Quoted:

I've reloaded for my AR & pistols; I'm just starting to load for a .308 bolt gun & intend to load for accuracy. I want to chech in here on trimming / not trimming if the brass (after full length resizing) is between the manual's case trim length and the max case length. I don't intend to crimp.



This is once fired (by me) Nosler brass. The shortest length I've got is 2.008 and the longest is 2.014. Will this variation in length - if I'm not crimping - have an effect on accuracy as I work up my loads?

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

[email protected] wrote:

: 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:


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.

Quoted:






5. Go to the range. shoot for groups- two groups of each load and determime the best of the bunch. Note- shoot some cheapo ammo for fouling shots.