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Joined: May 2023
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Originally Posted by Mark II
In theory if you bore the monobloc for the barrels so the barrels touch at the muzzles and the center of the bores are level you will be really close.

You want to explain that to me? They would be tapered inwards in relation to each other by a couple degrees just due to the outer profiles of the barrels. If each barrel is angled towards center just 1 degree (2 degrees included angle) the center line of the theoretical perfect pattern will be offset to the other side about 2 feet at 40 yards. The patterns might overlap at 40 yards, but the center of the patterns would be four feet apart. I would say, "if I did the math right," but I used a triangle calculator on my cell phone. If the ap did the math right.

My plan was to basically bore the block and make spacers such that the center line of the chamber was almost exactly the same distance apart as the center line of the muzzle.

P.S. I've been playing with the idea of using a clamshell design as opposed to a one piece mono block. It would simplify some of the setup issues for machining. I'll probably cut a mono block anyway, but I've got to setup and figure out the basic geometry and do some basic plans before I decide the best approach for me. The center line of the chambers is restricted by the firing pin locations on the frame.

Last edited by Bob La Londe; 06/03/23 08:18 PM.

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Bob La Londe - CNC Molds N Stuff
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Bob, I am given to understand that it is more than a math problem, the effect of the shooters body and "hold" affects the point of impact and is very difficult calculate. Granted, this is likely easier to see with a double rifle than a shotgun, but it is still there. Milling the rear part of the top rib is often done as you described. One thing I recall and didn't mention is when drilling the holes for the barrels in the monoblock, they would leave one of them smaller than the other. This is to give them the ability to move it slightly, if final measurements show movement is necessary.
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There is also the effect of recoil that moves the movement laterally and slightly down. If you have access to other barrel sets you can measure the center of the breach and the center of the muzzles and find the basic angles. When relaying ribs I use a jig that has level surfaces to lay the barrels on so the center of the bores are parallel at the muzzle. That leaves the patterns vertically, and the angles of convergence will overlap the patterns horizontally. The rub comes in the fact if you just bore sight the barrels you may not get where you want. They may appear to crossfire sooner than you want .

Joined: May 2023
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Originally Posted by Mark II
There is also the effect of recoil that moves the movement laterally and slightly down. If you have access to other barrel sets you can measure the center of the breach and the center of the muzzles and find the basic angles. When relaying ribs I use a jig that has level surfaces to lay the barrels on so the center of the bores are parallel at the muzzle. That leaves the patterns vertically, and the angles of convergence will overlap the patterns horizontally. The rub comes in the fact if you just bore sight the barrels you may not get where you want. They may appear to crossfire sooner than you want .

I only have two other doubles. One is an OU and the first barrel tends to shoot a little low. The second shoots dead on. Both are centered. The other is a .410 side by side. One barrel shoots a little low left. Second barrel shoots dead on. This is based on fixed patterning with a sheet of butcher paper stretched between two posts. If I adjust I can hit with them in the field.


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Bob La Londe - CNC Molds N Stuff
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Haven't done a darn thing with this in two months. I've just been sort of thinking my way through it. More and more I like the idea of a mono block, but fitting the barrel lump seems like it could be its own issue. Would it be strong enough to make a mono block for the barrels and the barrel lump that engages the frame separately and bolt and solder them together? I think if it is made with just the right tolerance the recoil force will all be on the faces of the frame, not on the barrel lump.


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Bob La Londe - CNC Molds N Stuff
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The circle on the lump is fitted to distribute recoil and help keep it on face.look up joining a sxs on the internet also the book guncraft has a good explanation.

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