Cylinder Throat Diameter & Hard Cast Bullets

[mdi]

I have never heard of a "perfect" throat diameter to groove diameter ratio. For my revolvers I slug the barrels to make sure the groove diameter is smaller than the measured throat diameter ("drop through", "push through", "snug", "loose" are not measurements and just a hair better than a WAG. I prefer to measure). Very accurate measurements of a cylinder throat can be done with slugging. I have Pin/Plug gauges for a couple of my guns and have used quite successfully expanding ball gauges. I know what my throats measure to the .0005"

 

[Walkalong]

The truth is most people buy a revolver, get some or load some ammo, and go shoot it. Many or most without issues.

Is it best to know your groove diameter and the diameter of all your throats of course, but if the bullets are a tight slip fit to the throats, and they are over groove diameter, within reason, the owner will again shoot his ammo without issues and be happy and blissfully ignorant.

When the owner shoots their ammo and gets leading, then the only recourse is to measure the groove diameter, and all the throats so they can fix the problem.

 

[Carlitos]

Assuming the throats are .452 or larger, size/buy bullets to fit the throats and shoot some. No leading? Your golden and the groove to throat relationship is fine. Get leading? Time to slug the bore and measure throats.

Hello Walkalong, Thank you for your post; concise, to the point, and precisely what Bruce at Montana Bullet Works said to me. I reloaded some .452 diameter, 255 grn Keith SWC that fit the cylinder throats but do not push through. Finished reloads, with starting powder weights and 1.594”-1.598 C.O.L., chamber in the cylinder without issue that I can discern without firing them. Bruce suggested I fire them to test for accuracy and leading (I have a chronograph). I told him I plan to have the barrel slugged and cylinder throats measured by my gunsmith.

 

[Varminterror]

It’s pretty simple for me - the revolver is a pipe pouring into a funnel...

The groove diameter sets the throat diameter, the throat diameter sets the bullet diameter. The forcing cone will taper from larger than groove diameter down to groove and bore diameters, and the throat diameter should as big or within a couple thou larger than groove diameter. If the critical diameters of the bullet’s path are appropriately sized per this description then bullets can be sized to slip fit in the throats, minimizing distortion in the forcing cone into the bore, and minimizing leading/fouling.

 

[Carlitos]

It’s pretty simple for me - the revolver is a pipe pouring into a funnel...

The groove diameter sets the throat diameter, the throat diameter sets the bullet diameter. The forcing cone will taper from larger than groove diameter down to groove and bore diameters, and the throat diameter should as big or within a couple thou larger than groove diameter. If the critical diameters of the bullet’s path are appropriately sized per this description then bullets can be sized to slip fit in the throats, minimizing distortion in the forcing cone into the bore, and minimizing leading/fouling.

Hello Varminterror,
Thank you for your post. That makes sense to me, based on my research and conversations with cast bullet handloaders/manufacturers. I need to slug my RH’s barrels and cylinder throats and go from there.