I've posted something like this before but just typed this again.

Any corrections or additions?

***

Regulation is about bringing the point of impact of two barrels together or parallel at a certain distance. For example, a side by side, might have it's barrels adjusted or regulated to impact two barrels points of impact approximately 30 mm apart at one hundred metres, inches, yards etc. Right barrel to the right, left to the left.

An under and over similarly, the top barrel a little higher than the bottom barrel at a given distance.

Some people incorrectly believe double rifles are regulated to cross at a certain distance.

This is in theory. Actual practice may not be theory.

Point of impact depends on how long a bullet is in the barrel and any initial recoil movement. Physically a side by side might move upwards and to the right with the right barrel. Upwards and left with the left. The bullet while in the barrel might experience some of that movement.

Of course the projectile, bullet weight, load, velocity etc affects regulation. The warming and barrel movement from heat expansion of the first shot barrel, impacts on the impact of the second barrel. The Heym Gunmakers regulator said to me they use a standard of a seven second delay between shots when regulating. More or less time might affect the regulation impacts as the heat expansion might be different.

The weight and balance of the double rifle can also have an effect. The double rifle might be regulated with open sights, a dot sight, or a specific scope. The weight of a scope or change in balance might affect the regulation.

Sighting or shooting a double rifle should be done with the forehand resting on a rest, not the forend or barrels directly resting on a rest . It simulates more a standing shot. How the double rifle moves under recoil affects regulation bullet impact placements.

Regulation is done with the two barrels temporarily fixed in place. And temporary wedges used to alter the angles between the barrels, left and right, up and down. Test shooting is done until the wedges are right. Then the barrels are affixed together, wedges fixed in place, rib soldered in etc. One of the reasons double rifles can be so expensive is this hand regulating and test shooting. (Engraving and brand name, other big expense factors. Hand fitting is not cheap either.)

Unless the double rifle has non affixed barrels. My Tikka/Valmet 512 under and over double rifle has a gap between the barrels. A wedge at the front to adjust for right and left movement. Another wedge half way along for adjusting up and down the barrels, changing the angle between them. I regulated my 9.3x74R barrels with the 286 gr bullet and chosen hand load to be about 25 mm apart at one hundred metres. Similar to the muzzles. With the scope attached.

A side by side would be the opposite. The midway wedge to alter left and right. The front wedge for up and down. The Blaser S2 was another drvwith barrel gaps, unaffixed barrels. The Baikal is another.

Regulating loads. Owners talk about handloading and "regulating" their double rifles. This terminology is incorrect but acceptable. Unless they are moving the barrels, what they are actually doing is trying to duplicate a regulating load. They are not regulating their rifle by tinkering with handloads.

Regulation and double rifles are not exact sciences.

Some rifles will still shoot ok if say the left barrel barrel is shot first instead of the usual right barrel. On a side by side. Some demand the right barrel first.

Some will regulate still with different loads and bullets. Some claim a 75% rule works, use eg the same powder charge in a .450 as used for a 480gr/500 gr bullet, but for a 75% weight bullet, a 350 gr and it will also work and shoot to regulation ? In a .375, 300 grs and 235 grs. Trial and error is the rule.

Some rifles might regulate with the open sights and a scope. Don't depend on it though. A lighter small dot point sight might work better for an express sight regulated rifle.

The main thing, is to have a double rifle shoot well for regulation with an effective bullet and an effective cartridge for the game desired. And the ability to shoot it well, at close range, in a hurry. Running or still shots. All the extras are pluses.

Trial and error until hopefully the desired result or an acceptable result is achieved. Trial and error supersedes theory.

Lastly, sometimes an existing rifle just has to be sent back to the factory, or a very good double rifle gunmaker/gunsmith and bevre-regulated. The rib removed, the barrels separated, the whole regulation process repeated. Not a cheap option. I remember one gunsmith charging $5000 for this. I think a Brit premium brand would charge a multiple ?

Have fun. It's not some crap plastic stocked mass produced bolt action.