I will try to help you out.
First off, The Beretta is a fine reliable weapon, not the most accurate witht the fixed combat sights, but then again, it wasent intended for bullseye competition. JMO.
First off on your aftermarket barrel, i would have the mfg. confirm that the chamber fully supports the .357 Sig round round. If it does, that would eliminate the possibility of damage due to a ruptured case.
The second thing to consider , is, will the round in itself damage the pistol. The main contributing factors(using a factory guiderod and spring) are 1. Bullet weight and velocity 2.pressure curve. Both can have detrimental effects on a handgun frame.
The faster heavier bullets are pushed, more recoil is generated, thus more stress on the frame as it is pushed back and upward in your hand, this affects alloy frames more so than steel frames. The 357 Sig typically shoots 125gr bullets, very similar to the 9mm, so bullet weight wouldnt concern me too much.
Now to pressure. The .357 sig generates a higher operating pressure. With a factory spring that is not strong enough, or has been used a lot, the action may open a few milleseconds early as the slide travel is faster than the firearm was designed for. This causes consistant and excessive frrame battering. This would be my biggest concern.
You can tell if your having this problem by looking at the primer of the fired case. If the primer strike is not totally round but elongated, this is a sign that the action is opening with the firing pin still heavily engaged, or prematurely.
If the barrel you bought is to correct specifications, you can shoot a steady diet of .357 Sig without any problems.
The easiest and most inexpensive way to accomplish this, is to use a good strong spring on your guiderod (WOLFF is a good quality spring) and change it every 300 to 500 rounds fired.
Another option are the specialty guiderod/spring assemblies from Sprinco (
http://www.sprinco.com/index.html). These are expensive, but well worth it, and have a lifetime guarantee against breakage. They offer standard and Cor-bon(for +p loads) models. The guidrod with spring will run you about 85.00 for the Beretta 96.
I hope this was of some help.
Best Of Regards To All, Todd