DOUGHNUT without neck turning?

[Vassal]

HEllo,
I was loading some 6.5BPC rounds this morning (Grendel in a bolt gun) and realized an issue that has been happening for the last couple of loadings - at least - Dreaded Doughnut. based on pictures and reading I have done it is DEFINITELY the Doughnut, a .264" bullet slides right in a fired case, tightly, but comes to rest on the doughnut, and loading rounds I can feel it BIG TIME. I can't believe I didn't realize last time I loaded the cases. The problem is this brass has never been turned! I shoot Lapua brass in a chamber that allows about .001" on each side of a loaded case. WHAT? Can it be? I don't get it. The cases are around 13-15 firings, and I go pretty hot.

I guess I have to get a neck reamer, so I can firgure out the solution to this brass, BUT I don't know why it happened,,? Can someone enlighten me?

 

[BoydAllen]

Perhaps a review of what happens when a round is fired and subsequently sized will be helpful.
When you pull the trigger, the firing pin strike, and the pressure of the primer's explosion against the front of the primer pocket, push the case forward in the chamber. The pressure created by the burning powder creates enough pressure so that it is held there by friction between the body of the case and the chamber wall. As pressure continues to rise it exceeds the yield strength of the brass near the head, and the case stretches at that point, sufficiently so that the head of the case comes back into contact with the bolt face. This stretching causes a slight thinning of the case just in front of the web. The greater the stretch, the greater the thinning. This is why paying attention to shoulder bump is important.

When the case is FL sized. the shoulder is moved back, and the diameter of the body of the case reduced, and in the process the brass at the top of the shoulder is forced into the bottom of the neck. Since the shoulder is thicker than the neck, the neck is thickened at its base.

Another issue is incorrect design of close neck chambers. If you section an unturned case, you will see that the wall thickness tapers, including the neck. For this reason factory chambers have necks that are tapered, so that a constant clearance between the loaded round and the chamber can be maintained. Often, those who specify reamers for close neck chambers forget to put in this taper, with the result that the loaded round has a decreasing clearance from mouth to where the case meets the shoulder.

I see no merit in your .001 clearance on unturned necks. Some of the best short range benchrest shooters do very well closer to .003 than .002.

I am curious about how much you bump your case's shoulders (from what a tight case measures) , how much your FL die reduces the diameter of fired case at the shoulder, and how much case length growth you are getting per sizing.

If your chamber was large enough at the neck shoulder junction, and you had a FL die that was larger than normal in this area, the thickening would not matter, because it would not intrude into the ID of the base of the neck. I have discussed this approach with a reamer manufacturer, and he tells me that it has already been done, and evidently works. With a bushing die, and a larger chamber, at the base of the neck, spacing the bushing a little higher than the shoulder should be a usable workaround. Perhaps a custom chamber neck reamer...?

 

[rcw3]

How about another aspect - there's corner at the junction of the neck and shoulder of the brass that does not blow out or hold the blown out shape as easily as the basic neck metal (i.e. like angle iron is harder to bend than flat iron), so a donut is almost mechanically inevitable unless you have enough chamber clearance there (by way of a bigger radius at the junction of the neck and shoulder or raw neck clearance) so it blows out there and stays out of the way.

Robert

 

[Vassal]

OK I am not QUITE sure I understand WHY a "less than loose" chamber at the neck shoulder junction would cause a doughnut but I DO think that is what you are saying.

OK I double checked the measurements and I actually have about .003" TOTAL neck clearance on a loaded round - I was going off memory this morning and must have been confused. The Reamer is the standard 6.5BPC reamer from Pacific tool& guage. My die leaves the shoulder Diameter about .003" smaller than after firing. I bump the shoulder about 1 thou when I resize.

This has been a gradual thing. assuming I don't know how I haven't noticed untill now but it didn't get bad untill after at LEAST 9 firings.

I am using standard grade Redding FL die with a Lyman M die to expand.

Do you think It will make sense to do some chamber modifications (Me=no lathe and I'm not willing to experiment by hand on this barrel even with the new hand reaming guide from P T&G.) or to rather just remove the ring somehow and monitor the brass?

What is the best way to remove the ring? My neck turining tool won't work because the brass springs back too much! Eveen AFTER expanding the ring is still pronounced enough to act as a ledge for a bullet! I guess the neck turner MAY work in several stages - meaning expand, neck turn the amount that was pushed outward, expand again, repeat, repeat. HMMM Is that reasonable?

I really appreciate you guys taking time to thoughtfully read and write inforamtive replies. thanks
VASSAL

 

[rcw3]

Additional item - The more you fire and re-size brass the more it work hardens and has more spring back after firing.

Robert

 

[MrMajestic]

If it took 13-15 reloads to develop this why not toss it and buy new. With that kind of life it don't owe you nothing.

 

[BoydAllen]

Have you settled on a particular bullet? Do you have magazine length issues to deal with? The reason that I ask is that there are a couple of approaches that you may want to consider. I understand that Forster will make inside neck reamers in custom diameters. I have not used them, but have read that others have, for the same problem that you have. Depending on whether you have a magazine that imposes a maximum length, or if you have some room to spare, you might consider having a gunsmith cut your throat to a length that sidesteps the issue. BTW how does the rifle shoot?

 

[TonyR]

Wilson will also make inside neck reamers to whatever diameter you want.

 

[Vassal]

OK this is a bit embarrasing but I got some thoughtful replys so I will post it. After thinking it through some more, and repeatedly running the brass over the M-die mandrel (which "fixed" the issues at the range the next day) I realized what it was - CARBON. I had not been using any sort of real cleaning device, simply cleaning by hand and a soak in BC case cleaner. It is REALLY stupid of me not to have realized, because there has been a permenant black ring on the outside of the case which I rubbed on a little when I cleaned but did not remove totally. When I sized it (repeatedly) into the case geometry it caused a bulge on the inside big enough after so many firings to cause trouble. DERR! I got an ultrasonic cleaner and have resolved the problem.

PS - I do anneal my cases and anyone fortunate enough to have dealt with PPC type case can tell you Lapua brass properly cared for and sized will last A LONG time. I may only be about HALF done on these; and they are pricey! THANKS

 

[BoydAllen]

I try to avoid removing all of the powder fouling from the inside of case necks. I run an old bronze bore brush in and out of fired case necks three or four times, leaving a translucent layer of powder fouling. I find that this gives me more uniform bullet seating force. No one, that I am aware of, that shoots short range benchrest, wants his(or her) case neck interiors spotlessly clean, for the reason that I have stated. I clean the outside of case necks with 0000 steel wool. It works just fine, and does not take long. I just clean enough to remove the powder fouling, no more. IMO the whole spotless brass thing seem to have taken on a life of its own, that is independent of any measurable difference at the target.

 

[mikecockcroft]

Sounds like he said the carbon ring was on the outside of the neck? Causing the sizing die to push the brass or donut into the neck. Did I read it correctly?

 

[BoydAllen]

Yes, I agree, that that is what he said. What I was trying to point out was that by using ultrasonic cleaning to remedy the problem that there was the possibility that he was creating another problem....neck interiors that are too clean.

 

[robhen]

Yes, I agree, that that is what he said. What I was trying to point out was that by using ultrasonic cleaning to remedy the problem that there was the possibility that he was creating another problem....neck interiors that are too clean.

 

[robhen]

I am 80 years young and don't understand why to clean inside neck is the problem. Any help would be a preciated. Bob

 

[savagedasher]

Donuts happen when the brass It being stretched foward from the bolt face .
The neck is 3 times smaller then the case . Any time you move three time the materal in a smaller area its going to build up That is the same as trimming the neck if you neck down you should trim up is another story.
Larry

 

[Eddie Harren]

Donuts happen when the brass It being stretched foward from the bolt face .
The neck is 3 times smaller then the case . Any time you move three time the materal in a smaller area its going to build up That is the same as trimming the neck if you neck down you should trim up is another story.
Larry

I need that darned translator again!

 

[weejohn]

I had hoped to avoid neck turning for my .284 using necked up 6.5x284 brass but after the first firing I had the dreaded doughnut .

 

[mikecr]

I am using standard grade Redding FL die

So you're FL sizing the neck, including donut area?
On firing the neck hammers to your chamber neck, and as Boyd mentioned, unless tapered chamber neck this will drive the thicker near shoulder brass inward(donut). Ideally, partial neck sizing leaves it alone for the most part, and neck expansion would then drive most of the donut back outward. Then you would seat a bullet with it's bearing clear of this area.
But FL sizing of necks muddles the whole plan here, as you're further driving the donut brass into play with this. 'Into play' whether seating bullet bearing into donut or not, as you're still bringing the donut into tension contributing (which is never good)..

IMO, it's best to pick up a bushing-body die(merchandised as FL bushing die), and follow posts about adjusting the throat(if needed) to get seated bullet bearing clear of donuts.

 

[BoydAllen]

Given what you have told us, I believe that the best way for you to deal with this situation is to have the freebore area of your chamber's throat lengthened to the point where the full diameter part of the bullet does not touch your doughnuts. In my 6PPC's none of the bullets' shanks reach cases' neck shoulder junctions. If you are managing your shoulder bump as you stated, your doughnuts are the result of the cumulative effect of the amount that your cases' bodies are being reduced in diameter each time that they are sized.

I have a PPC did that does not change the shoulder diameters of cases. It has interchangeable bushings for the base are of the case. Typically the amount of reduction in diameter from sizing, in this part of cases is .0005-.001. Case length growth is extremely slow. The same forces that make cases longer from being sized will eventually produce doughnuts, if the brass just below necks is substantially thicker than that of the necks. This is typical of Lapua brass.

 

[RonAKA]

I do seem to have a bit of a "donut" inside my 6BR cases. I shoot flat base bullets that have a pressure ring at the base. Bullets are a nice slip fit into a fired (and unsized) case. However, they stop when the pressure ring hits the end of the neck. But my case preparation is to neck size only 2/3 of the neck, while bumping the shoulder (Forster Bushing Bump), and I use an expander ball that just kisses the ID of the case on the way out. And, when I seat bullets they are never further back in the neck than the sized portion.

Short story? There may be a donut there, but I don't see how it has any effect on anything.