The thing with the 130-vs-P-3 is not just ROOM, per se, because obviously the 130 has plenty. What it doesn't have is space UNDER the main deck in which to put a bank of PSLT's (Pressurized Sonobuoy Launch Tubes), which is where a P-3 carries its sonobuoy loadout. To be effective, an MPA aircraft needs to carry at LEAST 80 (and that's just to equal the P-3, the MMA carried far more). The tubes are mounted vertically, and are about 8 feet long and have to fire out the bottom. Now, look at a 130 again...where's the landing gear? Right...the bottom, which is also where you need to put the bomb bay. On an ORION, the gear is out on the wings, under the #2 and 3 engines. That leaves the bottom of the fuselage for the PSLT's and the bomb bay. The deck inside is flat, and thus leaves the space under it for those things, plus some electronics.
You need a bomb bay because the torpedoes need to be kept warm to function right. So does other ordinance we can carry. You can't carry them on the wings, like missiles.
Look again at the 130...the main deck in the cargo area is simply too low to allow the PSLT's to be installed correctly to function right. The aft section is all cargo door, so no help there.
Also, like I said, it's not manueverable enough. ASW and marittime patrol ops require hard turns at LOOOWWWW altitude, often in stormy, rotten weather. 130's aren't made for that kind of abuse. It rolls slower and turns slower than an ORION.
You could probably, with a LOT of design work and metal-bending, work a 130 to carry the loads, but then you add weight, which further degrades the agility. Plus, you've basically created a new airplane anyway. In any case, with the electronics and systems getting smaller, you now have lots of wasted internal space.
Hope that helps. I've got nothing in particular against the 130...it's a great airplane with an outstanding history of service. It's just not what you need for ASW. Believe me, if it was, it'd be used for it.
Cool! Thanks for the good information. That's what I love about FR.
There is a great photo of the underside of the C-130J at post 19 on this thread: http://www.freerepublic.com/focus/f-news/1331873/posts
There is plenty of room to put in a center line bomb bay between the pods holding the landing gear. The cargo bay is also tall enough to accomodate 8 ft vertical tubes for the buoys.
You are probably correct that an updated P-3 would be a better bet, but a 130 conversion would probably be more suitable than a 737.