Wolf Killer, the jdj series of cartridges have been with us a long time. The 375 JDJ is a pretty awesome cartridge in a Contender. I beleive it has taken the biggest critters on earth. I have had one for about 15 years now and use it mostly for South Texas hogs. I have talked with Larry several times and he does like the 375 version too. The 309 JDJ and 6.5 JDJ are also popular rounds,but the are based on the 225 Win, not the 444 Marlin. Johnch's statement on velocites are right on. My favorite bullet is probably the 225 gr Hornady, with A-2520 and IMR-4064. It's all that is needed for hogs. I have played with the 300 grain Sierra SBT and Hornady 300 RN too, it's a little too much but a hoot to shoot.
I can tell you that a good ring/scope mount is a must for this one, I learned the hard way....shearing the base off twice, I then fixed it with using 6 number 8 screws instead of the 4 #6's. No problems since then.
doubleA.......here's a 100 yard group with some 300's