Guys, just for the record, I've built 8 1911s from scratch, I've done all kinds of tuning on Glock triggers, furthermore I'm an engineer and have had numerous deep discussions about force, momentum, friction, impulse, and energy. So if you want to discuss physics of a gun, I'm your guy. Just so we're on the same page, let's review some FACTS, not "I think", not "It makes sense but I've never tried it", but FACTS.
Let's first give a little credit to gun manufacturers. Don't you think they know as much about the physics and mechanics of their gun as anyone? In the Beretta 92FS and PX4 owner's manual, they describe the direct chamber loading method as an approved way of loading the chamber. They give no warnings that this is hard on the slide or anything of the such. They describe dropping the slide, not lowering it. Sig described it to me in the same way and I asked three times if that would damage anything, especially the extractor. The answer was an no, with no hesitation.
There is a bit of difference in slide momentum in direct chambering and mag chambering. The real issue is does that matter at all. But again, the manufacturers are every bit aware of this as we are. The question is, what absorbs the slide momentum? The case or a stop in the gun? As I recall, and I'll try to confirm this tonight, the slide doesn't space or stop on the round, but on a mechanical stop in the gun.
If there was a difference in impact ON THE ROUND, and I'm not so sure there is, all that means to direct chambering is that we don't have to release the slide from as far back, and direct chambering becomes more gentle than mag stripping. QKS just posted how little motion and hence force/momentum it takes to slip the extractor over the rim of the case. I do it as he described, i.e. I lower the slide to about half its range and then let it go. But let us keep in mind that at least three manufacturers, Sig, Ruger, and Beretta, approve of direct chambering with a full force slide drop.
Some will have no option because their manufacturer does not approve direct chamber loading, e.g. Glock. But to those of us that carry Sig, Beretta, Ruger, and H&K (using their modified direct chambering method), we have an option. That's not to say that those are the only manufacturers that approve of direct chambering, it's just those are the ones we know right now.
So here's the options: As some have suggested we SHOULD buy a precision caliper and test all of our rounds for setback. I presume that would mean every SD round we chamber from the mag. I presume it would also be wise to keep up with how many times a round has been chambered from the mag. I presume when we eventually see setback that we buy an inertial bullet puller to adjust the round back to it's original dimension????
So how many are gonna rush out to buy a caliper? I'm not, well that's not fair for me to say that, I already have a 4 digit 6" caliper and a SS 12" dial caliper, two dial indicators, and a long stroke dial indicator, so I really don't need any more precision measuring equipment. I use them for measuring final cuts etc. on my lathe and milling machine.
It has also been suggested that we rotate our rounds to minimize multiple chambering of the same round. Let's think about that one for a moment. I return to my car from visiting someone in the hospital and I want to reload my gun. I empty the mag, put the last chambered round in the mag first, being careful to keep track of the order the rounds came out of the mag so I can maintain systematic order. I then leave out the next to last round so I can insert the proper round in the rotation into the mag. I insert the mag, chamber the round, remove the mag, insert the final round and reinsert the mag. Smooth.
Or I could simply drop the loose round in the chamber, drop the slide from about half it's range, insert the mag, holster and drive off.
This has been blown way out of proportion. It is depicted by some as a disaster waiting to happen, yet we know of three manufacturers, and that's all we've confirmed at this point, that approve of direct chambering.
So it's very simple, if your manufacturer approves of direct chambering, you can use it at your option and convenience without being concerned about the method damaging your gun.
If the manufacturer of your gun does not approve of direct chambering, then you don't have direct chambering as and option - maybe. As QKS has demonstrated, direct chambering doesn't take a lot of slide force, IIRC, he said about an inch and a half slide drop will do it.