Slug the barrel, or the forcing cone?

[easyrider604]

1. Why bother to slug the barrel and then size your bullets to match, when the bullet has to pass through the cylinder throat before entering the barrel through the forcing cone?

2. Isn't an oversized bullet re-sized by the cylinder throat anyway? In this case, shouldn't bullets be sized to throat diameter, or perhaps .0005 larger?

3. Would sizing bullets bigger than throat diameter place undue stress on the cylinder, because upon firing the cylinder will have to extrude the bullet as the bullets passes through?

4. Am I missing something?

BTW, I tried slipping a .357 Speer JHP through the cylinder throat from the cylinder mouth.

Results:

1. Ruger GP100 and NM Blackhawk: Bullet passed through the throat when bullet dropped vertically into the chamber mouth. I could wiggle the bullet very slightly in the throat.

2. Two S&W 686-3s and one S&W 586-1: Bullet had to be pushed with slight pressure needed, using a brass rod. I didn't feel the bullet wiggle in all 18 throats of 3 cylinders. I'd say the .357 Speer JHP was a perfect match for S&W throats.

 

[rcmodel]

You are correct.

Slugging the barrel on a revolver will only tell you the bore diameter, and whether or not there is any tight or loose spot in it. Interesting to know, but not real useful when picking a bullet sizing die.

The real critical measurement for best accuracy is the cylinder throats.

Lead bullets need to be sized to fit them, not the bore. Even if the throats are much bigger then the bore.

The bullets will slug down to fit the bore, but if they start out loose in the throats, they start out crooked, and don't get any better later on.

rcmodel

 

[easyrider604]

rcmodel,

Thanks for your answer. It's clearer to me now.

1. It seems that my S&W 686s and 586 throats are right at .357, does this mean that this is the correct diameter?

2. The .357 Speer JHP bullet wiggles just tiny bit in my GP100 and BH, does this mean that I need to resize a little larger than .357?

3. Who makes a .3575 resizing die?

BTW, I've been wanting to ask about your "rcmodel" handle because I used to build and fly RC model aircraft which we referred to as "rc model planes". Do you by any chance fly rc?

 

[rcmodel]

Well, the resize part only applies to cast lead bullets.

You can't resize jacketed bullets and make them any bigger then they already are.
(It is possible to make them smaller, but I don't recommend it!)

Jacketed bullets for the most part, have a very soft, pure lead core.
They will slug up to fit the throats at .357 Mag pressures.

Of topic, but:
My rcmodel name is one I have used since there were computers and DOS bulletin boards.

I used to be VERY involved in R/C flying.

I wrote a monthly column for one of the R/C mags, and did kit testing & such for Top-Flite and other companies.
I was also president, newsletter editor, and chief cook & bottle washer for a 75+ member club for years.

It got to be a job, and all the work & politics were no fun anymore.
SO, I got really burnt out, and am no longer involved in R/C at all.

rcmodel

 

[Walkalong]

The real critical measurement for best accuracy is the cylinder throats.

Lead bullets need to be sized to fit them, not the bore. Even if the throats are much bigger then the bore.

Absolutely correct.

 

[243winxb]

Info from Lyman, follow link.

http://www.lymanproducts.com/lymanproducts/images/Bullet%20Guide.pdf

B. Choosing a Bullet Sizing Die
The correct sized diameter for your cast bullet
depends upon the groove diameter of your gun. The
exact sized diameter is a determination that only you
can make. Experienced shooters usually prefer
bullets sized above exact groove diameter.
Figure 5 shows how to determine the groove diameter
of your gun. Carefully drive a SOFT lead slug
through the lightly lubricated bore. The slug should
be large enough so the some excess lead is shaved
off as it enters the muzzle. Insert a cleaning rod into
the breech and carefully tap the slug back out of the
barrel. With a micrometer, measure the diameter of
this slug at its WIDEST POINT. This is the groove
diameter of your firearm at the smallest part of the
rifling. Using a sizing die up to a couple of thousandths
of an inch larger
than the slug will be
best.

For revolver, auto pistols i size on the + side. My .357 die sizes to .3575" my grove diameter measures .357" Hand guns, no larger than +.001" For rifles +002" to +.003" over bore diameter. :banghead:

 

[rcmodel]

Hand guns, no larger than +.001"

That is only true if your cylinder throats match your bore diameter.

On many guns, they do not.

My Colt SAA for instance has a .452" bore with .4545" throats.

It "patterns" .452" bullets, and shoots tight groups with .454" (.002" over bore size) bullets.

If bullets sized .455" would go in the chambers I would use them, but they won't.

rcmodel

 

[Walkalong]

You must size lead bullets to fit the throats. You would like them to be, for best accuracy, .001 or .002 over bore diameter. If they are more than .002 over you can be OK, but if they are under bore diameter you are screwed, unless you ream the throats larger.