-3). ~.2365 = BH
The goal would be a solid repeatable QD mount where the bases only protrude from the rib .160” or less.

The gun will only be used for deer blasting with a 20 gauge slug and 7x57R in the tubes.
In addition, I don’t want to spend $700 for pivot mounts because they just won’t add that much to the value of the gun,

I am thinking about having the rib machined to use the bases and retain use of the iron sights (which are really of no use to me these days).

Figuring 2 rectangular cuts in the rib at a depth of .090” with corresponding screw holes (4) to secure the bases.

My question is when leveling the barrel assembly for the cuts...

I’m assuming the setup would be to level against the flats on the mono-block.....Right?

Dave,
If I were going to do that, that is the way I would level it. You should be careful, though, where you place the bases. The rib, itself, is likely soft soldered to the barrel, and I would not trust it to stand up to the recoil. The rib extension, over the chambers is likely either solid( if an actual monoblock) or hard soldered(brazed) to the barrel ( if they are bundled), and would likely be OK. A close inspection should show where the joint is. Chose the scope to fit the spacing of the bases. Good luck, let us see how it works out.
Mike

Good advise on the rib extension. I would also determine if the remainder of the rib is solid or hollow before you mill or drill. You could drill a 1/16 inch hole in the bottom rib just behind the forend loop and use a wire to determine if the rib was hollow or not. Just use a mail set (small) to knock in a piece of shot to close. And lead shot (while it's still legal)

Good point Dennis, I just presume an O/U rib to be an upside/down channel section and a SXS to be basically solid, but radiused, with narrow solder joints on the sides.
Mike

Here are a few pictures of the rib in question.

There is obviously a separate section of rib that runs maybe 1.5" from the rear of the barrel
assembly to about the back of the 0 on the 20 engraved designation.

The rear sight is maybe 7.2" forward of that and is obviously dovetailed into that section of rib.

Based upon the rear sight I'm thinking the rib between it and the breech is pretty robust.
However, when ever I see pivot mounts atop this gun there seems to be a reinforcement
(that the front pivot base gets dovetailed into) applied to the area of the front mount?

Dave,
The rear sight is not heavy enough to create much force in recoil, so it is set into the rib. A scope is different, because of the weight. When claw mounts( or swing mounts) are mounted, a section of the rib is cut out and a base(sattel) is fit to the barrels and soldered to them( sometimes glued). The joint is much larger than the joints with the rib it replaced, so it resists the recoil. I have seen one or two guns that had the front plate dovetailed directly into the rib, but since the scopes were "lost" I don't how they would withstand a good bit of shooting. Also, it is a real "trick" to cut the section of rib out, due to the curvature of the barrels. You can only saw it part way, then you have to cut the rest of the way with a very thin capeing chisel. You would have a similar problem with an end mill to cut the rib out. You might have to be satisfied with the Talley base standing a little proud of the rib. If you have or can find a Heym O/U combination gun, take a look at the dovetail for mounting a scope with "slide on" mounts( 1960-70 era). You may have enough room to set a similar section of dovetail in your rib and use one of the Recknagel mounts made for Heym or Brno( same era). From time to time Simpson Ltd has a Brno with a mount for sale at a good price. Of course an O/U is not as "pushed" for room as a SXS (between the barrels), and the dovetail for this idea may have to be higher than the rib to mill the notch for the "lock" to fit into. See what you think. BTW, the 7x57R is one of my favorite cartridges.
Mike