You are not alone...I am going to build a 1911 A1 (Essex most likely) and I was wondering about the same; I have put together an AR15, and headspace is a big issue, so I guess that is the same here, we are dealing with pressure anyway...[
.45 ACP headspaces on the case mouth. That's why it's important to TAPER crimp your rounds, not roll crimp (should you reload). A roll crimp or a short case length could cause headspace problems - although too short brass is rarely a problem with quality factory loaded ammo, and quality factory brass (again, if you reload).
Most centerfire auto pistol cartridges headspace on the case mouth, a notable exception being the semi-rimmed .38 Super (and even then, most match barrels headspace that one on the case mouth also). That is why trimming your auto pistol cases to length is important after you're reloaded them more than a couple times.
I've heard people joke that you can keep reloading a .38 Special case until it becomes a .357 Magnum.
Yes, the .45acp was designed to headspace on the cartridge mouth. In reality though, most .45acp cases are too short to reach the headspacing shoulder in the chamber. Most of the time the cartridge head is simply held against the breach face by the extractor. I have measured once fired cases as short as .884; a full .014 shorter than max. I am definitely not an expert. I've never built a 1911 or fit a barrel. But it seems like headspace would be the distance from the locked breach face to the shoulder in the chamber. I suppose this might be determined by the fitting of the barrel link/lugs and the barrel hood. Don't take this to the bank, it's just a guess on my part.
Hows 'bout some of you 1911 smiths jumping in here.-TR
I know that I should be letting a gunsmith answer this, but . . .
An AR-15 like many rifles headspaces with a threaded joint to the reciever. Because this joint can be mismatched in the headspace direction(front to rear), headspace must be checked after assembly.
A 1911's headspace is much simpler. The barrel butts up against the breechface with spring pressure. The locking lugs also hold it in place during firing. Basically, headspace can be mostly controlled by the barrel machining, if we assume the breech face to be roughly flat and the slide lugs to not be too far out of position. I suppose if the slides lugs were too far forward, there could be a problem, but then the gun would not go into battery. Basically the 1911 is much simpler in terms of headspace and proper barrel machining can guarantee headspace. In a threaded joint there is also some assembly control over headspace, thus the check.
Basicly this, not enough headspace or chamber depth and you won't reliably chamber a round, too much and your accuracy might suffer a little, way way way too much and she might not fire at all. There's a lot to putting together a 45.
Most rounds headspace in a range of ~.006" min to max. The 45ACP has a range of .022" from min to max. It is obviously better to have it close to min, but I've seen very accurate guns running at just below max (.920"). You can have a barrel that is chambered at min plus a couple thou and still have chambering problems due to not enough throat ahead of the chamber. Usually shows up with wadcutters or other lead bullets. A throating reamer will fix that problem.