Cast bullet and revolver throat dimensions.

[MSD MIke]

So, it seems the generally excepted rule when shooting cast in revolvers is to size throats and bullets larger than bore size. For example, if I have a 38 special revolver with a bore diameter of .357. The rule seems to be that I would want throats and my cast bullets sized to .358. The idea, I guess is that the bullet will seal the throats and ultimately the barrel better this way. This reduces leading and generally is supposed to improve accuracy.
For the record, I have several revolvers set up this way and there are no issues, in fact it has fixed leading problems in a couple.

My question is why not size the throats and bullets to the same size as the bore? Why oversize? I feel like a .357 bore, .357 throats and a .357 bullet would seal equally well and deform the bullet less. Possibly being even more accurate.

What got me thinking about this is that I purchased an old three screw Super Blackhawk. Accuracy was mediocre and I experienced leading with cast bullets. Investigating dimensions revealed that I had a bore size of .432 (Largest I have seen in a 44) and throats sized all over the map, but all smaller than .432. It performed like you would expect with these dimensions. The largest sizing die I have is .432 so I honed my throats to .432. End result, with .432 throats, .432 bore and .432 bullets and I now have my most accurate revolver.

So why run larger than bore size bullets and throats rather than the same size?

Interested in your thoughts.

Mike

[Larry Gibson]

The four most accurate revolvers I have now have cylinder throats the same diameter as the barrels groove diameter.

[TD1886]

So, it seems the generally excepted rule when shooting cast in revolvers is to size throats and bullets larger than bore size. For example, if I have a 38 special revolver with a bore diameter of .357. The rule seems to be that I would want throats and my cast bullets sized to .358. The idea, I guess is that the bullet will seal the throats and ultimately the barrel better this way. This reduces leading and generally is supposed to improve accuracy.
For the record, I have several revolvers set up this way and there are no issues, in fact it has fixed leading problems in a couple.

My question is why not size the throats and bullets to the same size as the bore? Why oversize? I feel like a .357 bore, .357 throats and a .357 bullet would seal equally well and deform the bullet less. Possibly being even more accurate.

What got me thinking about this is that I purchased an old three screw Super Blackhawk. Accuracy was mediocre and I experienced leading with cast bullets. Investigating dimensions revealed that I had a bore size of .432 (Largest I have seen in a 44) and throats sized all over the map, but all smaller than .432. It performed like you would expect with these dimensions. The largest sizing die I have is .432 so I honed my throats to .432. End result, with .432 throats, .432 bore and .432 bullets and I now have my most accurate revolver.

So why run larger than bore size bullets and throats rather than the same size?

Interested in your thoughts.

Mike

First off let's correct you on bore size. I know what you meant, but the nomenclature is incorrect. The "bore" is the size initial hole they drill through the barrel and hone before they cut the rifling in it, which is the "groove". I believe that is what you meant to say. Here's a way to remember it bore=small, groove=larger. Notice the number of letter in each word. Kind of like remember port and starboard on a ship. Port=left, starboard=right. Again notice the number of letters.

Okay it shoots better because the bullet, when larger then the groove, fits it better and more fills out every bit of it. If it were smaller then the groove gas would leak by and the bullet would wander around in the large confines of the groove. Leading results as you have noticed.

[MSD MIke]

TD, thanks for the correction. I totally get the problem if the bullet is smaller than groove diameter. My question is why use bullets that are larger than groove diameter instead of the same size. I've had good results shooting bullets larger than groove diameter but had GREAT results when the throat diameter, groove diameter and bullet diameter were all the same. Of course, this is a sample size of one but correcting these dimensions on my SBH took it from being no fun to awesome.

[MSD MIke]

Thanks for the info Larry.

Mike

[TD1886]

Let me tell you about this very accurate Model 25 Smith I have in 45LC. It's an older pinned barrel one. My groove is in between .451 and .452, closer to the .451. I shoot cast sized to .452. Like mentioned very accurate. I was prompted to shoot the fattest bullet the cylinder throat would allow which in my case is .456. So I tried .456 and the accuracy just fell apart and I wasn't going to persue that avenue any longer. Don't fix what isn't broken so back to the .452. How accurate? Off the bench at 100 yards 1.5 inches. I use to stand up and shoot beer cans off of sticks at that same distance. Don't know if I can do that now and frankly I don't shoot it that much. Maybe I need to revisit that.

[shooting on a shoestring]

MSD let me take a swing at this.

Yes you’re right. IF all dimensions are the same such that barrel groove diameter = chamber throat diameter = boolit diameter. But…..there’s the unmeasured concentricity. Those inside diameters are not round. They are pretty close to being round. Probably more round than most people can measure. But they aren’t perfect. Neither are the outside diameters of the boolits.

By stacking the dimensions by a tiny bit (like about 0.001”) such that barrel grove diameter < chamber throat diameter < boolit diameter, the concentricities don’t matter anymore.

Of course if you’re lucky and your gun has a very concentric groove diameter and your chamber throats aren’t a little egg shaped and your boolits are good and round, then yes you can have excellent results with all the dimensions the same. In that case be happy and shoot! Don’t change a thing!

But….if your revolver is leading or is inaccurate, then the next step is to stack the dimensions and hope that brings happiness. It doesn’t always. It’s not a guarantee. It’s the real world and things happen.

[engineer401]

I have a 357 Blackhawk that slugged at 0.357". The throats were measured with pin gauges with 3 at 0.357" and the other three at 0.356". I contacted DougGuy here on the forum. He is full of great advice. He opened the throats up, cleaned out the chambers and polished the hammer. The gun which wouldn't give any decent targets before now shoots very well.

[DougGuy]

I haven't heard the term "stacking" as applied to the stages of dimensional changes the boolit's path takes it through, but I have said for years "In a perfect world a revolver's boolits are .001" to .002" over groove diameter, and cylinder throats .0005" to .001" over boolit diameter" so same basic ideas.

This works in every centerfire revolver regardless of caliber, for shooting cast boolits.

There are also a lot of shooters that go by the same dimension school too, so if something is working and the gun is proving itself with same dimension cylinder throat, boolit, and groove diameter, whatever works, works.

Didn't S&W in the 1930s make a revolver with no forcing cone at all, it was fitted and timed so precisely?

[Ed K]

Whereas the bullet is swaged into the barrel under great pressure I can visualize the throats wanting to be a smidge larger for a slip fit (tenths).

With the revolver I'm setting up now the barrel lands represent 35% of the internal circumference and are 0.005" tall. Doesn't that displaced metal improve the seal?

[TD1886]

Whereas the bullet is swaged into the barrel under great pressure I can visualize the throats wanting to be a smidge larger for a slip fit (tenths).

With the revolver I'm setting up now the barrel lands represent 35% of the internal circumference and are 0.005" tall. Doesn't that displaced metal improve the seal?

Usually makes the bullet longer.

[Jtarm]

The four most accurate revolvers I have now have cylinder throats the same diameter as the barrels groove diameter.

Interesting.

The most accurate revolver I own is an S&W 66-7. It’s has throats just a hair over .357. Also a very tight .003 bc gap.

It’s only been fed jacketed bullets, but maybe I’ll try feeding it some cast.

Do you size to throat diameter?

[Larry Gibson]

"Do you size to throat diameter?"

For those 4 revolvers, yes I size to throat diameter which is also groove diameter; .312 for the Ruger original run SS in 32 H&R, .410 for the Ruger Bisley 41 magnum and .429 for the Ruger OM Vaquero in 44-40 and the Colt Anaconda first model in 44 Magnum.

Note; All throat dimensions are pin gauge measurements.