Ideal diameter for pc'd bullet in cylinder throat?

[bmortell]

I just got my first custom mold an accurate molds 43-300R for sbh and srh. I powder coated them and was testing expansion in water since i want them for large game. They worked amazing i might add, but the coating was gone off the bearing surface even though it was still on the front and base. So i found some vernier micrometers and my lee sizer was making them .4296 and hand tight in the throats across both guns is .4312 giving .0016 room for gas blow by then they enter the barrel for a better fit.
My theory is the hot gasses (since it was enough for gas cutting the lead) was melting the PC then upon entering the barrel the hot PC would basically wipe off. I got varius grades of diamond paste from sharpening blades so i can roll up paper on a drill rod and lap it larger no problem. I could do trial and test but i used up all my catch jugs for now and dont wanna accidentally go over ideal size.
So anyone got experience fitting PC'D bullets to throats and recovered them still looking pretty and no leading? Also they have a bearing suface band in the throat when chambered so .4312 or bigger wouldnt chamber, and i was considering spring back if i were to size hardcast later. So i was thinking maybe .4310 would be good? Thanks

[OS OK]

When I was testing in water, I shot into a 10' long 4" PVC pipe filled with water. I leaned it at an angle that let me stand in front and shoot straight down the pipe. One end was on the ground, I put a plumbers test cap on that end with a radiator clamp to keep the water in.
On the other end I clamped an old towel over the open end with another radiator clamp...helped keep the blowback down but you still get a little wet.
You get all your casts back this way.
When I dumped it out I put a 5 gal. bucket over the open end in case some cast fragments wanted to slip by, I was testing HP's and got some fragging. Water is tough on casts.

[Bzcraig]

A couple of thoughts/questions; did you try the smash test on your coated boolits? Did you get any leading? I have opened up most of my Lee sizers because they were under sized. However, if you have no leading, I have to ask, are you trying to solve a problem that doesn't exist?

[DougGuy]

A couple of things.. Calipers aren't really reliable in doing cylinder work, very easy to get an erroneous reading. If you are working 4 places past the decimal, you need Z minus pin gages in half thousandth increments, and you need a good 0" to 1" micrometer that also reads in .0001" increments.

Most here will size 44 boolits to either .431" or .432" for revolvers, and perhaps .433" or even .434" for leverguns.

Likely, your throats are a little on the tight side, and if you want to size .431" they need to be .0005" to .001" over boolit diameter. At hunting velocities, unless you are shooting super hard alloy, BHN 22 or so, pressure at ignition will try to shove the bottom of the boolit violently toward the nose which will cause it to obturate and fill the throats. I doubt you are getting blowby in the throats. Sounds to me like your boolit is just a tad too small to make a good seal in the bore if you are getting any leading in the bore.

For PC I would probably suggest sizing to .432" and sizing throats to .4325" ~ .4328" as you do need a little "wiggle room" for growth due to age hardening, not uncommon for a heavy hunting boolit with long bearing surfaces to pick up .0003" to .0006" in the months after casting and sizing. You would want these loads to chamber without having to force them into the cylinder.

[bmortell]

For the questions: yes they were leading and i smashed one in a press it sticks fine. And i used micrometer reading of sized bullet for die size and throats mic'd size of a soft lead bullet that needs lightly tapped out.
So i lapped the die with 3 micron diamonds, it now makes em .4306 giving .0006 throat clearance rather than .0016 and i made a dummy round and pulled it measuring .4303.195140195141 results from firing 5 new ones after cleaning shows muzzle end leading, very little in beggining of barrel. And bullet on the left is the .4306 right is the old .4296 not seeing a differnce besides how nice it feels sizing bullets now lol. Any ideas ? Is my barrel just larger in the front ?