I'll try to field this one.

Since the bullet holding force is relatively small the bullet starts to move out of the neck at a somewhat low pressure. The distance it must travel to clear the end of the neck is very short. The bullet clears the end of the neck before the neck can form at the low pressure. The pressure than is equal on the inside and outside of the neck and the brass will not move as it normally would to seal the gas.As the burning continues and the pressure rises it does so inside and outside of the neck; again the brass does not move.

Since the elastic limit of the brass was not reached in the sealing direction the residual bullet holding stress returns the brass to near it's original free holding diameter. This is the short story. Have a nice Thanks Giving and remember WHO we thank. roger

Henrick, If your accuracy is that good learn to live with a dirty neck. We in the states still live with a lot of red necks. roger

http://www.varmintal.com/arelo.htm

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FITTED NECK CASES.... I take another 4 or 5 cases and adjust the neck turning tool so that the neck diameter (with the bullet seated) is the same or 0.0001 inch larger than the "spring back" diameter above. This is a cut and try process and takes some very careful adjustments on the neck turning tool. After the neck turning tool is adjusted correctly, I load and fire the 5 cases and neck size them and repeat the neck turning without changing the setting on the Marquart tool. This final neck turning will usually take a slight skimming cut of less than 0.0001 inch. I load the 5 cases again without doing any resizing and fire them and then clean the neck IDs with a brush and load them. I feel the neck tension by the force it takes to seat the bullet. If there is ample neck tension so that I can't pull the bullet out or seat it deeper with my fingers, I have the correct neck turning setting. I make no further adjustments to the neck turning tool. At this point I turn all the cases in the lot without changing the turning tool setting. I fire-form the cases, neck size them, and take the final skimming cut. At this point, I have "fitted neck" cases for this particular custom-chambered rifle. These cases give me the optimum in accuracy and I never have to resize them again. If they start to get the least bit sticky in the chamber (hard to open or close the bolt), I reduce the load and check for accuracy. The cases should last indefinitely, since, after the first firing, they are only exposed to elastic stress levels. Most cartridge cases have a limited life because of high stress levels and the plastic deformations of the brass during firing and resizing. I have given up shooting maximum loads. They burn out the barrels and don't give me the reliable accuracy I require when the cross hair is on a critical target.

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bartsche:

Your theory may well be correct, though I don�t know how it could be conclusively proved without the aid of some very complex engineering laboratory equipment. Even then, it might be difficult to do.

I haven�t kept a lot of notes about this non-expanding necks situation, when I have encountered it. Once I had satisfied myself that provided loaded cartridges definitely had an acceptable amount of neck clearance, it didn�t really matter how much the necks did � or did not � expand on firing, I gave up recording all the fine details.

I have found some notes about a 6mm wildcat I had many years ago. With a load consisting of 75gr Sierra HPs and 44grs of 4895, the necks expanded fully on firing. With a charge of 44grs of Nobels No.1 (a powder with a supposedly similar burning rate to 4895, now long gone from the market place) and everything else the same, the necks didn�t expand at all. I never could figure out a logical reason for that. I was using the same lot of cases, so there shouldn�t have been any significant difference in neck tension, and with the burning rates of the two powders being similar, I would have expected the pressure rise-time to also be similar. But clearly, something was behaving very differently.

With my Sako 6PPC, the load that resulted in no neck expansion was 70gr Sierra MKs, moly coated, and a stiff load of H4198. With 70gr Hornady SXSPs, moly coated, and everything else the same, there was some neck expansion, usually enough to just allow a bullet to fit through the fired case necks. With either of these bullets and a near-maximum load of Benchmark, the necks expanded significantly more, though still to several thou less than full chamber neck diameter. There was no abnormal blackening of the case necks with any of these loads, and they all shot very accurately. Because I was only re-sizing about half the length of the necks, I suppose the lower half of the necks formed a �gas seal� even though the case mouths didn�t fully expand.

clark:

Certainly, some benchrest shooters do use the �fitted neck� method, but in places like Precision Shooting magazine or the BR forums you read of a lot of others who have tried it and then given it away for one reason or another. Too fiddly, too accident-prone, or whatever.

I doubt that many people would use fitted necks for any other form of target shooting, let alone hunting. I certainly wouldn�t � sailing far too close to the wind for my liking!

I had the habit of pushing a bullet into my fired cases for a while. Sometimes I find some that wouldn't freely enter the neck. They weren't all that tight though. If I had five I'd load and shoot them as a group, considering them as fitted necks. They usually formed a very accurate group.
When I'd meausre them I'd find the neck thickness had indeed increased. You can turn or ream the necks. If you ream the inside of the necks to remove brass your cases will probably be out of round though. I don't find it a very good solution. It's much better to turn the necks if you have the equipment. Best wishes.

Cal - Montreal

with 32,5 grains of RL 7 the case body had some blackening on it. ( common with too low of pressure)

at 33.5 grains they did not.

seafire