+1 - I personally don't trust less than 4" on a .45ACP 1911 platform gun for that reason. IMO the barrel angle gets too radical when the slide is in full recoil. I think the EMP in 9mm may have enough different geometry to be OK, kind of like the old Star BM.
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Many years ago, the department armorer, (a Marine armorer that served on the Marine Corps Pistol team) explained the issues with short barrel 1911's. I will try to relate it as best I can after 15 years.
While that is part of the issue with short topends on 1911 platforms another major part of the issue of poor reliability is the combined issues of short stroke travel, less reciprocating mass, high recoil spring rates and it's associated short "out of battery" time.
In a full or commander length slide setup, the slide travels longer than in a short slide or "officers" length. This is obvious, shorter parts move less. However the shorter length also unlocks the slide from the barrel sooner, (related to the angle of tip on the barrel, freeing the slide slightly sooner, imparting less energy to it) The slide continues to the rear very quickly, ejecting the spent case and reaching either the end of stroke by contacting the stop on the frame OR expending the energy imparted on it by the recoil impulse. (This is a very important part of shooting pistols with high rate recoil springs, if you don't hold them firm enough, the recoil energy that would normally be stored in the recoil spring and slide are dissipated in your hand and arm resulting in less reliable cycling, short, light pistols are the least forgiving of a weak grip or poor shooting form.)
As the slide begins its forward motion, the next cartridge 'should' be ready to be stripped from the mag and fed under the extractor by the breach face. (All these motions are timed in thousands of a second. In the 5" 1911, these events take place in the longest time span, the Commander, the middle ground and the short barrel lengths, the quickest.)
If the short slide has short-stroked from either a slightly under power round or a weak grip, the slide may not clear the rear of the mag well and no fresh round will be picked up resulting in the hammer falling on an empty chamber. If the speed of the short slide has over run the ability of the mag spring to push up a fresh round to the top of the feed lips in time for the breach face to contact and strip it, the same empty chamber will result. The third scenario is where the short slide returns with excessive speed and when the fresh round is almost at the top of the feed lips, but not contacting them to provide resistance to slow down the feed cycle, the breach face strikes the round with such force that the round bounces forward free of the extractor and fails to align sufficiently to allow entrance to the chamber resulting in a stoppage.
OK, I know it's wordy but I related it as best I could from memory.
Tom