Before 1950 all Marlin barrels were made by
drilling the bore, and then cutting the rifling one groove at a time on a
sine-bar machine. This required multiple iterative passes of a cutting head for
each groove (at first this was done with a "scrape cutter", and later a "hook
cutter"). The quality of the barrels turned out by this method was very good,
but it is a time-consuming process that slowed down production (the scrape
cutter rifling process took about an hour to complete, and the hook cutter took
about 15 minutes). Marlin likes to call this process "Ballard rifling" because
this was how the barrels were cut on the Ballard target rifles that Marlin
produced, which were famous for their accuracy. Most of Marlin's cut-rifled
barrels from this period had 6 grooves, but Marlin also made 4-groove (some of
their .30 and .32 caliber guns), 5-groove (.22 caliber) and 7-groove (.35
caliber) barrels.
For .22 caliber
barrels, these grooves were generally in the range of .069-.090" wide (usually
about .075"), and .0015-.003" deep (these specifications changed over time). For
centerfire rifle barrels the grooves were .0785" to .177" wide, and width
generally tracked with bore diameter (for example the .25-20 had grooves that
were .0785" wide, while the .45-70 had grooves that were .141" wide). On the
older centerfire rifles (i.e. pre-WWI), these grooves were typically cut
.002-.0025" deep. However, on the post-WWII .30-30s the specifications called
for grooves that were .004" deep, and for the .35 Remington’s, grooves that were
.005" deep.
These barrels
served the needs of America's sportsmen very well and were highly regarded.
However, America was entering the "baby boom" years of the 1950s, with their
unprecedented growth and opportunity, and America's manufacturers were looking
for ways to speed up production to keep up with demand. Remington was
experimenting with a new rifling method called "button rifling" in which a
hardened swedge was pulled through the barrel. In 1950 Marlin tested some of
Remington's .22 barrels and studied the rifling method and found that it
resulted in more uniform dimensions, better bore finish, longer tool life and
much faster production time. Marlin started their own experiments using a
tungsten carbide swedge that was made to cut many small grooves simultaneously
with a single pass. Marlin could now rifle a barrel in 2-5 seconds, instead of
15-60 minutes. Wow!
In 1953 Marlin
applied for a patent on Microgroove rifling (US Patent #3,100,358 was granted on
Aug. 13, 1953). In this patent, Microgroove rifling was described as having 5
grooves for every 1/10th of an inch bore diameter, and that the driving side of
each land would be "tangentially disposed" (i.e. beveled, presumably to prevent
the accumulation of fouling). On July 29, 1953, Marlin introduced Microgroove
rifling in their .22 rimfire barrels, with 16 grooves that were .014" wide, and
nominally .0015" deep. In their 1954 catalog, they outlined numerous advantages
that this new form of rifling had, including better accuracy, ease of cleaning,
elimination of gas leakage, higher velocities and lower chamber pressures. They
also mentioned "... a bore of greater than standard size..." in their discussion
of how Microgroove rifling did not engrave (distort) a bullet jacket as deeply
as conventional rifling. Hold that thought...
Marlin continued
to experiment with Microgroove rifling in their centerfire barrels, primarily in
the Model 322 chambered for the .222 Remington (the highest velocity round that
Marlin chambered for). The results of this testing convinced the Marlin
engineers that Microgroove rifling was indeed suitable for high-velocity
centerfire rifles and in early 1956 Marlin announced that all of their
high-powered rifles would henceforth be equipped with Microgroove rifling.
For the common
centerfire calibers, the initial specifications (circa 1956) for Microgroove
barrels were as follows:
- .22 centerfire -- 16
grooves, .015" wide, .001" deep
- .30 caliber -- 16
grooves, .030" wide and .002" deep (this would be changed in 1958 to 22
grooves .024" wide, and .002" deep; and then again in 1968 to 12 grooves,
.040" wide, .0028" deep)
- .32 caliber -- 16
grooves, .035" wide, and .0015" deep
- .35 caliber -- 16
grooves, .040" wide, and .002" deep (this would be changed in 1968 to 12
grooves, .055" wide, .0028" deep)
Note that by
conventional standards, all of these grooves are fairly shallow. Microgroove
rifling makes up for this by having many lands/grooves to grip and spin the
bullet.
In 1968, the .444
Marlin was introduced, and of course it had Microgroove rifling. It was given a
1 in 38" twist appropriate for the short, stumpy, high velocity bullet that
factory ammo was loaded with. There were 12 grooves, .056" wide and .0045" deep.
It is worth noting that the factory specifications called for the .444 Marlin to
have a groove diameter of .433" (and a bore diameter of .424"), indicating that
oversized bullets would be necessary for best accuracy when shooting cast
bullets.
Also in the
1960s, Marlin introduced the .44 Magnum to the Model 336. This rifle was fitted
with Microgroove barrels, containing 12 grooves that were .062" wide and .0043"
deep. Factory specs for the .44 Magnum barrels also called for a 1 in 38" twist,
but that this time nominal groove diameter was to be held to .4315", and a bore
diameter of .4230", so oversized cast bullets are once again called for. It is
interesting to note that given the difference in groove width, groove diameter
and bore diameter, suggesting that the .44 Magnum barrels were produced using a
separate process and tooling than were the .444 Marlin barrels.
In 1969 the .44
Magnum was used to re-introduce the Model 1894. Ten years later the .357 Magnum
was added to the 1894 line. This easy-handling carbine was also made with
Microgroove rifling. In this case it had 12 grooves, each of which was .055"
wide and .003" deep, and had a 1 in 16" twist. The groove diameter specification
was listed as .3577", so .358" cast bullets generally shoot just fine. Later (in
1987) the .41 Magnum was added to the Model 1894. The .41 Magnums had 12 grooves
that were .072" wide and .0035" deep, with a 1 in 20" twist. Factory
specifications for groove diameter is listed at .4107", but these barrels
sometimes run a little bit larger than this.
In 1972, Marlin
introduced the Model 1895 chambered in .45-70. Initially, these rifles were made
with 8-groove Microgroove barrels, with grooves that were .060" wide and .003"
deep. The next year this was changed to a 12-groove barrel, with the other specs
remaining the same. Again, groove diameter is spec-ed out at .4587", so
oversized cast bullets are called for in these guns. All of these .45-70
Microgroove barrels had a 1 in 20-" twist.
There has been a
great deal of concern over the years about whether or not Microgroove rifling
would shoot cast bullets well. One group of folks says that Microgroove rifles
can't hit a barn from the inside with cast bullets, one group says that
Microgroove barrels shoot cast just fine just so long as velocities are kept
below 1600 fps, and one groups says that Microgroove barrels shoot cast just
fine at full throttle. The key to success with cast bullets in a Marlin with
Microgroove rifling is to keep in mind that these barrels tend to have oversized
groove diameters, and that the grooves/lands are shallow. Therefore, in order to
get proper engraving (and minimize "slippage" of the cast bullet as it enters
the shallow rifling), it is necessary that the bullet completely fill up the
groove diameter of the barrel, and engage the maximum amount of the driving
surface of the lands. Since Microgroove barrels are commonly oversized, this
means that the cast bullet must also be oversized to effectively "fill up" the
grooves. Other factors that also contribute to good accuracy with cast bullet in
Microgroove barrels are the use of a GC bullet (which helps to provide a better
grip and seal), and making sure that bullet are cast hard enough for the
application (generally a BHN of 12 or more). It is also worth remembering that
throats in modern rifles are almost always cut larger than groove diameter of
the barrel (to insure that ammunition can chamber easily), and that best cast
bullet accuracy is obtained by fitting the bullet to the throat, hence
reinforcing the need for oversized cast bullets in Microgroove barrels (for
example, the throats on my .30-30s run almost .311", so I size cast bullets to
.310" for these guns).
Examples of
Microgroove barrels shooting cast bullets accurately are easy to find. For
example, I have a Marlin 1894 in .41 Magnum that was made in the first
production run about 20 years ago. I don't know the throat diameter on this gun,
but it shoots it's best with .412" bullets. Bullets sized .410" don't shoot well
at all, and while .411" bullets group so-so, they have a really annoying habit
of throwing flyers wide of the group by a notable margin. In this levergun the
NEI .410-260-GC (#215F) is the key to success. I cast these bullets to a BHN of
13, size them .412" and lube them with homemade moly lube. Loaded over 19.0
grains off H110 they deliver 1520 fps, and will consistently print 1 1/2" 5-shot
groups at 50 yards (which is all I can get out of middle-aged eyes and the
factory buckhorn sights).
Another example
is found in the .35 Remington shooting the Lyman 358315 GC-HP (which weighs 206
grains as it drops from the blocks, when cast to a BHN of 13). Sized .359",
checked with a Hornady crimp-on GC, and lubed with moly lube, a very accurate
load is obtained by seating these bullets over 36.0 grains of H4895. This
combination generates right at 2000 fps, and will routinely group 5 shots into
an inch at 50 yards from a Microgroove Marlin 336. The cast HP expands violently
at 2000 fps.
Other examples
can be found in the .30-30, .32 Special, .444 Marlin, etc., but you get the
point. Microgroove rifles can shoot cast bullets just fine -- just keep them
oversized, GC-ed and hard enough.
Back in the late
1990s, with the rising popularity of cowboy action shooting and its emphasis on
historical accuracy, a whole new group of shooters started picking up Marlin
leverguns. Because of their desire to load cast bullets in them, and because of
the on-going, nagging (and inaccurate) rumors of poor accuracy from Microgroove
barrels, Marlin eventually caved in and dropped Microgroove rifling for a number
of their leverguns in the late 1990s. Sales of the cowboy guns have been brisk
(especially those chambered for period correct cartridges, like the .45 Colt and
.45-70). Marlin helps to feed this nostalgia by referring to these guns as
"Ballard rifled".
As of fall of
2007, Marlin still makes a number of guns that have
Microgroove barrels on them (according to the 2007 Marlin catalog posted on
their website; Marlin Firearms). All
of their .22 LR and .22 Magnum rifles still have Microgroove barrels. The 336s
in .30-30 and .35 Remington are still made with Microgroove barrels, however the
stainless 336XLR in .30-30 and .35 Remington have Ballard rifled barrels. All of
the big-bore Marlin leverguns (e.g. 444, 1895, etc.) are now fitted with Ballard
rifled barrels (both blued and stainless guns), as are the 1894 and 1895 Cowboy
models. Interestingly, in the 1894 series, the.44 Magnum and .357 Magnum guns
are Ballard rifled, but the .32-20 1894CL and .41 Magnum 1894 FG are both fitted
with Microgroove barrels.
Microgroove
rifling is a fascinating, and misunderstood, innovation by one of America's
leading arms makers. It speeds up production time and significantly extends the
service life of the tooling (thereby holding production costs down). It provides
very uniform bore dimensions and a very smooth bore finish. And the bottom line
is, as generations of American hunters can testify to, Microgroove rifling
provides fine accuracy in the hunting fields. Because of the specifications
adopted by Marlin, Microgroove barrels tend to have oversized groove diameters,
and as a result, Microgroove barrels early on earned a reputation for not
shooting cast bullets well. In fact, when loaded with bullets that are the
appropriate size for the barrel (and cast to a suitable hardness and wearing a
GC), these guns are capable of fine accuracy with cast bullets. Some of my
favorite cast bullet rifles wear Microgroove barrels.
The historical data used in this article was taken in large part from William S.
Brophy's book "Marlin Firearms: A History of the Guns and the Company That Made
Them". For more information on Marlin firearms, this extraordinary book is
highly recommended.