Why Random Flyers?

Why Random Flyers? 1

EWA Thoughts

Without the throne.


  1. The chambers are concentric.
  2. The cartridge exhibits zero headspace in the rifle.
  3. The bolt face or breach block is 90 degrees to the bore axis.
  4. You have good Extreme Spread and Standard Deviation numbers.
  5. The cartridges have good concentricity.
  6. Heavy weight stable barrels are being used.

Article 1:  Why Random Flyers?

I have observed the phenomenon over the years, after spending many hours perfecting a consistent load for a firearm, of what seemed to be random flyers from otherwise tight grouping ammunition.  I have also noticed that having cartridges that exhibiting near zero runout, occasionally throwing flyers.  What can the cause of this problem be?  Not being an original thinker, but pretty good at assembling solutions to problems from various available products (adapting), I took a look at what others were doing about making the flyers go away.  Turning necks, body sizing, neck sizing only, setting minimum headspace, annealing, and all the other mechanical processes written about, videoed about, and cussed about, I never found the magic bullet to stop the Random Flyer Phenomenon.

Most every one who reloads rimless cases realizes that there are factors we, as reloaders, have no control of, and have to rely on manufacturers to get it correct.  Things like primers, powder, and precise bullets are out of our hands for the most part.  We can vary charges, shift to different primers, select different bullets, and look for better cases if available, and play with barrel harmonics, but it is really hard to assemble a load that is consistent over many rounds, and when we get a five-shot string to group today it will not repeat tomorrow.  So, is there a factor that we do have some control over that will give us more consistent results?  Actually, I think there are two that are overlooked.

The two factors I have come upon are both within the same device, and that is the cartridge case construction and dimensions.  Factor #1 is mostly interior, and Factor #2 is exterior.  #1 is very hard to control, and #2 depends on the manufacturer.


Factor #1.      Cartridge body thickness uniformity.  You may have noticed that new cases, after firing, show body bulging about ¼” from the rim.  If you carefully observe you may see that on some cases the bulging is lopsided.  Others show more consistent bulging 360 degrees around.  I believe this indicates either the lopsided case is thinner on one side, or the case was held tight to one side of the chamber, and proud of the opposite side.  This non-concentric chambering forced the proud side to expand until it touched the chamber.  This action may have shifted the bullet axially at the time of firing, thus cocking the bullet a little as it entered the barrel lead.  Once the case has been unbalanced shape wise, it is very difficult to get the cartridge truly concentric again.  This mostly affects those that believe in neck sizing only.   See Figure 3 below.

This is Figure 3, sorry it is numbered out of order.  These cases were from the same lot of 500, and I have observed this for many years.  These cases measured .469” across the widest part of the bulge.  Figure 1 is farther below, and there is no Figure 2.

Why Random Flyers? 3


As it is hard to determine the uniform wall thickness cases from the not uniform ones, how do we modify our loading procedures to reduce the problem.

  1. Use a concentricity gauge (rim to mouth) to separate the brass by runout and mark the high sides. Then load the cartridges by the clock method with the high side always facing to a common clock position.  This is very time consuming.
  2. Take your new cases and roll them over an even surface slightly canted. As the cases roll you will hear the rolling noise evenness.  The consistent sounding ones will be the more consistant thickness cases, and the rrrrrr rrrrrrr rrrrrr ones will be the less consistant ones.  The unbalanced ones will roll unevenly as well, kind of like an out of round tire.
  3. If you are rich maybe you can find a ball micrometer with a long reach. I am not that rich.  Then you could cull the brass to get more uniform interior dimentions.
  4. Buy better brass. However, it is very hard for manufacturers to make cases that are really uniform in radial wall thickness.  It comes down to profit margin and lawyers.


Factor #2.      New case outside dimensions.  If you look in most reloaders manuals you will find case drawings with dimensions.  If we concentrate on Mauser derived cases you will see that most of the base dimensions are close to .470” +/- .001”.  Now you need to measure your actual new case base dimensions.  I believe that in recent years these dimensions have been reduced by several thousands of an inch from what I have measured in the past.  By “In the Past” I refer to my youth when I was under the instruction of a friend in the USMC.  We had, in those days, thousands of FA 58 and 59 Match 30-06 cartridges available as well as standard M2 Ball ammo.  Besides the bullet and powder charge the Match ammo all had .470” diameter case base measurements.  The Ball ammo was usually .467” diameter bases.  The small bases were considered proper for field issue and was likely to be more reliable in automatic weapons.  We also had some short chambered M1 barrels that we used for replacements.  Sarge had a special reamer he used for the match rifles that would finish the chamber and leave a base end measurement of .471”.  That left only .001” clearance when the match rounds were chambered.  As long as we chambered brushed the weapon every 80 rounds, we never had any problem with sticking cases, and the issue ammo ran fine even when the rifle was very dirty.  Needless to say, these Sarge Specials all ran MOA groups and were highly sought after.

So back to the subject, I started to look at modern cartridge cases, especially the Mauser derived ones.  What I found was that the manufacturers were producing smaller based cases.  Some of the cases are as small as .464”.  I then started measuring factory chambers, and I found them to be as large as .473” at the base of the case.  See Figure 1 below.  Therefore, in some cases the difference was as much as .009”.  That is pretty loose to my way of thinking.  I looked at SAAMI drawings, and I found that they often list up to .008” case base dimensional allowance.  .008” is considered a safe working limit for case expansion within a chamber.  I then obtained a case gauge that was made to SAAMI spec and inserted complete cartridges that were properly head spaced and concentric.  These cases measured .465” at the base.  I could grab the bullet and wiggle the case all around the base end of the of the gauge.  I was getting up to .004” equivalent bullet tip displacement with cartridges that measured .0005” runout in my runout tester.

It became apparent that this induced runout from the small base dimensions of the cases could override any of the steps I was taking to insure my cartridges had close to zero runout.  It all depended how the cartridge chambered.  Did the base lie against the bottom of the chamber, or the top, or the sides, I had no way to control it.  I looked all over to find cases that were closer to .470” base diameter, and could not find any until I remembered I had some old FA 59 30-06 cartridges in a trunk.  I took them and measured them.  They were .470” -+/- .0005”.  I broke them down, annealed and reloaded to match the commercial cased ammunition I was loading.  I picked the ones closest to zero runout and shot 20 rounds out of my Gibbs 03-A4 which has a tight chamber.  The 100 yard groups went from 2” to ¾”, with no flyers.  All in an even pattern, and ¾” is about all my eyes will permit.


Why Random Flyers? 5


Summary:  It is my contention that at least some of the flyers experienced are caused by under diameter cases not chambering in repeatable axial concentricity.  Until we can obtain properly dimensioned cases, we will always be chasing repeatable accuracy.

Imagine a chamber horizontally orientated.  It is .008” larger than the base diameter of the cartridge, and the shoulder is also somewhat larger than the cartridge shoulder.  The cartridge lies off center to the bore as gravity tends to pull the cartridge off center.  What will center the case axially as we push the cartridge into the chamber?  It is not the neck, as the neck area of the chamber has to be somewhat larger in diameter than the neck of the cartridge in order to allow the bullet to leave the cartridge.  The cartridge neck should not touch the chamber neck until the cartridge is fired.  The cartridge is centered by the shoulder at the forward end, and by the chamber / bolt interface at the rear end.  Due to bolt face clearances, extractor pressure, ejector pressure, etc. the bolt cannot be relied upon to center the rear of the cartridge.  The only true rear alignment tool we have is the cartridge base diameter.  If the case base is much undersize as compared to the chamber base, we will have a difficult time getting repeatability.

Having just started loading for 6.5 Creedmoor I have obtained several brands of brass, and here are the base measurements for the types I have.  Those in bold italics are primary component only manufacturers, the rest make complete ammunition as well.

Alpha                    No Samples Available (Maybe someone can comment with their measurements)

Hornady                .4655 – .4660”

JAG                       .4655”

Lapua                    .470″ (as reported by Southpaw)

Norma                   No Samples Available (Maybe someone can comment with their measurements )

Peterson               .4680”

Remington            .4660”

Starline                .4680”

Winchester           No Samples Available (Maybe someone can comment with their measurements )


What can we do:  If you have a factory barrel, I am afraid it comes down to buying the largest base diameter cases you can find, no matter the cost.   Savage owners could turn off the chamber end of the barrel and get a special sized chamber reamer ground to .001” – .002” over the base diameter of their chosen brass manufacturer and recut the chamber.  Semi-auto users would most likely have FTE problems with chambers this tight however.  Custom barrel users should be able to request the manufacturer to chamber tight to sample new cases.  You would want to purchase a life time number of cases at one time to insure you never ran out.  You will need a custom size die that will remove the above the base bulge.  If the cases are tight enough there will be very little bulge anyways.  Personally, I use Redding small base shoulder dies to set minimum headspace, and a Lee collet die to size the necks, but the original case base diameter needs to be within .002” of the chamber base diameter.  That will force an otherwise concentric cartridge to only allow .0005” bullet bore concentricity error in a cartridge like 6.5 Creedmoor.

I know everyone will not agree with my thoughts on the subject.  As an argument for my case, I would like to point out a 6.5 Grendel / 264 LBC example.  The drawings show a base diameter of .437”.  My Hornady cases measure .437”.  That is exactly to the drawings.  So why do the case manufacturers make the Mauser .470” pattern cases up to .006” smaller than the drawings?  My guess is that Grendel came about recently, and the makers have constructed the chambers to tighter controlled specifications.  Mauser pattern cases have been around a long time, and the chambers to fill vary widely.  Thus, the case manufacturers build to fit the tightest chambers that can be found.  This prevents customer satisfaction problems.  I would bet most manufacturers start their process for many Mauser based calibers with the same machinery and therefore the base diameters come out small on all.  Only those manufacturers that cater to reloaders would chance making cases with base diameters matching the drawings.

Comments are welcome.

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