Constructed as an improvement over the P's, the USS Salmon, commissioned in 1938, was the first of six boats in this class. The following year, ten boats in the Sargo class,  virtually identical to the Salmon's, were launched. Larger in total length and displacement, the Salmon class subs were also one knot faster then the P's. Silent Hunter depicts the Salmons as having four forward and four aft torpedo tubes, carrying 20 torpedoes.  Information that I have found lists the Salmon class subs as having six forward and two aft tubes, and internal stowage at 24. The USS Stingray (SS-186) was fitted with two additional tubes on the external part of the pressure hull giving it ten. 

A serious design flaw with the main induction hatch accounted for the death of 23 crewmen aboard the USS Squalus. The main induction hatch's function was to seal the large air intake valve to the diesel engines when the boat submerged. Due to its poor design, this hatch would occasionally fail to properly close. The sinking of the  Squalus was nearly repeated with the USS Sturgeon and the USS Snapper but fortunately without any fatalities. Once the cause of the tragedy was discovered, it was easily resolved, but not before casting permanent damage to the reputation of these subs. Most of the boats in these classes conducted at least eight war patrols. The USS Stingray has the distinction of holding the record for running the most patrols of any US sub during the war (16).