What ship?

Well, seeing as you can't use the second set Ls on the Gneise or O-Pro because they take up too much space (just noticed that) then i guess the question refers to the Bismarck.

Bismarck: second set L's.

thanks but why? What is it actually that makes them better? range? if so, how much more? spread? reload?

First set is a BB2 gun.

Second set is a BB3 gun.

Second set gives better range. Without looking, I am not sure about reload or damage per shell. Spread for both is fantastic.

Damage is more and reload is longer for the second set.

14.96"/52 SK C/34 L = Opro and Gneisenau dual guns
14.96"/52 SK C/34b L = Bismark dual guns
14.96"/52 SK C/34o L = Opro 2 trip guns

OK makes sense, thanks.

Is the range difference substantial? Like I said I found a source that gives them identical range but maybe it's out of date?

Also, as a side note, would it be acceptable to run a O-Pro II with Bismarck dual guns to save on weight (more speed) and gain more range?

Yes it is, but you do loose quite a bit of damage doing it.

14.96"/52 SK C/34 L - Slightly slower than C/34o L, Slighter faster than C/34b L
14.96"/52 SK C/34b L - Between Gneisenau and OPro2 gunset reload times, ~half tick more range than both
14.96"/52 SK C/34o L - Fastest of the lot according to the given values, but just a bit faster IMO in-game, range is roughly the same as the SK C/34 L (OPro1 set), maybe a hair more.

Bissy guns seem to be more consistent in spread., but all the gun sets have good spread. Not sure about the weight differences between the SK C/34b L and the SK C/34 L if you want speed.

many thanks everyone! I look forward to using these guns/ships =)