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Cutting Into the Shoulder When Turning Necks

We customarily cut slightly into the shoulder when neck turing in order to prevent a "doughnut" from forming in the inside neck wall. I have seen two reasons advanced for that: (1) to assure consistent neck tension and (2) if there's a doughnut it could restrict or impede the flow of gas up the neck.

Even with my simple layman's knowledge, point 1 makes sense to me -- we want to have consistent neck tension. But it seems to me that if the bullet is seated out far enough that the shank does not contact the neck/shoulder junction, then a doughnut would not be an issue as far as neck tension . . . unless I'm missing something here.

I'm curious, though, about point 2. Does a doughnut really have an effect on gas flow to the extent that it would show up either in terms of uniformity of velocity or the accuracy of the load? Or is it an old wives' tale?


Thank you,

Dave Rabin
 
Not being an engineer, I can't provide mathematical proof of my theory, but when I look at the cross section of a case with "donut" influences I see that the path for the gases from burning powder have to pass through a space that is narrower than intended. It seems to me that the pressures developed during ignition would be greater with the donut than without it. Without the donut the path for burning gases is unobstructed, except of course for the bullet. With the donut the path narrows to an opening that is smaller than the bullet's diameter before the gases reach the base of the bullet.
I'd sure like to get some input from an engineer that either proves or disproves my theory. ::)
 
It would be interesting to test. There might even be a tiny benefit to #2 (efficiency).

Problem with #1 comes into play even if seated away from it -if FL sizing necks(which makes no sense to do).
 
I've thought about this and it seems to me that a donut could act like a mini venturi, which you kind of got going on anyway with the shoulder and neck.
 
I'm no fluid dynamics guru but if keeping a donut out of my cases has a positive effect on reducing loss of accuracy, I'm for it.

I suppose there could be some level of turbulence induced by a donut at the shoulder / neck junction but I have no way to convince myself one way or the other whether my supposition has any merit....
 
mikecr said:
It would be interesting to test. There might even be a tiny benefit to #2 (efficiency).

Problem with #1 comes into play even if seated away from it -if FL sizing necks(which makes no sense to do).

Mike,

OK, you've piqued my curiosity: why does #1 comes into play even if seated away from the donut if I am FL sizing necks? I assume you're referring to sizing the neck all the way down to its junction with the shoulder?

Thanks,

Dave
 
Lapua40X said:
Not being an engineer, I can't provide mathematical proof of my theory, but when I look at the cross section of a case with "donut" influences I see that the path for the gases from burning powder have to pass through a space that is narrower than intended. It seems to me that the pressures developed during ignition would be greater with the donut than without it. Without the donut the path for burning gases is unobstructed, except of course for the bullet. With the donut the path narrows to an opening that is smaller than the bullet's diameter before the gases reach the base of the bullet.
I'd sure like to get some input from an engineer that either proves or disproves my theory. ::)

I sort of look at it that way, too! I have been shooting a couple of wildcat 7mm RSAUM cartridges for F-Open since June 2011 utilizing either Norma .270 or Norma .300 WSM brass to build from. In each instance of case forming, the shoulders are pushed back in a standard 7mm RSAUM FL die. ALL of the Norma .270/.300 original shoulder that is laid down to become "new neck" on the wildcat case by this process becomes one broad 3/8 to 1/4-inch donut (and not something that you can easily press a bullet into; nor would want to). I don't bother to cut it out...it can be; I have, but found no value in doing so. My reamers are named "7mm RSAUM Gooseneck" and "7mm RSAUM Gooseneck Improved" (Ackley Improved).

I do get by with a up to 1/2 grain less powder with these two wildcats. (Yet, I can use up to 1/2 grain powder more than with my original straight 7mm RSAUM load without encountering pressure.) Possibly, derived by a ventori effect?? Or, possibly, from the "Bryan Litz effect"...recall his articles this Summer listing the advantages of less pressure/increased velocity/improved accuracy as a consequence of seating the bullet further out into the case neck? For my Goosenecks, no part of a seated Berger 180 grain Hybrid protrudes below the case neck into the shoulder area, only using a portion of what was the "original" pre-wildcat neck. So, who's to know from where the benefit comes? Maybe both have a share?

Danny Biggs
 
brshtr said:
why does #1 comes into play even if seated away from the donut if I am FL sizing necks? I assume you're referring to sizing the neck all the way down to its junction with the shoulder?

If a neck expands .000001” the bullet is totally free from it’s grip. Hardly nothing, but it takes energy do this.

Bullet grip amounts to springback energy of the neck against bullet bearing area. Springback can be changed regardless of bullet seating.
When you partial NS with a bushing, the springback is added only to the sized portion against bearing.
When you FL size the necks, you add springback from the FL of the neck(to apply against seated bearing).
Even if not seating bullet bearing to junction, the additional energy is still in play, and pressure rise will have to overcome more to release the bullet.

Now, it's hard enough to produce consistent tension with necks of the same thickness. Adding to this the energy of springback from varying thickness donuts makes no sense, as there is no potential benefit in it. Why would anyone actually want to size the FL of necks?
I can think of nothing but bad in this.
 
Here we go again with the donut thing, I for one have always been of the belief that donuts caused problems and will stay there, however there are a lot of real brain teasers on this forum and they make you think out ones beliefs. As for turning the necks into the shoulder most good neck turning tools have a taper on the cutting blade that sorta insinuates that it was designed to at least go to the edge of the shoulder. Just be careful not to go to deep as it will cause shoulder separation.
P A Larson
 

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