From: Norman Johnson <nfjohn@flash.net>
Newsgroups: rec.guns
Subject: The problem with Micro-groove - here we go again
Date: 26 Jun 2000 09:00:45 -0400

#"Micro-groove", like polygonal rifling in some pistols, is very
#shallow.  This can lead to serious leading problems when shooting cast
#bullets, particularly at any type of velocity.  In the Marlin guns this
#is primarily a problem with the guns chambered in .45-70 as that
#cartridge is more likely to be used with cast bullets than most of the
#other normal loadings.  For this reason Marlin now offers the .45-70
#guns with Ballard style cut rifling. 

Experienced cast bullet shooters who have come to understand the
strategies needed to make them shoot well in any other rifle use cast
bullets in micro-groove barrels with the same success.  Some writer once
said that the micro-groove barrels do not handle cast bullets well and it
became conventional wisdom (I saw it in print so it must be).  Some
writers, seeming to have no idea what it takes to get reasonable cast
bullet performance, throw some loads together using cast bullets.  If
they happen to stumble on the right load, it is just by chance.  If one
will spend the time to learn how to make cast bullets shoot well,
micro-groove barrels are no particular challenge.

I would have not ever suspected that cast bullets do not shoot well in the
micro-groove barrels if I had not read it in the glossies who were, again,
pushing old wives tales.  It is patently obvious that the majority of the gun
writers have not the foggiest when it comes to understanding what it takes to
shoot cast bullets well, although Veral Smith is finally having some influence
on a couple of them. 

My 1895SS micro-groove shoots even pure lead bullets at the old .45-70
velocities without leading.  If you are having leading induced
inaccuracy, it is likely from an undersized bullet, powder not matched to
bullet velocity (use the slowest powder suitable to the velocity wanted),
or too hard (or too soft) a bullet for the velocity being developed.  It
is all in the technique, which holds true for any rifling design.  I
think even Marlin has swallowed this nonsense about the micro-grooves --
or perhaps it is just to feed on those who are gullible enough to have
done so.

I have found that no special requirements are necessary for good
performance of cast bullets in micro-groove barrels.  This is reinforced
by the NRA report that, when well executed, there is no superior or
inferior rifling type.  However, one must use the knowledge that we have
gained in the last 20 or so years, knowledge that was either lost or not
ever realized by the old time shooters.  The number one consideration is
bullet size.  The driving bands should be no more than .0005" under the
chamber's throat diameter, and the nose should be a moderate force fit
into the rifling.  This last can be checked by inserting the bullet nose
into the gun's muzzle.  If the nose enters easily, the nose is too small
in diameter.

********************************
#This is the same reason that Glock
#and some other European pistol makers don't recommend (or in some cases
#strongly advise against) the use of cast bullets in their guns.  The
#shallow rifling tends to not be able to hold the bullets and they "skid"
#down the barrel.  This results in two effects.  First, since they don't
#engage the rifling completely, the bullets may not properly spin and
#stabilize, leading to inaccuracy, tumbling, etc.  Second, when the
#bullet "skids" it tends to shave lead off and fill up the rifling,
#leaving you with a smoothbore barrel. 

Have you ever measured the depth of micro-groove rifling and compared it to
"conventional" rifling?

While the height of the lands results in about 75% of the driving surface
per land compared to conventional rifling, the number of lands in most
instances more than makes up for the smaller height.  At the opposite
extreme consider the 2 groove Springfield barrels that have such an
outstanding reputation for being cast bullet shooters.  The amount of
driving surface is at a bare minimum and no one complains about skidding
with this barrel.  Incidentally, I have two friends who have 9mm and .45
ACP Glocks.  All they shoot is lead - and with no problems.

I personally doubt that skidding is a problem at all with the velocities
used by the vast majority of cast bullets shooters given half way decent
bullet alloy selection.  Over the years I have seen articles that,
depending on who was writing them, varied from claiming that skidding was
the worst of evils to those who stated categorically that it cannot
happen, and supplied computer generated stress analysis diagrams to
support that claim.  I have tried to skid bullets and have not been
successful.  At least sometimes those who attribute slightly widened
grooves of cast bullets to skidding are looking at the results of flame
cutting due to undersized bullets.

I have a 7 Mauser Remington rolling block that is so worn that, as close
as I can measure, the lands stand all of .0015" tall -- and are very
rounded!  Back when I did not know better, I assumed that it would not do
well with either cast or <the other kind>.  However, it did so well with
<the other kind> that I decided to try cast.  Groups were virtually
identical with oversized cast; 1 1/2 to 2" at 100 yards - and that with
those turn of the century military sights and 65 year old eyes.

My ancient .47-70 rolling block takes .462+" bullets to realize its best.
Its lands are about .002" high.

My 1895SS .45-70 micro-groove needs .460+" bullets or groups open to 3-4". 

Same for my .45-70 Contender. 

*************

#This also tends to raise chamber
#pressures due to the fact that the effective diameter of the barrel is
#decreased. 

My experiments using Ken Water's method, show that for cast bullets of 13-15
BHN alloy, no pressure increases are generated when using bullets up to .006"
over bore diameter.  I stopped at .006" because larger bullets would not
chamber with sufficient neck clearance required for safe bullet release.

************

God Bless!

Norm