I was surprised to learn myself.  What nation and what gun?

IJN Yamato.

Why surprised?

I thought the 16" guns on the Iowa class had more range?

ETA-saw an article claiming the final Italian 16" guns has the best range but, lacked longevity.

Offhand I think it's either the USN 16"/50 or the IJN 18" on the Yamato. But I think the Yamato's guns had the longer range by a few hundred yards. Ours were more accurate because we also used radar targeting.

I think the IJN 18.1" had slightly longer range than our 16"/50.  Not sure about the 15" guns of foreign navies.  Not sure if I remember correctly or not but it seems like we were working on a 16"/56 or something like that but decided to stick with the 16"50 due to increased barrel wear with not enough increase in range to justify changing guns.  Seems like the 16"/56 might have had the longest range but not by much.  

Course I could be totally wrong.

The Pairs Gun was built with Naval Cannons, so this.

So OP, just gonna leave us hanging?

This

This seems to be fairly correct, except 15 for 16.

According to Naval Weapons of the World website, the Italian 381 mm/50 (15") Model 1934 and 381 mm/50 (15") Model 1939 had the longest range.

"A powerful weapon for its caliber, with a maximum range exceeding that of all other battleship guns despite its modest maximum elevation.  This superb performance had a price in that they suffered from excessive dispersion and a very short barrel life, which was only about half that of other nation's large-caliber guns."

http://navweaps.com/Weapons/WNIT_15-50_m1934.htm

Here's the rest of the data, broken out by country.

http://navweaps.com/Weapons/index_weapons.htm

ByNameRequest is right. Regia Marina's (Royal Navy in Italian) 15" guns were the longest reaching naval guns of WW II.  Trouble is, they had to delay the firing of the center gun lest the shot dispersion at long range gets too wide.

I was surprised because I was under the impression that the Yamato's 18.1" guns outranged everything else.  I knew the British 15" on their last battleship, the HMS Vanguard didn't win the prize because they were WW I gun turrets recycled on a newer hull (based on the KG V).  I knew our 16" guns outranged the HMS Nelson's 16".

Well then. Looks like I have some reading later this evening. Thanks. [:-)]

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Interestingly enough, the Warspite, a WW1 battleship, and the Scharnhorst, using "only" 11" guns, were roughly tied with each other for longest naval gunfire hits in combat @ ~26,000 yards. Warspite scored her hits against Guilio Cesare (who I think had those long range 15" guns we mention above) recorded as 26,000 yards. Scharnhorst scored her hits against the Glorious alongside Gneisenau, calculated to be around ~26,400 yards. They opened fire @ 28,000 and based on the recorded time of hits and firings the longest hit would have been around 26,400ish yards. Given the accuracy of rangefinders, charts, and timekeeping there is some margin of error, so one could say that Warspite and Scharnhorst have a dead heat for hitting a moving target.

If you include stationary targets there's some evidence supporting Massachusets hitting the Jean Bart while it was at anchor with distances out to 29,000 yards.

And if you really want to see some internet browbeatings, start googling on Japanese claims for Yamato making hits @ something like 34,000 yards, I believe at Samar. Lots of flak about Japanese overinflation, but there is some merit to the claim. Another detraction from that claim I think was that a near miss caused damage to one of the escort carriers, but it wasn't a direct hit. Shell splashed short but continued under the keel and exploded on a delayed fuse (as designed), knocked out machinery and cause the White Plains to swerve out of formation.

The real fun stuff is cruisers making hits out past 18k yards, like Prinz Eugen vs Hood with"dinky" little 8" guns.

So then a bit overbore.

HMS Warspite was a Queen Elizabeth class battleship. She had 15" guns (like the Hood, Renown class CB, Royal Sovereign class BB).

Sounds like they built their naval guns like they built their sports cars.

I guess my 11" quote wasn't clear that it was only referring to Scharnhorst. I just think its cool that a WW1 battleship and a compromise battlecruiser design, both with WW1 pattern guns made those kinds of hits while moving and firing on a maneuvering opponent.

You mean, their GUNS leaked oil all over the deck?

Don't matter how far you can shoot, only how far you can hit.