I have been told American customers often ask for barrels to cross. Euro customers prefer the barrels to shoot parallel. Which the gunmaker thought proper.

What 'your' particular rifle does? Let's say my rifle is shooting impacts one inch apart at a hundred yards. Left and right barrels to L and R. Are they still parallel at 150 metres? At 200 metres? Or crossing somewhere?

Let's assume they magically stay parallel. 1 inch at 100 yards apart. Let assume 1 MOA groups from each barrel. Magic!

. So theoretically at 100 yards two bullets, one from each barrel could be touching. Or 2 inches apart. 2x 1 MOA side by side. Combined a 2 MOA grouping. In theory.

Let's go parallel to 200 yards. 2 inches apart. 2 MOA groups each. Similar result, combined 4 MOA group.

Make the original individual barrels accuracy 2 MOA. At one hundred yards, one inch apart, the impacts might cross like a Venn diagram. Creating a 3 inch wide dual group. At 200 yards the groups are 2 inches apart, but each group is 4 inches. combined group is 6 inches wide.

If crossing the combined group will be more mixed. Unknown, many variable factors.

All very theoretical!

Trial and error.

If one has ones "regulating" load say at 100 yards. Shoot 1xR, 1xL. Reload, repeat but say on a second target. Keep track on which is the left barrels and right barrels.

Superimpose the two targets to get 2x 2 shot groups. One could keep shooting each barrel, on new targets, 3, even 5 shots from each barrel. Would be revealing on each barrel's accuracy. How effective the regulation is. How both the accuracy and regulation changes as the barrels heat up.

Give your shoulder a rest. Let barrels cool down. Repeat. Repeat until you see stars ...

Do similar at 150, 200 yards. If 200 yards is encouraging, try 250, 300 ! After all my 450 NE's express sights go to 500 yards! Might need a very big target for groups ... A barn door?

Definitely shoot at 50 yards as well. Might rebuild confidence after shooting at 200 and 300 yards,

I think I would limit it at 2 barrel 3 shot groups. Maybe 3 of each. At 100, 150, 200, and 50 yards. Maybe a couple of shots using the 200 yard and 300 yard leaves as at a test. That's 28 shots. Using up bullets and bruised shoulder.

Would be much easier for a medium bore.

If scoped, try some 2 Barrel groups scoped and unscoped.

Try shooting the left barrel first as well. Any great variation?

Using a different target for each R and L, but superimposing each new target, allows a progression of shots to be analysed. A computer graphic could be created. More simply copy the impacts onto clear sheets of plastic. Labelling each. Identifying each barrel.

One could examine a single group shot from each barrel, left's and rights. The first, second, third shots together. Or one could examine the first two shots of each groups of shots. Creating an average composite grouping of first shots from each barrel. Or one could compare the progression over different ranges. If the impacts are crossing, this should become evident.

If doing all this, you're probably better familiarised with your dr than 98% of owners.

One thing I like theoretically is for one barrel of a double rifle to be well sighted in. The other barrel nearby hopefully close enough. Instead of the compromise sighting. Theoretically if say the left barrel was sighted in like a single shot, longer shots could be possible.

Express sights are good for short range. Less for longer. A scoped double will be far more accurately shot at longer ranges.