I personally wouldn't do a pound casting of the chamber on any of mine. I'd rather do Cerrosafe, or in a pinch a wax casting than pound lead into my chambers.
But until you can set up a bench, or a good solid rest, it's going to be tough to determine accuracy.

Rotometals in San Leandro CA sells a Cerrosafe clone for cosiderably less. Having used quite a bit of it, seems to be the same, but in name only.

Yes, it will shrink, but I've found it so easy to do that I simply melt the casting back down, and do it again.
I always wondered why guys saved the Cerrosafe castings since they're really useless once they've shrunk anyway. I bought two bars years ago, and quickly realized one was plenty for a lifetime.

vall in my position at a camera company where we made Plastic lenses. When we were making new cavity sets we would use Cerro Bend to get cavity dimensions. The process was actually easy. We just heated up the Cavity's and then the Cerro Bend and poured it into the Cavity. We set them aside to cool off and left them over Night. We had found that Cerro Bend initially shrank quite a bit, but then went into a series of shrink and expansion as it went through temperature changes.
The end result was that 12 to 15 hours later the casting would be very close to the actual cavity numbers within .0002 to .0005and we could use that information to modify our machining system to get the product we wanted. It worked very well and we were able to create multiple cavity's that were all very correct to small numbers. The important issue was giving the sample enough time to go through the shrink and expansion cycles until it settled down and was stable. FITZ. OLD TUCK

fitz is Cerro Bend the same as Cerrosafe?

RotoMetals also has the shrink rate chart for their product on their Web site. I have been using it for years and find it to be a very good product for my own use.