Two guns, coming of the Husky assembly line in around 1900, identical in all ways except one might have normal spiral rifling in it's 20 bore (or 12.2x47R), the next might have straight rifling. The US equivalency was the .50/70.

Straight rifling was introduced by Husqvarna for hunters and farmers(mostly) to meet a Federal law, stating that only rifles were allowed to be used for moose. Most farmers has SxS shotguns, either 12 or 16. Husquarna then started making SxS's with one barrel, usually the right, either 20 or 16 bore with a rifled tube - spiral rifling or straight as requist to meet the law for someone wanting to have shot in both barrels if he decided to, yet shot round ball OK.

Some people bought bore-size guns, 16 or 20 bore, that, having straight rifling shot just fine with shot for rising birds, but also shot ball - legally for moose also at close range.

Show did poorly if fired from a normal spiral rifled barrel, but did OK from a straight rifled tube.

Husqvarna loaded black powder round ball loads in brass hulls, but shot in paper so it was easily determined between the two types of charge.

At one time, I had just such a gun, but mine, a bit different, had the right tube straight rifled, instead of the left tube. The chamber was the proper size for a paper or plastic 2.5" hull, but also for the 16 bore brass case & bought from Buffalo Arms, USA.
The other oddity, of my gun, was the groove diameter was .705", while the bore was slightly oversize for 16 bore, at .675", which is actually lose to 15 bore(.677").

Due to the .015" depth of rifling, I experimented with balls of differing sizes. My base accuracy came from using 16 bore round balls of .662", patched (like a ML round ball)with .030" Neetsfoot oiled denim. These fit perfectly in the brass cases, and with the case crimped over the ball, shot very well indeed, using a card & fiber between the patched ball and 3 drams of 2F GOEX powder.

That is my understanding on these guns, be they Model 20 SxS shotguns and shotgun/rifles or the single shot variety of bore or rifled guns.

They shoot well with modern smokeless powder shotguns (SxS's) or with black powder loads. I tried both, but mostly use factory 16 bore Federals due to my shotshell chamber being 2 5/8" long, almost exactly the length of a fired 16 bore plastic Federal case. They shot beautifully even large patterns from the straight rifled 13 bore barrel(right one), unless most SMOOTH, cylinder bores.

I call it 13 bore, as .710" is 13 bore, mine being .005" undersize, but larger than 14 bore, which is only .693".