What is the correct bore diameter for .357 magnun pistol barrels?

[JAMESGR]

Anyone know the correct bore diameter for .357 pistols and rifles? Today, I received a new pin gauge set. When I measured the bore of three .357 magnum pistols they measured .347 and .348. The chamber throats measured .356 and .358. I thought that I should get at least .354 for the bore diameter. Pistols are SS Ruger Security Six 4" barrel; Tarus 608 61/2" ported barrel;and a Rossi 4"barrel.

[megasupermagnum]

.347" bore is about right. Most of my handguns have about .005" deep rifling. Groove diameter should be .357" plus or minus a little. .356" throats will probably be an issue for accuracy, especially with a .002" variation between them.

[dbosman]

You can, if the revolvers are worth it otherwise, get a throat reamer to make them the same diameter. That can improve accuracy. You'll probably find you want to ream both cylinders.

[DougGuy]

I usually size 35 caliber revolver throats to .3585" and you use .358" boolits, 9mm I size to .358-ish whatever the throating reamer finishes at. Most want to shoot .358" in the 9mm, same reamer will cut a slightly larger throat in a barrel that is not real hard, it will finish sometimes a tad less than .358" if the barrel is hard. Not salt bath hardened, just some barrels will cut easily, and some don't. I shoot for .3585" if and when I can get it, and size 9mm to .358" same as a 357/38 revolver.

In a perfect world, a revolver's boolits need to be .001" to .002" greater than groove diameter, and cylinder throats need to be .0005" to .001" greater than boolit diameter. This lets you deliver a correctly sized boolit to a slightly smaller barrel so it gets a good seal and everything works like it should. If you can get this same boolit to barrel arrangement in an autopistol, it works for them too.

As far as bore diameter, the pilots I use for reamers are just like pin gages only hollow, they go on the nose of the reamer and fitted snugly ensures the reamer cuts dead on center. In 35 caliber, .3475" is my most used pilot, .347" is common, as is .348" I have a .3485" pilot that is the largest of the 35 caliber collection, and I rarely ever use it in fact it may still be brand new. Groove diameter for 9mm is all over the map to be honest. There really isn't any standard that is readily adhered to. Barrels can run .355" .356" .359" in the groove. Most revolvers are much more consistent and very close to .357" groove.

[Hick]

Just in case you are actually asking for what the bore of a 357 Magnum should be, SAAMI specs say 0.346 bore and 0.355 groove diameter. SAAMI specs on ammunition suggest 0.359 for lead bullets (although my 357 Magnum Ruger seems to like 0.358 lead).

[Outpost75]

The .346 bore and .355 groove is minimum spec. which the pressure test barrel is made to.
The allowable manufacturing tolerance on diameters is +0.003"/ -0.0000"

[Catshooter]

At one time I had seven .357s, all quality. Their bores varied from .3555 to .3598.


Cat

[mdi]

This old guy can get confused when posts use the same term for different items. "Bore" is the inside diameter of the barrel aka "Land" diameter. "Groove" diameter is the barrel's ID of rifling cut into the bore. Perhaps I just found a nit that needed picking...

[TRX]

If it's a pre-SAAMI .357, the bore could be .356, .357, or .358, depending on who made the gun, and the bullet sizes in commercial ammunition varied as well. Every manufacturer had their own idea as to the One True .357.

In practice it seldom mattered; a .358 bullet got swaged down by a .356 forcing cone and a .356 bullet obturated to fit a .358 barrel.

[DonHowe]

It is my belief that a revolver bullet cannot obdurate larger than the chamber throat. A revolver with throat diameters less than groove diameter is challenged, hence the earlier comments about throat reaming.

[mdi]

It is my belief that a revolver bullet cannot obdurate larger than the chamber throat. A revolver with throat diameters less than groove diameter is challenged, hence the earlier comments about throat reaming.

This is why I slug the barrels and cylinder throats of all my revolvers. First to make sure the throat diameter is larger than the groove diameter and so I can size the bullets to the same diameter as the cylinder throats. Sizing bullets to the same diameter as the throats has given me a very good starting point for finding a clean shooting (no barrel leading) load, and often is enough...

[fredj338]

Trying to pin gage the bore is sort of useless. Slugging is the way to go if you really care. I find 0.358" sized works for just about any revolver. More important are the cyl throats. Small throats will reduce any size bullet to that size, often resulting in leading & poor accuracy. Uniform all the throats to 0.358" & run 0.358" bullets, should be golden.

[Petander]

Bore & groove... I just slugged a -94 mod 66 with a groove dia under .356.

[tazman]

I own an early model 60 S&W that has a small groove diameter like that. It shoots just fine with .358 boolits. It would probably do better with smaller diameter boolits, but I also have several revolvers that have larger groove diameters.
I just size all my 38/357 boolits to .358 and call that good enough.
They don't all need to have custom ammunition. Just the match guns.

[Petander]

Yes,customizing everything can get very complicated.

The tight barreled mod 66 I mentioned also has one "large" throat .3585, others being .3575. I'll see what happens with coated .358's first.

Nice little guns.

[PositiveCaster]

Here are the SAAMI specifications for the .357 Magnum.

[tazman]

Ooo...K, here. My GP 100 Match Champion revolver throats all measure 358...except one that is a tiny smidge tighter. It doesn't increase the force needed to run a .358 dia. boolit through it enough to worry about, IMO. My barrel groove diameter is .3564, measured by Matt Dardas. So, if I am correct, boolits from .3575-.358 should be proper for my uses?

I have a bunch of boolits I ordered from two of the more "famous" on-line casters; ordered .358, and what I got measures .3575 for one batch, and .357 for the other. The only real .358 boolits I have are some I cast myself, and a batch I got from a forum member. But, seeing as how the diameters are not so much different from my throat diameters, could .357 sized boolits work also?

I am thinking about ordering unsized/unlubed boolits, lubing with BLL, running them through my .358 Lee sizer and lubing again. Other than the extra work, is that a good idea?
Thanx

I think you have the right idea. Actually, the boolits that are .357 will probably be fine also but sizing to fit the throats is normally the correct approach.
If you have some that are too small, you might try powder coating them. That usually adds a thousandth or two and then size them back to .358.

[Petander]

Bullet sizes can vary a lot for many reasons. The same sizer gives different size for different alloy. I just recently noticed how my oven heat treated bullets grow in size after a couple of weeks. All that jazz.

Measuring those barrel slugs is not exactly easy either.

[tazman]

Measuring those barrel slugs is not exactly easy either.

That is so right. The barrels that have an odd number of lands make it difficult.

[tazman]

Comparing SAAMI specifications to actual firearm dimensions is like comparing climate and weather.
Climate is what you expect, while weather is what you actually get.