15 MOA Inside Case Taper

Now there is a way to put a very small taper on the inside of the cartridge case at the shoulder to neck junction.  This junction in the cartridge case is more known for the suddenly appearing little “Dreaded Doughnuts”.  The importance of this junction in the terms of performance and accuracy seems to come up in many of the shooting articles.  So, is there a tool available on the market to apply a taper inside the cartridge at the case neck to shoulder junction?   If so, why not put a small taper at this junction, and rid yourself of any luring “Dreaded Doughnuts at the same time.  What if I told you that the tool to perform this operation is already on the market, has been on the market for some time, and that many of you may already have this tool.  It just happens that the K&M Neck Turner with the Fluted Mandrel is the tool that performs this operation.  Using the K&M Tool in this matter is not well known or yet advertised.  So this story is about how I discovered and used the K&M fluted mandrel to put a 15 minute of angle (MOA) taper on several hundred 308 Winchester and 221 Fireball cases.

Having been a subscriber to Precision Shooting for around twenty years now, I have read many articles mentioning the “Dreaded Doughnut”.  I being a more realistic standup silhouette shooter, this doughnut thing is probably something that I will never encounter.  Wrong, but a little more history first.  Over the years, I have enjoyed the bench rest articles in Precision Shooting.  Many of the articles gave me an idea or tip that I could use for silhouette shooting.  Would they actually be of any benefit is another story.  Back in the early 1980’s, Precision Shooting Magazine had many articles and discussions on shooting 30 caliber bench rest rifles by some of the real stool shooters.  Therefore, in my stack of old Precision Shooting Magazines are some with yellow post-its® sticking out and flagging a 30-caliber article. In the event, I ever replaced my factory barrels, those flagged articles were my future references.

That day did come a few years ago.  Now I have new Hart barrels on my two standard silhouette (308) rifles.  They both have chambers with a 0.340-inch neck diameter.  A new barrel deserves new brass.  I opted for the IMI Match Brass based on good reports from some local high power shooters just before they all got wimpy and started good scores using AR-15’s.  In addition, the IMI Brass weight was very close to the Lake City brass weight.  I was also hoping that the IMI Brass would have neck walls thick enough and actually require some neck turning to fit the 0.340 neck.  Close but no cigar.  However, better to error on the side of safety, I will turn the necks and have a 0.340 bushing around to test the loaded rounds.

When should one turn the necks, after resizing or after fire forming with some sort of filler?  With 1000 IMI cases to do, easy decision, after full length resizing.  First I full length re-sized all the cases to around –0.004 to –0.005 on the RCBS Precision Mic scale and trimmed cases for over all length.  Then I turned or just cleaned the necks on the IMI Brass.  I set the neck cutter for a 0.015-in. neck wall thickness.  I set the cutter on my neck turner using an old Craftsman automotive point feeler gauge.  The neck wall uniformity on the IMI Brass appeared to be very good.  The case turning effort was minimal and felt uniform all around the case neck.  Turning the brass for a 0.015 neck wall thickness, produced an extremely light cut on the neck area which seemed to be only removing the tarnish or annealing stain.  I was pleasantly surprised on how many cases completely cleaned up all the way around the case neck with such a light cut.  Only about 80 of the 1000 cases seemed to cut somewhat harder on one side, I segregated these cases from the lot.  I also set the neck turner so it would just cut the neck area and try not to go too far into the case shoulder.  In addition, this neck turning operation was not actually required because the IMI brass would fit the 0.340 chambers.  Therefore, it made no sense to be very aggressive in the shoulder area, only to over cut some with shorter necks at this time.  So all the case prep and initial loading went just fine.  Barrel break was a breeze.  Now just re-set the scope knobs and off to the silhouette matches and the world of stand up shooters.

 308 Winchester Cases - Left Case has doughnut
Figure 1.

Everything was just fine until one day I struck a bullet in the fired case.  As shown in Figure 1 (Doughnut.jpg) and describing the cases from right to left side is as follows.  First is a typical loaded round with a moly coated 175 Sierra MatchKing.  Next is a fired case with a regular 175 Sierra MatchKing placed in a fired case and resting on the little case neck doughnut.  Then there is an unfired but prepped case, which I am trying to show the amount of neck turning done during the original case preparations.  Last and the case on the far left, is a case which had some of the neck and shoulder filed away to show the little bright ring where the doughnut was removed.

Therefore, I guess my 308 cases have doughnuts.  I can only detect them by placing a bullet into a fired case.  About 90% of the cases used in one of my rifles are indicating the doughnuts after five firings.  While only 10% of the cases are indicating the doughnuts after two firings in my other rifle.  The boat tail shoulder of the bullet will come to rest at the base of the neck just in front of the case to neck shoulder, as shown in Figure 1.  The bullet fits into the fired case like a shaft going into an air bearing.  It even compresses the air in the case.  In fact if I hold the case horizontally, I can lightly tap the bullet into the case and the air pressure build up will pop it back out some.  Now I have a better understanding of why I am getting the same muzzle velocity with 2.0 grains less powder with the 0.340 verses a 0.346 neck.

The bullet just seems to hit a little ridge just above the shoulder top in the neck.  I have not been very concerned about the little doughnut since I have been using the new Sierra 175 MatchKings.  As shown in Figure 1 the 175 MK boat-tail resting on the doughnut in the fired case is well below the normally seated bullet.  The little doughnuts have been something to talk about and demonstrate.  However, after seeing many ads for the new and improved K&M Neck Turner, the fluted mandrel appears to be exactly the tool I need to remove my doughnuts.

Actually, my first assault on these little doughnuts was going to be with an inside reamer.  I order a 308 inside reamer.  Cute little reamer but it would not fit into the fired case.  Funny that a 308 bullet would fit but a 308 reamer would not.  Therefore, this reamer has been pigeon holed, lost, and found over the past few years.

The second assault on my not so dreaded doughnuts came as a result of reading Mr. McPherson’s article in the September 2000, Precision Shooting Magazine.  The article by Mr. McPherson on doughnut formation, doughnut harm, and doughnut cutters is the root cause for this article.  After reading Mr. McPherson’s article, I just had to get me one of those little K&M Fluted Doughnut Cutters.  Mr. M. L. McPherson’s article gave me the impression that using this tool is a no brainier.  Just follow the simple directions and no more doughnuts.  After all if Mr. McPherson can make it work, it should be safe and easy enough for even a silhouette shooter.

My K&M Neck Turner System came with a little note stating that I may need an Expandiron if the inside diameter was not correct.  Well that’s it for instructions and seems very simple to me.  So, let us stick one of my doughnutized cases on this K&M Doughnut Cutter and see how it works.  Well the 308 K&M doughnut cutter is too small and misses the doughnut.  What is going on?  I get a Forster 308 reamer and it is too big.  The Forster 308 reamer will not fit into a fired case neck hole that a 308 bullet has suddenly vacated.  Then the K&M 308 Fluted Pilot Doughnut Cutter is too small and can not find the doughnut that the 175 grain Sierra bullet can find.  Now what?

How may I rid myself of these little doughnuts?  Use the K&M fluted pilot?  Is it also an inside reamer?  Only that this fluted pilot appears to be too small to function as a 308 Doughnut Cutter in my cases.  However, may I still be able to use it somehow?  Can the fluted pilot cutters be positioned in the case somehow, then also be forced to cut the doughnut area.  Answer is YES.

How to position the case neck on the fluted pilot so that the fluted cutters can attack the doughnut area.  My first attempt, I held the fluted pilot by hand so the flutes on the pilot would contact just the doughnut area inside the case.  Then I turned the case and pilot in opposite directions by hand.  I was able to cut away the doughnut.  The main part of the pilot is just a mandrel without any cutting teeth.  This part of the pilot or mandrel may contact the upper part of the neck inside and will not scratch or cause any harm to the case inside surface.  The little flutes on the tip of the pilot, this is where the cutting action takes place and where the cutting teeth are located.  Just the tip of the fluted pilot must contact the case in the neck to shoulder area to remove my not so dreaded doughnuts.  Then comes the hard part, how do you force the pilot tip to ride on one side of the case neck wall so that the cutter portion will cut into the doughnut area?  I did it on one case by hand. May I do it again?  Yes I may do this by hand, but not for another 800 or so cases.

 Case position for cutting taper
Figure 2.

The case must be position so that short flat cutting section of the fluted pilot aligns with the doughnut area inside the case.  First, I found a rubber grommet that fit onto the K&M 308 Fluted Pilot, this arrangement which got me started.  Figure 2 shows the rubber grommet on the K&M fluted pilot for positioning the case correctly to attack the little doughnuts.  What this procedure ends up doing is cutting a taper on the inside of the neck at the shoulder.  The angle for this taper is small as illustrated in Figure 3.

15 MOA Drawing Showing Calulations

Let us say a case neck is about .240 in. long and the pilot or mandrel is 0.001 inch smaller then the neck inside diameter.  This would allow the mandrel to be angled somewhat in the case neck.  The angle being at a 1 to 240 rate.  Now lets us expand that angle to 1 ft for 240 feet.  That is the same as 15 inches at 100 yards, which is an angle 15 minutes.  Therefore, this turns out to be a very small angle.  So having a fluted mandrel, which is slightly smaller than the inside diameter of a fired case neck, does not mean the K&M fluted mandrel is the wrong size or will not work.  In fact, it may actually be better than being lucky like Mr. Wright and Mr. McPherson and having the mandrel just slip into the case neck and remove most of the doughnut.  After all and theoretically speaking, if the fluted mandrel just slipped into Mr. Wright’s and Mr. McPherson’s case without cutting or marking the case neck and removed the doughnut, it only removed the top of the hill has it passed through.  There is a very small portion of the base remaining.  However, if the mandrel is actually a few thousands smaller and angled slightly in the case neck, you not only remove 100% of any doughnuts, you add a small angle taper leading into the case neck from the case shoulder.  That has to add more to the accuracy and performance than all the primer pocket stuff.  The amount of material removed at the shoulder to neck junction is very small.  The depth of the cut is only about 5% of the gap between to mandrel diameter and the case neck inside diameter.  Again, see figure 3.  Looking back at Figure 1, the material removed shown by the little shiny band is more than just the doughnut.  It is the 15 MOA inside taper.

Now I have a few hundred cases to due and must get some power into this operation.  Somehow, I have to get my 3.2-Volt Skil electric cordless screwdriver involved.  This led to mounting the K&M case holder on my cordless screwdriver.  Now I may slowly power turn the case.  Next, how to position the case’s neck, so the cutters on the tip of the fluted pilot will align with the doughnut.  I need some sort of a positive alignment.  First, I mounted the K&M Fluted Doughnut Cutter Pilot onto a small wooden block.  Using the right combination of washers along with the rubber grommet, I was able to position the case on the fluted pilot so the small reamer like cutter would align with the doughnut area.  Now the case may be power turned using the electric cordless screwdriver in one hand, while holding the small wooden block with the fluted pilot.  Next, force the doughnut area of the case neck into fluted cutter.  First, I just thumbed it with hand holding the block.  Now, the doughnut area inside the case neck is being power reamed.  Then I discovered that slightly tilting the case with the cordless screwdriver, I could also force the cutting action and create the 15 MOA tilt.  Now this was very easy to do.  The process even follows the basic best machining principle, turn the work not the tool if possible.  Turning the case slowing with a low torque cordless screwdriver and holding the tool will keep the cutting action uniform.  After all, you are not removing enough doughnut or case material to measure.  My final no/go test was to have a bullet pass in and out of the case by gravity action only.  Because of the close fit, it takes a second or two for the bullet to settle into the case and a couple of extra seconds for the bullet to back its way back out.  Therefore, I repeat the cutting action until the bullet goes in and out by only gravity.  The process worked well for me.

The Fireball Barrel for my Contender has a match chamber, which only really means the case necks must be turned to fit the 0.245 neck.  Checked some fired cases with a flat base bullet and sure enough, doughnuts.  Same little critters as were in the 308 cases.  So tried the doughnut cutting procedure by hand and doughnut was gone.  Bullet slips into and out of the case by gravity.  Second generation idea, the .22 caliber fluted mandrel was in the K&M neck turner.  I was leaving it set up for the final cut on the fireball cases.  The little cutting teeth on the mandrel stuck out past the edge of the turner housing.  I cut off a small piece of 1/8 inch thick Plexiglas (screen door stuff) the same size has the housing and drilled a .221 diameter hole in the center.  Placed it over the fluted pilot and reamed the hole to fit.  See Figure 4.  The Fireball Case stops at the correct point to taper the inside of the neck at the shoulder junction.  Best of all, all the neck trimming settings were not changed.  Holding the K&M turner housing was easier than the wooden block.  The fluted mandrel maybe adjusted in and out to place the flute flats right on the case neck shoulders.  Now this arrangement really makes the K&M fluted mandrel a first class doughnut cutter.  So my bottom line is K&M Fluted Pilot is a Doughnut Cutter for lucky and one great inside taper tool for everyone else.

Plexiglas case locator added to K&M Neck Turner  
Figure 4.

I noticed a couple of things while doughnut cutting on the Fireball Cases.  I am mentioning this only as some thought starters.  First, while cutting the doughnut on some of the cases, it felt like I was hitting something hard on one side of the neck inside.  Felt different than just a heavy cut on one side.  No visual signs of anything on the outside of the case to cause it.  I had about half dozen cases do this.  Next, while checking the cases by letting the bullet slide to and out of the case by gravity only, I noticed on a couple cases that the bullet would yaw as it left the case.  The bullet would yaw some just as it released from the upside down case.  Looks like I need to re-crown a couple of cases.  Additionally, on a couple of cases, the bullet was tight at the mouth.  It appeared to have a little bur or doughnut inside the neck right at the end of the lead in taper.  I am mentioning these items because of the article on the Secrets of the Houston Warehouse by Dave Scott some time back, in that article something was mentioned about sandpaper and cases.  Article never really explained anything about using the sandpaper other than indicating that it was something like the final tuning step for those 0’s groups.

Larry Medler

Rifle Silhouette Shooting

Silhouette Ballistics

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July 8, 2003