... some more pics of the P40 ... disassembled, showing markings on the frame.
Is the marking on the frame expected? Aside from one mag of WWB 165gr functional testing, this doesn't appear to have been fired much at all.
Those markings are perfectly normal. It looks like yours hasn't been shot much. Not unusual with P-40s. People would get one, shoot it a time or two, become convinced it's too much, or not reliable, then sell it. They are not for everybody. I got my first one as a novice shooter while looking for a P-11. Fortunately, I was a member of this forum, and was able to find out what to expect from it, and how to deal with it. I eventually did get a P-11, but I shoot the P-40 better. Maybe my P-11 is just not as good a specimen as are my two P-40s.
Make sure you grip it firmly. A light, powerful gun needs to work against your grip to function reliably. You are essentially the mass that it doesn't have. (Supposedly, the reason this pistol was discontinued was that Kel Tec got tired of people sending in guns for service that were actually functioning correctly--when held firmly enough. It puts the company in the awkward position of having to tell customers that they don't know how to shoot this gun. It seems that experienced shooters often have more problems with this gun than do novices. It's not a 1911.) Loading JHP rounds from a full magazine might result in the bullet nosediving into the feed ramp and not going into the chamber. I don't know why some (most? all?) of these do this, but I load the chamber with just one in the magazine, then load the magazine to capacity and insert it. This doesn't seem to be a problem when actually firing it. Also, don't get too upset if there are some failures early. Until you know
it has 200 rounds on it, a few hiccups are not out of the question. Clean and lube it frequently during this period.
The advice to stay away from 180 gr ammo in these guns is wise. I have shot some 180 gr in mine with no ill effect or malfunctions, but it does better with 165 gr or less. I carry 135 gr CorBon in mine. Another one I have carried is Remington Golden Saber 165 gr. I have seen bullet setback with Speer Gold Dot 165 gr. This can cause excessive pressure, and happens because of recoil banging the nose of the bullet against the magazine. The 155 gr ammo you have been using sounds like an excellent choice.
Too long a range session will leave your hand feeling like a claw. Once, I shot a whole box through the P-40. I switched to my Glock-17, and my hand was so stiff and numb, that the Glock got to bouncing against my trigger finger, and I essentially bump fired it. I ripped off 10-12 rounds before I stopped it. The other shooters thought I had a full auto Glock-18. That's one way to get stared at on the range. (I haven't been able to reproduce that grip.