Generic word for firearms with long barrels

What's the generic word for a firearm that has a long barrel? Like in the following picture, what is the man holding?

I've always called it a rifle, but recently I realized that this generic meaning is not in the dictionary. So either I've always been wrong, or there's something wrong with every English dictionary I consulted.

Merriam Webster: a shoulder weapon with a rifled bore; a rifled artillery piece; rifles plural : soldiers armed with rifles.

Wiktionary: a shouldered firearm with a long, rifled barrel to improve range and accuracy; a rifleman; an artillery piece with a rifled barrel; a strip of wood covered with emery or a similar material, used for sharpening scythes;

Dictionary.com: a shoulder firearm with spiral grooves cut in the inner surface of the gun barrel to give the bullet a rotatory motion and thus a more precise trajectory; a cannon with such grooves; etc.

Similar definitions can be found in other dictionaries (American Heritage, Collins, etc.), and they all specify that the barrel is rifled. The word shotgun would have more or less the same problem, that is, it's a specific type of firearm according to the definitions, although I have to say that Merriam Webster defines it as "a usually smoothbore shoulder weapon [...]".

‘Long guns’ and ‘long arms’

Although in non-technical use any long-barrelled firearm is often enough referred to generically as a “rifle”, no matter whether it actually sports a rifled barrel, the preferred technical term for these is long guns — or sometimes in older writing or in historical contexts, also long arms.

As with nearly all such double-barrelled monikers in English, both terms can be (and have often been) variously spelled as either open compounds (long guns, long arms), or as hyphenated words (long-guns, long-arms), or with both words run together (longguns, longarms).

The OED provides these two entries and accompanying citations:

long gun n.

a gun or cannon with a barrel of (relatively) great length; spec. (in later use) a handgun with a long barrel, such as a rifle, as distinct from a pistol or other handgun of similar size; cf. long arm n.²

• 1530   John Palsgrave Lesclarcissement de la langue francoyse ɪɪɪ. f. xlvᵛ
  Longe gonne, flevste.
• 1644   Kenelm Digby Two Treatises ɪ. xi. 99
  Take the barrell of a long gunne perfectly bored.
• 1757   John Muller A Treatise of Artillery ɪɪ. 101
  Long Guns are very inconvenient on board of ships, on account of the difficulty of loading them.
• 1838   The Civil Engineer & Architect’s Journal 1 263/1
  The Gorgon will be fitted with sixteen 32-pounders (long-guns).
• 1922   Army & Ordnance Nov. 160/1
  A long gun wears rapidly.
• 2002   Esquire Mar. 183/2
  O’Neill..requested that his agents be commissioned to carry ‘long guns’—rifles and automatic weapons—instead of just sidearms.

long arm n.²

Compare firearm n., small arms n.

rare before 19th cent. Now historical.

A long-barrelled gun, as a musket or rifle.

• 1675   in J. H. Trumbull Public Rec. Colony Connecticut (1852) II. 270
  Such Troopers as shall neglect to prouide themselues with long armes, viz. a carbin or muskett..shall be disbanded.
• 1854   J. J. Thompson Hist. Feud between Hill & Evans Parties xi. 86
  ‘Why don’t you take long arms then?’ ‘Because I can use short arms better.’
• 1972   D. W. Bailey British Military Longarms 1815–65 9
  The barrels of military longarms were officially ‘browned’ from 1815.
• 2011   A. Ewert in D. Shideler Gun Digest 2012 90/1
  Undeniably, this three-lives long arm had some interesting history.