@LHSmith , I have a question that I've never seen answered, maybe you have an answer. How great is the increase in precision between turned and unturned, assuming that your reamer is spec'd to shoot unturned brass with out issue? or is a tight necked chamber a necessity to realize the benefits? (and again, pls quantify)
On the latter question, there was a write-up of some quite involved experimentation with a short-range bench gun in 6PPC in Precision Shooting magazine many years ago. The question asked was on the lines of .... does the benefit come from the small neck to chamber clearances used, or does the undersize chamber with matching thin necks confer any intrinsic benefit on top of limited clearance? Starting as usual with 220 Russian brass, necks were first built up with some sort of metal spraying technology and turned to be true and at the same time give the usual BR 0.003" overall clearance, fireformed and extensively tested. Then a basically no-turn chamber was cut to match brass out of the box with the slightest clean-up turn needed to produce consistent neck thickness values. Then the barrel was set back and rechambered with a 0.262" neck chamber and brass turned to the 0.0085" thickness or thereabouts as the individual would use normally. The thin-neck / tight chamber option gave substantially better groups than the others, despite their all using the same case-neck to chamber clearances. Short range BR rifles and the 6PPC are pretty esoteric animals though. What works here might make no difference to a 308 even if built to full match / BR spec.
I've had people say to me that when the chamber is limited to a minimum floor neck section diameter - which applies to our FTR 308s and 223s in the UK, minimum SAAMI dimensions being demanded (unlike US practice where 0.340" necks are not uncommon) that one should use brass that is best matched to the chamber subject to its overall production quality and that neck turning should be minimised or even omitted as any metal removal increases clearances = bad. So IIRC minimum 308 Win SAAMI is 0.344" (I stand to be corrected here), a consistent but thick neck lot of Lapua 308 is what you want and need, thick here being 0.016" so an overall neck O/D of 0.340-0.3405" on the loaded round. Most Lapua lots are a little thinner at individual readings taken at three points of the neck varying from a bit under 0.015" to a thou' more taking a box of 100 examples, the variance smaller in individual cases usually. I still neck turn for consistency, even if by so doing I'm increasing clearances to the 0.005-0.006" level. Moreover, I have several hundred of the 1980s Norma 308 Win '160 cases', so-called because they are very thin-walled and weigh around 160gn compared to most makes which are 20-40gn heavier. 13 thou' neck-walls are the norm, but some are 0.012 to 0.0125" thickness. So I have batched lots turned to 0.0120", 0125", and 0.013" from these elderly cases (all brand new on starting out). Apart from needing lots of expensive die bushings, I've never seen any disadvantage to using these cases compared to thicker Lapua despite running anything up to 12 thou' total clearance (0.344" chamber and 0.332" neck O/Ds on the 12 thou' examples). However, I'm not talking rigorous test programmes here and as I switched to small primer Lapua 'Palma' cases years ago, all of the large primer models effectively became obsolescent around seven years ago.
This issue of brass make, quality, prepping / batching in high spec but more normal (than BR) competition rifles has always interested me, and one of my planned test programmes is to see how much difference (if any) I can get out of changes / differences here using many makes of brass and comparing batched, prepped, turned etc v out of the box.
My test vehicles will be a 260 Rem FN SPR just rebarreled with a heavy Palma profile 28-inch Bartlein and my 7mm-08 Savage PTA based F rifle with a full F-Class profile 31-inch Bartlein. Makes of brass will be Remington, Winchester (necked-down 7-08 for the 260), Lapua, Norma, Federal, Sako, PPU, and Peterson (260). The two rifle chambers were cut with PT&G 'minimum SAAMI neck' reamers. All brass will be new or once-fired to start with.
I've seen many tests over the years in gun mags that saw primer pockets uniformed and various other mods whose conclusions were that all such work is basically a waste of time and effort. However, these tests were of 3-round groups fired from skinny barrel sporter 30-06s, 308s and suchlike and were talking about single groups that could be up to an inch C to C or more. The only ones I've seen carried out recently and scientifically are a couple in Applied Ballistics'
Modern Advancements in Long Range shooting Vol II which look at neck tension changes including annealing, and flash-hole deburring / uniforming. The neck tensions had very little if any effect on MV / ES / SD levels, that applying also to a very limited annealing trial with an AMP machine too (to be revisited in depth, Bryan Litz says). Flash-hole debur / uniforming really works - likely the cheapest and easiest improvement you can make to your cases.
