I was at the range today shooting the pistols. I started shooting a SW 1911. After 4 shots the slide jammed about halfway and it seemed to be locked up. After looking it over for a few seconds I noticed that the slide lock pin was missing. Looking around everywhere I saw it laying on the ground a few feet in front of the table. I was at an indoor range so I had the range boss help me and he took it to the onsite gunsmith. He extracted the spent cartridge and inspected it. He determined that when I had cleaned it and put it back together that I had failed to put the slide lock pin in all the way. He showed me the proper way to make sure it doesn't happen again.