vaguru, No offence but we aren't playing from the same deck. About 10 years ago, I decided I needed a super accurate 1911 so I went to my friend in LeClaire, IA ... Mr Les Baer. He sold me a nice stainless target model for a mere $1600 that was guaranteed to shoot 1/2" groups at 50 yards from a Ransom Rest. After I bought the gun, one of Les' snuffies took me to the range and proved my new gun would indeed hold groups under 1/2" at 50 yards. When I got home, I tried to replicate the load ... the best I could do was about 3/4" @ 50. I weighed each powder charge, use high quality Speer bullets that did not vary a tenth of a grain, and I was using Winchester large pistol primers. When I chronographed these loads, the max velocity spreads were under 10 fps from fastest to slowest in a 10 shot string. That's when the lights came on .... I was using new Starline brass .... known for its good quality but when I checked case lengths, I was hard pressed to find more than a dozen out of 500 that were at the true SAAMI max length of .898". Nearly all the new Starline brass was .005" .... a few as much as .010" too short. I dug through my resources and found a bag of once fired GI 45 ACP cases with a WCC 86 headstamp. Most were at the SAAMI max or just a few thousandths more. I ran a batch of them through my case trimmer set for exactly .898" then went on to load up a test batch. Sure enough, that did the trick .... just that .005~.010 extra headspace caused a considerable difference in accuracy. Granted, if I didn't have something to prove to myself, I would have been plum tickled with 3/4" 50 yards.
What I learned from this exercise was ... headspace can make a notable difference in accuracy. Using this same logic, I tested my CZ75b 9mm pistol and found headspace with a full SAAMI length case (.754") was about .005". Most 9mm pistols have .010" or more headspace, assuming full length cases .... which means the CZ is pretty tight. I sorted through a bunch of once fired range brass and came up with about 25 that measured the full .754". I loaded these with Nosler 124gr JHPs and off to the range I went. I've never seen a 9mm pistol shoot better than 1" at 50 yards by my CZ did .... my best 5-shot group was just over 3/4".
On to my next adventure .... my 10/22 build. I had improved accuracy tremendously by installing a Green Mountain 20" bull barrel, 3~10X Nighteater scope, and a Fajen target stock ... plus a very light trigger pull. At 50~70 yards it is a "one holer" but at 100 yards, even with the best match grade ammo, about 3" was the best I could muster. I measured headspace (Ruger factory bolt) and found it was excessive ... about .030" too much. I machined the face of the bolt until I got headspace down to .005" .... a bit tighter than I intended. Wow! At 100 yards with Wolf target match ammo and shooting from a sturdy bench rest, I was pounding them in the bullseye with groups measuring about 1". Wow, a 1 MOA 10/22 .... at least on a very calm day!!!
I guess the point of all my rhetoric is ... excessive headspace allows the cartridge to move in the chamber and its movement will cause the bullet to enter the bore at a microscopically different angle. This results in minor bullet damage .... virtually no difference at 50 yards in my 10/22 but at 100 yards it really showed up .... just like a handgun at 50 yards. Is this level of accuracy needed? Absolutely not! When the Les Baer 1911 was in my hands, I doubt if I ever shot a group much under 1" at 25 yards. That said, my shooting buddy could shoot one tattered hole at 25 yards so a few years later, he became the proud owner of the Les Baer 1911 ..... and my old Ransom Rest that I bought new in 1979.