Recutting the forcing cones in both barrels today.
Usually, except for benchrest rifles, I indicate the barrel on a snug fitting gage pin. In theory a snug fitting gage pin's error should be less than 0.001".
On expensive benchrest rifle barrels I use a gage pin to do an initial dial-in, then follow up by indicating directly on the grooves with a test indicator.
Since these were cheap barrels and a revolver is not a benchrest rifle, I had previously used a gage pin to dial in the barrels. Today I started out using a gage pin in the No-Name barrel, but my forcing cone turned out visibly off-center even though I had dialed in the gage pin carefully, so then I switched to a Mitutoyo indicator with a 1.5" long tip reading directly on the grooves, as shown in the photo.
In theory you should get the same results with either method, but today that was not the case. Well, several things can go wrong using the gage pin method. The gage pin must have some clearance inside the barrel, so that clearance can cause a few ten thousandths error. Next, the gage pin rides on the lands while the test indicator method reads the grooves, and in a cheap barrel there may be slight variations in the groove depth from one groove to the next. Finally, if the bore is curved -- not an unusual thing even in expensive barrels -- that will cock the gage pin and throw the readings off.
Indicating directly on the grooves is not perfect, either. If the groove depth varies from one groove to the next, that will throw the readings off. If the bore is curved then the best you can hope for is zero runout where the indicator tip is positioned, but there will still be runout at any other position.
So dialing in a barrel can be complicated, and gunsmiths have endless campfire debates about which method is best. My personal take is that as long as you are using a reamer with a snug fitting pilot, then the reamer will try to follow the bore even if the dial-in is off a thousandth or two.
But I'm using a boring bar to cut these experimental forcing cones, and there's no pilot to keep the forcing cone concentric with the bore. If my dial-in is off, then my forcing cone will be off the same amount.
After today's unsatisfactory experience using the gage pin method, from now on I'll probably use the Mitutoyo to dial in all my barrels, even non-benchrest barrels.
After re-dialing in the No-Name barrel with the Mitutoyo, I recut the forcing cone, and now it appears concentric -- though some of the diameters ended up bigger than I intended. The main freebore section was supposed to be 0.358", but it was actually big enough to swallow a 0.358"+ gage pin, so it's probably 0.3585" or thereabouts. That's not the end of the world, but I'm just noting it here for the record.
I made the freebore long enough to swallow the 180 gr. BB bullet, as shown in the photo.
The freebore is followed by a 1.5 degree leade into the rifling.
Like many of my projects, this "Whole Hog Taylor" forcing cone is just an experiment and I make no guarantees that it will work.
Next, I recut the Green Mountain barrel's tight 5 degree forcing cone, turning it into a "Taylor Junior," though slightly different than the No-Name barrel's "Taylor Junior." If I have time I'll post a sketch later, for now let's note that a 0.358"+ pin will go in 0.200" deep, and that freebore section is followed by a 1.5 degree taper.
I had used the Mitutoyo to dial in the Green Mountain barrel, so the new forcing cone should be perfectly concentric, right? Well, it's close to concentric but not perfect -- it looks to be off about 0.0005" relative to the bore. How is that possible? Because I positioned the tip of the Mitutoyo a good inch inside the barrel rather than at the end of the barrel where the forcing cone is. That wouldn't be an issue if the bore is straight, but if the bore is curved then all bets are off.
If I knew for sure which forcing cone design was best, I'd make a piloted reamer to cut that forcing cone and that would make it easier to cut the cone concentrically. In the meantime a boring bar is more cost effective for these experimental cones.
While I had the GM's bore dialed in with the Mitutoyo, I also double-checked the runout of the tenon. I had originally cut the GM's tenon for 0.0072" offset as indicated on a snug fitting gage pin. But when indicated with the Mitutoyo, I measured 0.008" - 0.0085" offset at the tenon!
Which is correct? They both are correct relative to the way they were measured, but there is some uncertainty in the measurements for all the reasons I have laid out in this post.
I'm not losing sleep over the imperfections, just noting them for the record.
The GM's "Taylor Junior" cone will swallow only the nose of my 180 gr. BB, as shown.