Throat diameter

[Catpop]

Question: Should, in a perfect world, cylinder throat diameter be identical to barrel groove diameter?

 

[243winxb]

SAAMI has the answer.

 

[Catpop]

I knew 243winxb would chime in! Thanks 243!

Saami calls for, in the case of 44 mag, a throat of .4325 and a groove of .429. Meaning the throat is .0035 larger than the groove. Please correct me if I'm wrong.

Following these numbers, if I buy a new 44 mag revolver, I should expect these diameters to be present in the new revolver. Again, please correct me if I'm wrong.

 

[243winxb]

I changed the photo, to get all the info. . The bottom right has "chamber" info. Looks like there is a variance.

We need a gunsmith to explain it.

Should, in a perfect world, cylinder throat diameter be identical to barrel groove diameter?

I think no. The throat has to be .001" to .002" larger then the groove. The cast bullet diameter should be .0005" to .001" larger than the groove diameter.

When i started, Lyman said to match handgun bullet diameter to groove diameter. This i did, with error on the larger side of bullet diameter.

Rifles .002" larger then groove.

Just how i see it. Could be wrong.

 

[Vern Humphrey]

Question: Should, in a perfect world, cylinder throat diameter be identical to barrel groove diameter?

Short answer, no.

The throat exists to control the bullet as it leaves the case -- the cartridge has to fit fairly loosely into the chamber, and the bullet is inside the cartridge case. This makes the bullet diameter quite a bit smaller than chamber diameter. If we did nothing about that, the bullet would go down the chamber ricocheting from one side to the other.

What we do about that is to put the throat directly in front of the bullet so that as it emerges from the case, it enters the throat and is swaged down slightly. This controls the bullet until it reaches the face of the cylinder and enters the forcing cone. Once again, the bullet is slightly swaged down.

The rule is, a revolver should get smaller as it goes forward. Start with a large bullet, swage it down with the throat, and swage it down again in the forcing cone.

 

[Catpop]

All good points! Thanks to all for adding info.
I hope for more comments!

 

[Catpop]

The rule is, a revolver should get smaller as it goes forward. Start with a large bullet, swage it down with the throat, and swage it down again in the forcing cone.

If you're only talking about swagging a .0005 or so I agree provided the lead boolit remains at least .001 to.002 over groove diameter once in the barrel.
This is the reason I like to start with the groove diameter and work in reverse.

 

[243winxb]

Chamber/barrel
All diameters can be larger by + .004"

Length tolerance can be longer by + .015"

The posted chamber numbers in the drawing , can be larger by the above amounts. No for sure.

 

[Vern Humphrey]

If you're only talking about swagging a .0005 or so I agree provided the lead boolit remains at least .001 to.002 over groove diameter once in the barrel.
This is the reason I like to start with the groove diameter and work in reverse.

"It's all one, says the sapper." -- Rudyard Kipling

Of course you start with groove diameter -- because if you don't LIKE groove diameter, what can you do about it, short of re-barreling?

Now you want a throat slightly larger than groove diameter, and bullet sized a bit larger still.

And that leaves you with a revolver that gets smaller as you go forward.

 

[Walkalong]

It should be slightly over groove diameter. .001 to .002 is good, .001 is better. Size lead to fit the throats. Jacketed isn't as picky, and can shoot well when the throats are at, or even .001 under groove diameter. Lead is not so forgiving.

 

[Varminterror]

Question: Should, in a perfect world, cylinder throat diameter be identical to barrel groove diameter?

No. Diameters should shrink from back to front, with the exception of the "funnel mouth" at the inlet of the forcing cone.