The reason I posted this was to see what people believed.

I was of a particular viewpoint myself and found my belief was incorrect interesting.

Popular belief in general is double barrels are designed to cross. Perhaps this comes from double barrelled shotguns?

An internet "fact" is crossing barrels path is incorrect, and the bullet paths should fly in parallel. In theory at least.

In fact BOTH alternatives are CORRECT.

Per Heym, they make double rifle for the USA market to have the bullet paths parallel (in theory of course). They make double rifles for the continental market with bullet paths designed to cross at a certain distance.

So for at least the considerable number of double rifles made by Heym, the FACT is they make barrel designed to BOTH CROSS and shoot PARALLEL. Not theory, fact.

One of the advantages of having a bespoke rifle made by a maker such as Heym or L'Atelier Verney-Carron is one can presumably specify whether the regulation will have the bullet paths flying parallel or crossing.

My preference is to have the bullet paths being parallel. In any case bullet groups as the distance increases will in theory merge under a "parallel" system.