Measuring The Slug

[Judan_454]

I just slugged my S-W 629 44 mag. Do measure the lands or the grooves to get the correct cast bullet measurment? Im getting land measurement of .4230 does that sound right for this gun?
Thanks Dan

[geargnasher]

Most S&W handguns have five grooves, which makes measuring them a real pain, since the grooves aren't directly opposing on the diameter.

Unless you have a special V-block fixture for measuring such things, I'd stick with just measuring the cylinder throats and size accordingly, as long as the groove dimension is smaller than the throats, you'll be fine. I've measured a couple of Smith .44s and they were both right on the money .429", but the cylinder throats on one came out about .432 and the other .430. Both shot just fine with no further work provided I shot .4325" boolits in one and .431" boolits in the other (no hard rule here, just what my sizers and boolit styles worked out to be).

Gear

[MtGun44]

I've managed to just sort of catch the edge of one and the opposite edge of the other
raised portion on the slug, which is the groove of the barrel. But, as gear says - as long as
the throats are larger than the groove diameter, size to throat or sometimes +.001 (altho
I get a lot of static over that). Start with sizing to the throat diameter and if it works well you
are there.

Bill

[geargnasher]

Sometimes, with a good micrometer, you can carefully rotate the boolit between the anvils and adjust in half-thousandth intervals until you just barely catch the edges across from each other, provided two things: One, it isn't a GP100 that has the bottom of the grooves cut on a different radius than the bore (non-concentric radius, the grooves must be cut on a 1" or larger radius), and the grooves are wide enough to meet each other on the full-diameter chord.

BTW Bill, you'll never get any flak from me about the way you size revolver boolits.

Gear

[goofyoldfart]

Simple way to do odd number lands and grooves is to take a one or two thousandth auto motive feeler gauge or shim stock and wrap it around the bullet then measure. subtract two times the thickness of the feeler or shim stock and you have your groove diameter. God Bless to all.

goofyoldfart.

[MtGun44]

Thanks gear ! I try to report what has worked for me and several revolvers shoot best with
+.001 on the throats, but some folks just can't understand how it might work. My THEORY is
that there is some out of round that gets filled it, but I really only know that they shoot
tighter groups with larger boolits. I try to recommend that newbies at least recognize that
this is a possibility. Actually, some are really worried that anything larger than groove diameter
is going to drive pressures through the roof.

Bill

[mdi]

I was a machinist/machine operator for many years and I usr the "rotating" method. Lightly turning the boolit while bringing down the spindle slowly. This method takes some "feel" and getting used to, so I go along with the above gents and say size the boolits for your S&W to the cylinder (My 629 has .429" groove diameter; took me about 3 or 4 slugs to arrive at this dia., and throats of .431") So, .431" boolits work for all my magnums, except my Puma that has .432" groove dia...