I have a new type "S" neck die with a .266 bushing that sizes the neck to .264 with no sizing button on the decapping pin, and .265 with the sizer that comes with the die set on the decapping pin? I thought the whole idea of the bushings was to give your neck the O.D. of the bushing? My gun is a 6br, and my chamber is .269. When I do the math after I have turned my neck the seated round has a diameter of .267, subtract .001 and the .266 bushing should be the one.

My question is this; is the bushing .001-.002 undersized to let the sizing button expand neck to the final desired tension?, or may the bushing be labeled wrong? or just get a .268 bushing to get the .001 neck tension that I want with how things are turning out?
I want .001 neck tension.

Andy, I have been wondering about this as well. Redding claims this phenomenon - i.e. outside diameter of sized neck ending up SMALLER than listed bushing size - is most commonly observed where cases are sized down a whole bunch (e.g. from being fired in a sloppy factory chamber). See

Redding Article on Bushing Selection

However, I have also observed that it is quite possible to have sized neck diameters end up smaller than listed bushing size, even when brass has been fired in semi-tight chambers. Your chamber/brass is an example of this - you essentially have minimal clearance between your chamber neck and loaded neck diameter. I have a .250" neck .222 Rem. Fired brass neck diameter is about .249" (which would be understandable if you figure .001" springback). Both .246" and .245" bushings produce neck diameters BELOW bushing size - although I would have expected to get bushing size PLUS (about) .0005 - .001....

So I also observe what you do - at least that means there are two of us...

I don't have an explanation for this. Maybe Reddings numbers are simply out of whack (try to measure bushing inside diameter)?? Maybe my measuring tools or skills are not up to scratch?? Maybe there is some rational reason why this effect occurs?? The only (halfway) solution I have come up with, is not to figure neck tension based on the stated diameter of the bushing in question, but on measured diameter of sized brass. In any event, it is difficult to know exactly how much your brass will spring back from being sized - so in a sense you should always determine neck tension from observed (sized) neck diameters. Sadly, this means you may end up buying more bushings than you would, if you could simply count on the numbers with which they are labelled...

One interesting question, which I sadly cant answer: is this effect limited to bushing dies, or do fixed dies (with necks honed to a particular diameter) produce the same effect?? I know that bushing dies are not always optimal in terms of runout produced - maybe these effects come as a price we have to pay for the flexibility of being able to swap bushings??

- mike

P.S. I also wanted to add, it is actually difficult to know in advance, with which neck tension your gun/load will shoot the best. When I first set out to load for these semi-tight necked guns, I also figured .001" neck tension would be optimal - after all that is what Redding recommends. You may well find that to be optimal for you, but there is no guarantee this is the case. A lot of BR shooters shoot with quite a bit more neck tension (.002 - .003), in particular powders like VV N133 are rumoured to like more tension. True or not, I don't know, it has to be tested, I guess. My primary reason to normally want more tension than .001, was that I found it difficult to get CONSISTENT neck tension at such low nominal tension figures. Maybe this has to do with my not turning necks, but I keep observing that brass varies (it seems to be one of the few "universal truths" about reloading...), and I have a distinct feeling that varying "brass tension" will prohibit me from getting consistent neck tension at such low nominal figures. FWIW, YMMV...

Mike,

Thanks for the reply. Very informative

Andy.