Donuts.... Maybe not so bad after all?

[LongRanger]

Anybody conducted a velocity test over their chrono to determine affect of a donut on bullet speed? Logic would suggest a velocity increase since neck diameter at base-shoulder junction becomes smaller. Wonder how much, percentage-wise?

I try to prevent them with good neck turning but occasionaly get the odd one. In the past I have either fixed the donuts or scrapped the cases.

But I know that at least one top LR shooter has his 7mm's set up to "encourage" donut formation to get the speed increase. They sure work for him!

Frank B.

 

[Benjerman08]

donuts sort of turned out to be a blessing in disguise for me. I use a wilson hand reamer to trim out the donuts and it occasionally takes a little off the side wall of brass that is a little tighter than the rest. Has made bullet seating great afterwords. Strangely I never noticed increased mv, but it definately made seating more uniform.

 

[jlow]

In my experience, the concern is not whether donuts affects MV but their ability to consistently do this. If you can absolutely control how big and where the donuts form then it is the same as increasing you neck tension.

 

[Lapua40X]

Under some circumstances, the bullet doesn't seat low enough for the donut to have much effect. However, if the condition is ignored and the bullet is seated deep enough to reach the donut it is possible to create problems, especially in a tight chambered rifle.
An inside neck reamer cures the problem.

 

[jo191145]

wether or not donuts increase velocity I'm not sure.
I fail to see the logic of the wishful thinking though.
Increased velocity is gained by added pressure. So, if the gun/load combo is able to handle that extra pressure would'nt it just be easier add more powder instead of trying to grow a donut?

 

[amamnn]

donut = uncontrollable variable = sorry shooter at some point

 

[Quinc]

Sorry for being a noob.. what is a donut? ???

 

[mshelton]

Hmmm, got me thinking if the doughnut could add to the same principal behind the Ackley style improvement, ever so slightly but could add to it.

 

[Lapua40X]

Sorry for being a noob.. what is a donut? ???

It's essentially an accumulation of brass that forms at the inside base of the neck of a case over a period of time. The "donut" appears as a ring of thickened brass inside the neck of the case. They should, IMO, be removed using a neck reamer. Otherwise dangerously excessive pressures can be developed over which the shooter has no proven method of measuring without strain gauges etc.
Oh .... you're not a boob for asking a question. You're a boob when you don't ask questions and go on your merry way in isolated ignorance.

 

[LongRanger]

I forgot to mention that shooters using the donut to gain more speed are seating their bullets long so that bearing surface is OUT of the donut. (I can't see anything good coming from seating the bullet into the donut.) They are using special-ground reamers to enhance the effect. I assume the neck is smaller at the neck-shoulder junction.

As for consistency, I assume that since the chamber is cut to induce the effect the brass measures the same from shot to shot.... still subject to, of course, internal brass flow.

MShelton is thinking the right direction when he speculates: "Hmmm, got me thinking if the doughnut could add to the same principal behind the Ackley style improvement, ever so slightly but could add to it."

Frank B.

 

[Benjerman08]

The trouble is this: The donuts in your brass do not all grow at the same time/rate. When I ream them out (usually every 4-5 loads), about 25% will have them. Some have a donut just started, others have them more prominent. Cut em out for the sake of keeping your brass consistent.

 

[BoydAllen]

Some of my 6PPC cases have doughnuts, and some do not. The rifle shoots well with both, and with mixed. Bullets are seated well above them. On the other hand, if the bearing surface of your bullets fall within the part of the neck where they show up, I think that it is obvious that they need to be removed. For cases in which the shoulder is blown forward during fire forming, and necks are turned, simply turning far enough down the neck of the unformed case, so that the top of the shoulder is formed from the bottom of the turned neck should solve the problem. I would go far enough so that even with repeated FL sizings that the problem does not show up.

 

[memilanuk]

...at least one top LR shooter has his 7mm's set up to "encourage" donut formation to get the speed increase.

...shooters using the donut to gain more speed are seating their bullets long so that bearing surface is OUT of the donut.

These two statements would seem to contradict one another, unless you think the donut is creating some sort of 'nozzle' effect... not sure how that would benefit velocity.

Honestly... I think someone is yanking your chain just a wee bit.

 

[Lapua40X]

...at least one top LR shooter has his 7mm's set up to "encourage" donut ....

...shooters using the donut to gain more speed a...

These two statements would seem to contradict one another, unless you think the donut is creating some sort of 'nozzle' effect... not sure how that would benefit velocity.

Honestly... I think someone is yanking your chain just a wee bit.

Ditto.
Whenever someone offers a comment like "at least one to LR shooter..." without naming and shooter and, where it is not otherwise obvious, providing contact information for verification I'd suggest ignoring it. The entire concept of using the donut to provide advantages is, IMO, ridiculous. Furthermose, because donut development is inconsistent case to case and can be dangerous, isn't worthy of experimentation.

 

[TrapperT]

LongRanger:

You are referring to the change in pressure/velocity based on Beroulli's equation in fluid mechanics along with the effects of having a "venturi" in the system. The formation of a donut within the case neck does not have the dimensional change to have any effect on velocity of the gases existing the chamber and therefore velocity of the bullet. There would need to be a substantial change in cross sectional area to have any effect above and beyond what is there now. But you can substantially increase velocity by changing neck tension.

I "accidentally" tried an experiment one time when I loaded rounds for my .243 in new Lapua brass. I did not "expand" the necks using an expander mandrel. A load that was once under pressure and safe, became over pressure and faster. Can't remember what the speed differential was, but it was significant such that I quickly stopped shooting and pulled all the bullets.

I am making an assumption, but I feel that the person you are referring to who would encourage growth of donuts to get more velocity was probably seating bullets into the donut and essentially increasing neck tension which in turn produced higher velocities.