Quoted:
OK, I know that the max length according to SAAMI is 2.015” and recommended trim length is 2.005”.  I also know that one reason you trim is to keep within SAAMI spec to avoid possible jamming of the neck into the bullet if my chamber is cut to SAAMI length, so this is a safety reason.

The other reason to trim is to keep the case neck length consistent from round to round i.e. to maintain even neck tension for greater consistency between rounds.  So for that standpoint, I measured the case length of 100 of my new 308 Win brass that has already been FLR to see the range in length.  You can see them in this Excel graph.  So they range from a max of 2.0190” to a min of 2.0000” with an average of 2.0066”.

http://www.ijn.dreamhost.com/Photo/308.jpg

So now here is the question – if I trim to 0.01” below SAAMI which would be 2.005”, then they would certainly be safe, however, 34% of the cases will be shorter than 2.005”.  I imagine that this would make their neck tension also different than the ones trimmed to 2.005”.  So the question is what should I do?  If I trim everyone to 2.000”, then it is likely that almost everyone of my 308 brass (500 pieces) will likely have the same length, but would the 2.000” length be too short?  What would be the disadvantage of trimming them this short?

Quoted:
OK, I know that the max length according to SAAMI is 2.015” and recommended trim length is 2.005”.  I also know that one reason you trim is to keep within SAAMI spec to avoid possible jamming of the neck into the bullet if my chamber is cut to SAAMI length, so this is a safety reason.

The other reason to trim is to keep the case neck length consistent from round to round i.e. to maintain even neck tension for greater consistency between rounds.  So for that standpoint, I measured the case length of 100 of my new 308 Win brass that has already been FLR to see the range in length.  You can see them in this Excel graph.  So they range from a max of 2.0190” to a min of 2.0000” with an average of 2.0066”.

http://www.ijn.dreamhost.com/Photo/308.jpg

So now here is the question – if I trim to 0.01” below SAAMI which would be 2.005”, then they would certainly be safe, however, 34% of the cases will be shorter than 2.005”.  I imagine that this would make their neck tension also different than the ones trimmed to 2.005”.  So the question is what should I do?  If I trim everyone to 2.000”, then it is likely that almost everyone of my 308 brass (500 pieces) will likely have the same length, but would the 2.000” length be too short?  What would be the disadvantage of trimming them this short?

Quoted:
Thanks CCW!  I was always a biologist who was weak in stats and having professional statisticians on staff at our disposal did not help to strengthen that skill – LOL!  So with that caveat in mind, I did some analysis on the data set and here it is using Excel’s build in Stats operations.  Does not look too normal a distribution to my untrained eye….  Have you looked at this before and seen anything similar/different?

http://www.ijn.dreamhost.com/Photo/308a.jpg

Thanks for the advice as to going for 2.005”.  I made some calculations to see the percent difference in grip length between a cartridge that is 2.000” in length vs. one that is 2.005” in length and this is what I got:

SMK MK175 gr. bullet = 1.25”
COAL = 2.800”
Cartridge                = 2.005”
Length of bullet in round                = (2.005+1.25)-2.800”=0.455”
Length of SMK that is the boat tail                = 0.180” (approx).
Length of SMK in round that can grip                =0.455-0.180 = 0.275”
Length of cartridge neck                = 0.27” (approx)
Length of actual SMK that grips the neck                                = 0.27”
Grip length difference between a 2.000” and 2.005” cartridge          = (0.005/0.27)x100 = 1.9%

So the grip length difference between a cartridge that is 2.000” and one that is 2.005” i.e. the greatest extreme after my trimming would be around 1.9%.  How much difference would that make in total neck tension I am not sure because it depends on how much error can be contributed by the variables you name i.e. neck material thickness, hardness, diameter tolerance etc.  All of which I don’t know.  I mean the other option which is what I was thinking of would be to trim everything down to 2.000” in which case all the rounds will have a similar if lower grip length?  

ChrisGarrett  - I would certainly agreed going longer is better (<2.025”), but I am just struggling with the idea that if I go as long as 2.018”, I would basically not be able to use 98% of my brass - at least initially.

Quoted:
The process of trimming to 2.000 has its own set of tolerances.  I have the RCBS trimpro that references off of the case head surface, for trimming.  The best I can do for a batch set up is +/-.003.  IMO, the SAAMI numbers sometimes tries to anticipate the kind of process variation one is going to generate and sets the number accordingly to keep the operator out of trouble.

: Which of the variables within a reloader's control have the greatest
: impact on rifle accuracy?

Primer uniformity is probably at the top of the list.  This is about 60%
of accuracy.  But it matters more as the range gets longer.  Non-uniform
primers mean a greater velocity spread which means the vertical stringing
at the target will be greater.

Case length doesn't matter much.  As long as the mouth is square with
the case axis, case length can vary 10 to 15 thousandths of an inch and
accuracy will still be excellent.

Necks should be reasonably uniform in wall thickness, like no more than
a .0005-in. variance.  This helps keep the necks straight when the cases
are resized.  And they should not be sized too much.  If they are, the
bullet seating pressure will be too high; too much tension will cause a
higher spread in velocity due to the normal variable incountered as each
bullet needs a different amount of force to push it out of the cases.  If
neck tension is minimum, the spread of the force to push the bullet out
gets lower resulting in a lower velocity spread.

Case weight variance plays a somewhat important part.  Heavier cases have
less volume, hence slightly higher pressures and velocities for the same
set of components.  A 1% spread in case weight is typically good enough
for best accuracy.  Any more than that doesn't seem to make any difference.

Primer pockets and flash holes should be uniform.  Pocket depth needs to
be consistant so each primer has the same pre-load; more uniform ignition.
Flash holes need to be the same diameter so the same flame properties are
transferred to the powder.  But these two things only make about a 1/16th
MOA difference in group size.

Case body wall thickness helps too.  They shouldn't have a spread of more
than about .003-in. for 30 caliber cartridges; .002-in. for 22 and 24
caliber ones.  But again, this only has a small effect, like about 1/8th
MOA or a tad less.

Powder charge weight variance can also cause more velocity spread.  But
just as important is powder type and charge weight.  It's been my experience
that for a given powder, there's two charge weights that tend to produce
the best accuracy; one at about maximum, and one several grains less.  A
chronograph is needed to check the velocity spread to determine this.
Charges with no more than a tenth grain variance is plenty good enough.
Primers typically cause a greater difference in velocity spread than a
tenth of a grain of powder.

Bullet seating depth also has an effect on accuracy.  Best accuracy is
usually going to happen when the bullet contacts the lands.  A lot of folks
are reluctant to do this as they claim max pressures will be higher.  Well,
they're right, but cutting the powder charge a few tenths of a grain will
reduce that pressure.

Bullet concentricity is important, too.  In a .308 Win. for example, if
the bullet runout is more than about .003-in., the groups will start to
open up a bit.  If it's more than about .007-in., they probably straighten
out as they enter the leade; I've not seen any more accuracy reduction with
such rounds.  The more a bullet has to jump to the lands, the straighter it
needs to be.  But bullets that are seated out far enough to be pushed back
into the case a few thousandths as the bolt is closed can have a bit more
runout and be accurate as compared to bullets that jump a ways to the lands.
A square case mouth probably does more for letting the bullets seat straight
than most other things.

It's easy to get good cases, bullets, powder and tools to assemble them.
But the hardest thing is getting good, uniform primers.  Primers vary
quite a bit across several lots; some much more than others.  For ranges
of 200 yards or less, there's not much difference between primers, like
about 1/4 MOA in the groups they can produce for the most part.  When
longer ranges are used, primer uniformity becomes more important.  A recent
batch of match-grade ammo (about 300,000 rounds) was loaded from the same
lots of cases, bullets and powder.  Neck tension was quite uniform.  Primer
pockets were not uniformed nor were flash holes drilled to the same size.
Bullet runout peaked at about .004-in.  But several lots of primers were
used.  Some loaded lots of this ammo would shoot 2.8-in., 20-shot groups
at 600 yards.  Other lots would shoot 20-in. 20-shot groups at the same
range.  The difference was in the primer lots; some were very uniform,
others were not.

BB

Quoted:
Thanks CCW!  I was always a biologist who was weak in stats and having professional statisticians on staff at our disposal did not help to strengthen that skill – LOL!  So with that caveat in mind, I did some analysis on the data set and here it is using Excel’s build in Stats operations.  Does not look too normal a distribution to my untrained eye….  Have you looked at this before and seen anything similar/different?

From: [email protected] (Bart Bobbitt)
Subject: Re: Accuracy via microprocessor control ?
Organization: Hewlett-Packard Fort Collins Site

J. Spencer ([email protected]) wrote in response to my comments
preceded by : #:

: #In reality, primers are more responsible
: #for velocity variations than powder
: #is.

: How, and why?

Because of the way the priming mixture is prepared and put in pellet moulds
then put in cups, anviled and sealed.

: If the manufacturers can get powder to burn consistently,
: and we can get consistent charges so that their effect is minimal, why
: can't the manufacturers produce consistent primers?

Primer mixture creation and processing is almost a `black magic' operation.
Few folks can uniformly put the putty-like substance in a totally
homogeneous form that's very uniformly mixed.

: And a related question: how do know
: that it *is* the primers, how has it been measured?

Measured by velocity standard deviation, using various primer makes/types
with different powder charges show that primers have the greatest effect
on uniforming velocities.

: That is one variable which we can eliminate, can't we?

Yes. Seat the bullet to where it pushes back into the case when it jams
against the leade as the bolt is closed.  That uniforms bullet jump as
leade/throat wear lengthens this part of the barrel.  You may need to
increase the powder charge weight by about one-tenth of a grain every
1000 or so rounds to keep velocity at the same level; otherwise it will
drop several fps as the leade/throat dimension gets greater.

: #Some powders tend to cause more muzzle velocity variations than others.
: #Extruded powder tends to be the most uniform; both benchrest and highpower
: #competitors favor this type over ball powder for this reason.

: It's just a bugger that it doesn't meter as well, eh Bart? :-)

Yes, but a charge weight spread of up to two-tenths of a grain doesn't seem
to matter much.

: #When the most mild and uniform primers are used, muzzle velocity standard
: #deviations can be kept to 5 to 7 fps.  That's a muzzle velocity spread
: #of about 16 to 22 fps; darn well good enough for even the most demanding
: #shooter.

: Really? I'm genuinely surprised at that (not to imply I'm skeptical,
: just surprised). Which primers do you recommend (for 243 and 30-06 if
: it's relevant)?

If you can afford 'em, use RWS 5341 primers.  Otherwise, Remington 9-1/2
standards are also very mild and work very well indeed.

BB

From: [email protected] (Bart Bobbitt)
Subject: Re: [RELOADING] What makes Bench Rest Primers special?
Organization: Hewlett-Packard Fort Collins Site

`Benchrest' primers are typically more uniform in their ignition
characteristics than `standard' primers.  The process of mixing the
lead styphnate (the explosive), glass frit (the microscopic glass
particles that act as micro-anvils for the styphnate to compress
against) and a couple of other things is a kind of `black magic.'
About two quarts of this priming mixture is mixed together for each
lot of primers.  Getting the mixture homogenous (uniform percentages)
throughout the gooey mixture is difficult.  This mixture is spread
on a plate with little recesses in it that are the shape of the
primer pellet.  Getting exactly the same amount in each little round,
deep hole is also a type of `black magic.'  The mixture is much like
putty and a large putty-knife like tool is used to force the mixture
into each hole where it's left to dry.  Then the dried primer pellets
are punched out and put in primer cups, topped with a sealer, and
finally the three-legged anvil is pressed in place.

Interestingly enough, some folks do a better job of mixing and spreading
the mixture than others.  Within each lot of several thousand primers,
there's bound to be differences in the ignition characteristics from one
primer to the next.  As more care is needed to make `uniform' primers,
they usually cost more.

By uniformity, I mean the consistancy of velocity they produce.  Velocity
tests of the bullet is about the best test of primer uniformity.  Some
very uniform lots of primers will produce a velocity spread of only 15 fps.
At the other end of the spectrum are primers that produce velocity spreads
of 100 fps, or more.  Some folks have tested primer uniformity by shooting
BBs from a primed case (no powder) in a 17 caliber barrel; primers that
produce low velocity spreads with BBs do the same with powder and bullets.
Uniform primers tend to produce more uniform pressure curves, too.

Alas, not all the `benchrest' primers are as good as they're marketed to
be.  Many times a standard primer will be more uniform than benchrest ones.
And some makes of benchrest primers aren't as uniform as another make of
standard primer.  Even some standard primer brands are more uniform than
any benchrest brand.  Some benchrest primers are hotter than their standard
versions for the same make.

In many accuracy situations, a milder, standard primer will produce better
groups than a hotter benchrest primer.

BB

From: [email protected] (Bart Bobbitt)
Subject: Re: Accuracy via microprocessor control ?
Organization: Hewlett-Packard Fort Collins Site

Randy Howard ([email protected]) wrote:

: I'm curious what you mean by "mild"?

By mild, I mean primers that produce just enough heat to properly ignite
the powder.  As primer brisance (heat/energy) gets higher, it also gets
less repeatable from primer to primer.  Competitive shooters have found
that the milder primers give both better velocity uniformity and accuracy.
But they aren't after every last fps of speed nor ft/lb of muzzle energy;
just putting all fired shots in as small a group as possible.

: A lot of reloading articles I have
: read seem to indicate that it's better to use magnum primers in all
: rifle cartridges (starting with a lower powder charge than normal and
: working up of course) because they will result in more consistent ignition.

This is true with most ball powders, the type most folks tend to use in
rifles because they meter very uniform charge weights from measures.  But
when accuracy is the objective, extruded powders are used and so are mild
primers such as Federal 205M, RWS small rifle, Rem. 7-1/2 BR small rifle
primers.  The mildest large rifle primers are Remington 9-1/2 standards and
RWS5341s and they are also the most uniform of large rifle primers.  Some
lots of Federal 210Ms are very uniform and others are not; these are not very
mild primers.

But then virtually all folks writing reloading articles for magazines have
no real knowledge of what they are talking about in the first place.  When
you consider that most of 'em don't have what it takes to shoot 1/10th MOA
groups at 100 yards nor 6-in. groups at 1000 yards, how in the heck can one
consider their information worthwhile regarding accuracy.  They put
enthusiasm and egos in print; not reality.  But they get paid for it in
spite of not having any skill or knowledge in putting together a valid set
of tests to produce statistically significant results, then publish those
results in an unbiased manner to boot.

: It sounds like you feel the opposite is true.

So do hundreds of others who have observed the results of lots of primer
makes and types being used by the hundreds of thousands of rounds fired.

BB

Primers

I have come to the conclusion that primers account for 60% of accuracy. When you think about how a primer shoots up through your case and ignites your powder charge, you can realize how it doesn’t take very much variation in primer flash to change the way it ignites the powder charge.

During the last few years I have found that CCI was by far the worst primer, emphasizing as I said earlier that these are my own personal opinions. I have talked with other people around the country and they seem to confirm what I have observed about CCI’s hang firing. On a scale of one to ten, with ten being the best, I rate Remington 4-8, Federal 5-8, Winchester 4-6 and RWS 5-8, when you can find them. You will occasionally find one lot of primers from any manufacturer that rates a 10. What you must do is buy a 1000 primers and test them. If they work real well at 600 yards keep that lot of primers for 600 yards. If they are not satisfactory at 600, use them at short range or in practice since it doesn’t make as big a difference at short range.

Let me tell you about a test I ran that shows how important primers are. One time I loaded up several lots of primers purchased at different times. I must have had 15 or 20 different lots. I tested them at 100 yards using a bench rest gun. They grouped anywhere from a ¼” to 2”. The same bullet, same case, same powder charge were used, the only thing that varied was the primer. Primers can actually make that much difference.

I have also seen primers that would produce ¼” groups at 100 yards, but they were giving a velocity spread of 75 to 80 fps. on a chronograph. At 100 yards, they appeared to shoot fine, but at 600 yards they wouldn’t hold elevation. So if it is possible, you should also check your primers for velocity variation.

When you load primers in your brass, you should handle the primers in such a way that you don’t rub your fingers or any oily substance over the anvil of the primer. Any foreign substance you put on that primer is not going to help assure uniform ignition. Primers should be seated in the bottom of the primer pocket properly and firmly.

Primers should be stored in airtight containers. Humidity can dramatically change the way a primer functions. I know one shooter who had a bunch of bad primers. When he put them in a can with some desiccant and dried them out and then tried them again they worked real well. When he tried drying them even more, they didn’t work at all. This would indicate that there is a moisture content that is ideal, but I cannot say what the percentage is. Until we have that answer, just try to keep your primer storage as uniform as you can. I think an ammo can is the best place.

In addition, I think you ought to keep your loaded rounds sealed up too. I don’t think you can take a lot of loaded ammunition, place it up on the shelf and expect it to be the same six months from now. Two things are going to happen. The primer can change and if you haven’t washed and cleaned the brass properly a chemical reaction between the bullet and case is going to occur and the bullet may begin to stick to the neck of the case. I have even seen rounds that were loaded and stored over a period of time which, when they were fired, had the whole neck come out with the bullet. That is the reason I believe in cleaning cases.