There are 2 types of 5.25 AA guns - one with "RP" in the name and one without. Their ranges are the same, meaning

Range of 5.25N = Range of 5.25RP N
Range of 5.25L = Range of 5.25RP L
Range of 5.25D = Range of 5.25RP D 

And above all three, 5.25RP L (and 5.25L) has of course the longest range. The only differences of these 2 sets of guns are:
1) 5.25 set of guns are lighter and requires less turret space than 5.25RP;
2) AA shells of 5.25 set of guns have a little bit more damage (116) than 5.25RP shells (100).
3) 5.25 set of guns have significantly longer reload time than 5.25RP guns.

In battles, I use 5.25RP L for AA. This type of gun is for sniping scouts and DBs, and shooting around Ftrs when you are bored... I average at 2000 AA credits with this gun in the DD.L Class, 3000 AA credits with this gun in CA.County.

In many cases, your T turret space only allows 5.25RP D, and this type of gun is generally not designed for sniping scouts (because of its poor range). I use this gun on CA.Furious and average at 3000 AA credits for shooting down Ftrs and DBs.. (CA.Furious only allows 4 boxes of 5.25RP D shells...how ridiculous...)

Technically there's the 3rd type 5.25 gun, which is 5.25RP A, which is only operatable by UK AA Gunners. The range of this gun is a little bit shorter than 5.25D and 5.25RP D, and the reload time is a litte bit faster than 5.25D and 5.25RP D. Apparently this gun is not designed for sniping scouts either, but may be good for AA-ing DBs and Ftrs. Personally I would not use this type of gun because its range sucks too much. And it's too costly to reach the reload cap for this gun. You will need to level your gunners from level 22 to 120 directly, then class them to AA gunners. 

Actually I believe the 5.25" RP10 (the higher level one) has a faster reload. I would recommend it above the previous one, it worked wonders on my Malta Project. Freaking lifesavers, used it to sneak up the border and eliminate two enemy CV's with guns in GBII one time.

**Edit** After doing some digging in the Components section. There is actually only one 5.25" RP10, which is unlocked from level 30 to 32 depending on which submodel you're talking about. The other 5.25" are just 5.25"/QF 50, and while they probably do have the same range (I never used them) the RP10 has a faster reload. Basically, just echoing what jedizorro said.

@ Jedizorro;

You may strongly believe that 900 SD is not the cap and that more restore may help, but you are dead wrong about that.

Just to humor Nay sayers such as you I did a test a while back, you'll find the result here:
http://fm.en.kupaisky.com/Community/Forum/View.aspx?num=8925&searchtype=1&pagecount=0&searchvalue=danita&sort=5&category=D01&page1=1

im using rp 10 but cant seem to get the angle dialed in. what is golden angle to start with