Skip the "Extreme Shock"
.
Lemme get this straight: you've got a GP100 set up as a 38Spl and not a .357?
Interesting.
You can probably get a gunsmith to ream it to 357. Get confirmation over at Rugerforum.com but I'm pretty sure the metallurgy was identical. These 38spl versions of the GP100 were for police and security agencies (and some overseas sales) that didn't want that evil "Magnum"
label.
If you leave it as it, you'll have basically an unbreakable gun. And you'll be slightly more accurate now with 38+P loads than you would be with the same gun in 357 - the difference won't be much and might not be noticable at all. (Err...on edit, that might not have been clear. 38/38+P ammo will be more accurate in 38spl chambers than it will be in 357 chambers. In a 357 gun, you need 357 ammo (even if loaded on the lighter side of "Magnum" territory) to bring out full accuracy.)
The Cor-Bon 110gr and 125gr "+P" ammo is very hot but won't hurt that sucker any. The single hottest 38+P load is probably the Buffalo Bore 158gr gas-checked lead hollowpoint doing 1,000fps from a 2" barrel, probably better than 1,100fps from your four-incher. That critter is a rip-snortin' monster and probably more effective from a 4" tube than most 9mm+Ps.
http://www.buffalobore.com/ammunition/default.htm#38spl
Cor-bon used to sell a 158 just like it and at almost the same speed, but without a gas-check it leaded up barrels something fierce. The BuffBore is the only 158 lead gas-checked load you can get..."gas check" means it's got a copper base-plate to prevent the burning powder charge burning the back of the bullet and plating the barrel with what burns off. Other than that, it acts just like any all-lead slug. This allows you to drive a soft lead slug at rocket velocities.
The Remington 158+P lead hollowpoint isn't as hot as the BuffBore but it's a wee bit stronger (or maybe softer lead) than the Winchester variant. But then again, your 4" tube will probably drive the Winchesters fast enough to work.
The Federal is the wimpiest 158+P, avoid those period.
What else...oh yeah, that Winchester Supreme 38+P is usually described as a 130gr but it's probably 129. Another big-cavity design, good stuff, probably a step behind the Speer 135+P but not by much.
What else...
Are you aware that if you have two loads of the same weight in the same gun, one very potent and one fairly mild, they'll still go to about the same elevation? That means you can do most of your practice with milder 158gr loads, dial in the sights for that bullet weight and then practice just a little with (and carry) monster slugs like the Buffbore 158 or a respectable medium-power critter like the Remmie 158.
Me, I've set up my sights to the 130/135gr range, load 135 for defense, practice with 130gr jacketed ball.
