I have older 44 cal pistol
but standard .454 balls are too big
how to precisely measure right ball diameter ?

You could Slug the Barrel.

My .44 Flintlock takes patched .424 balls

Basically I did it few days ago.
But don't have caliper with 0.001 precision.

So next question is - which good quality and great priced caliper will you recommend ?
Thanks

I am pretty sure the Ruger Old army takes the .454.  I will have to wait until I am at home to see what size the rest of them use.  

The balls should be shaving a slight bit of lead as you seat them, otherwise they won't seal and can cause chain fire.  They shouldn't take a lot of force when doing this. I think you need a .451 ball.  

The Harbor Freight calipers work fine.  I had a set under NIST traceable calibration control for years and they passed cal every year.  

Pistol - not revolver.

I was thinking about .451 but not sure .003 will be enough ...
But smaller are quite rare, so maybe measure first will works better.
thanks

I presume you will be shooting your balls patched.  IIRC, I used .440 patched in my old CVA .45 flintlock pistol.  Maybe .445 with thin patches.

If your pistol IS actually .44 caliber and not .45, then it would probably need .430 or .435 balls and patches.

General rule:  Revolvers use oversized balls that get "shaved" and/or swaged down.  Single shots use undersize balls and patches.

Rob

Great update, thanks.
Must be that way.

BTW which kind/manufacturer patches should I use ?
It's brand new Sea for me :^)

BTW2 barrel is rifled
3.5"

Try .430 round balls, with either store-bought patches or NATURAL cloth made of 100% cotton or linen, available at Hobby Lobby; look for pillow ticking or twill material, and don't let the nice old lady at the cloth counter sell you any sort of synthetic blend. Try to use a  decent patch lube like TC, but Crisco shortening works pretty well. Keep your loads below 40 grains or so. With  a 3.5" barrel, you won't be getting gilt-edged accuracy, but you should get minute-of-soda-can. Is the pistol one of the Dixie Gun Works "Philadelphia Deringers," by any chance?

I got some cleaning cotton patches already, but not .430 balls yet.
I was thinking about 20 grains max.
Maybe 25 later, after few tests ...

Since pistol don't have any sights - it will be rather minute-of-left-barn-doors accuracy :^O
No Dixie - some pretty simple old CMC pistol, cute but very basic.