First off be VERY careful with the gun. A good gunsmith can tune a trigger to sub-2lb weights but most Government Issue 1911's would be very hard to lighten to that degree w/o creating a situation where the gun can go off w/o touching the trigger. Too light and the sear can release when you take it off safe or can go Full-Auto on you w/o warning.
You likely have a barrel that is worn out. If rounds that head space properly in a newer gun get "swallowed up" by your chamber, It is probably time for a new barrel.
The 1911 and most auto loading pistols use controlled feeding similar to a Mauser rifle or Pre-64 Winchester. As the round is pushed out of the magazine the rim slips in behind the extractor. You should NEVER drop a round in the chamber and close the slide on the gun. That WILL eventually damage the extractor. You are correct that the headspacing is on the case mouth so too much crimp is a bad thing.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball