Reader Frank B. Oudkirk recently asked how to make .243 Winchester cases from .308 Winchester brass successfully. He related that when he runs .308 Win. brass through his .243 Win. sizing die, it results in a ring bulge at the base of the resized case that will not allow the case to chamber.
My short, quick answer: You just have to use the right tools in the right sequence to avoid damaging the case.
Necking Down .308 to .243: A More Detailed Answer
Until I received his inquiry, I'd actually never tried to neck down commercial .308 or 7.62 NATO cases to form .243 Win. brass. I was surprised by his ring bulge problem because the 7.62 military round was the parent case Warren Page used to form his original .24-caliber wildcat that was eventually adopted as the popular .243 Win. I have encountered a doughnut ring formed at the junction of the case neck and shoulder when necking down brass. When that occurs, I can usually true up the internal neck diameter with a precision neck reamer.
To better answer his question, I selected a few pieces each of Hornady and S&B .308 brass and some Lake City military 7.62 brass. Before attempting the reforming process, I measured and recorded the case head diameters and neck wall thicknesses. The Hornady brass was unfired, and the case head diameter measured 0.468 inch and the neck wall thickness ran 0.013 inch. The S&B brass had been fired once, so the expanded case heads measured a bit larger (0.471 to 0.472 inch) and the neck walls measured about 0.014 inch. The Lake City military case heads ranged from 0.472 and 0.473 inch with the same 0.014-inch neck wall thickness as the S&B.
Using a Hornady .243 Win. full-length sizer die, I reformed a couple pieces of each. I had no problems with the new commercial brass; however, the case neck thickness increased to 0.014+ inch. The cases easily chambered in my Thompson/Center Venture bolt-action rifle.
The S&B and Lake City cases did not fare so well. Each time, the full-length sizer sheared a sliver of metal from about a quarter of the way around the case head, and one of the military case necks collapsed. After that happened, I resized the next ones in small increments, forcing the case into the die a bit, then withdrawing it and rotating it slightly before pushing it into the die a bit farther.
As the neck diameter was reduced step by step, the unsupported part below the smaller diameter bulged out severely before finally conforming to the internal die dimensions. Apparently, it can be hit or miss as to whether the neck will fully reform correctly without collapsing. Using an intermediate caliber sizer, e.g., 7mm-08 or .260 Remington, would be a prudent precautionary step in the process.
Then I took three more S&B and a couple more Lake City cases and pursued a more indirect process. The S&B case heads measured 0.470+ inch, and the Lake City brass measured 0.472 and 0.473 inch. I decapped the crimped-in primers in the military brass (to reduce total force required later when resizing), lubed the body and inside necks of all five cases, and sized them in an RCBS .308 Win. full-length sizer die with the decapping rod assembly removed. Afterwards, the case heads all measured between 0.469 and 0.470 inch.
I then full-length sized one Lake City case and two S&B cases in the Hornady die with no problems. I sized the other Lake City and S&B cases in an RCBS .243 Win. full-length die with no case damage. I also reformed a couple pieces in an RCBS .243 Win. trim die and then neck sized them. Using the .243 trim die first will almost surely preclude damaging a fired .308 case head when it's full-length resized.
Since I'd carried the process that far, I went ahead and loaded a few of the reformed .243 Win. cases to see how they functioned/performed in the T/C Venture. I used a charge of 40.0 grains of Vectan Tu7000 propellant, CCI BR-2 primers, Hornady 100-grain BTSP InterLock bullets, and I fired the handloaded rounds at 100 yards. Both three-shot groups measured just under an inch. The small ripples on the reformed case shoulders were pretty well ironed out when the rounds were fired.
So, as I said earlier, it's reasonably easy to reform .308 Win. brass into .243 Win. cases — or any other .308-derived cartridge case, like the .260 Rem., 7mm-08, .338 Federal, or .358 Winchester — if you use the right tools in the right sequence. The resulting increased neck wall thickness caused by necking down is usually of no concern because the throat diameters in most commercial rifles are typically large enough to accommodate the difference. However, it pays to measure the loaded neck diameter and trial fit a loaded round in the rifle. If it's difficult to chamber, the outside neck walls will need to be turned down to correct the too-thick condition.
Keep in mind you can buy a lot of new .243 Win. brass for the price of a trim die or neck-sizing die or neck reamer or neck turning kit. It may be better economics to just buy new .243 Win. brass to handload.