Boil for about two minutes, knead into a ball. Plug up barrel end of throat. Stuff the mixture (play dough consistency) in the chamber. Cram into the chamber with dowel rod or...….you have about 3 minutes or less from water. 

In about 5 minutes push out plug.

The questions I would have are does it keep it's perfect size after it cools? If it shrinks or expands and isn't an accurate casting, then it wont be a good product.
The next question would be whether it cools faster as it contacts cool metal? If it does, it wont be easily pushed into the shape of the chamber and give a good form.

I've never had any problems with Cerrosafe, and it can be reused numerous times.

Same thoughts as Vall pointed out and I use Roto Metals chamber casting material. Below is the shrinkage information Vall was referring too for their product. It's on sale right now 1lb for $9.99 and a hell of a deal. A 1/2lb. of Cerrosafe is now going for $14.99 and Midway's current price as an example. Click on the below RotoMetals link of which includes the shrinkage chart and how to use information it's what I have been using for years now and it's an excellent product. Make sure you click on view all to access all of the information including the shrinkage chart.

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Contraction expansion factor versus time, measured in inch per inch compared to cold mold dimensions. Test Bar x 10 (Below measurements are x/10,000ths of an inch. ex: 0.0025" is 25/10,000ths of an inch)
2 minutes - .0004"
6 minutes - .0007"
30 minutes - .0009"
1 hour + .0000"
2 hours + .0016"
5 hours + .0018"
7 hours + .0019"
10 hours + .0019"
24 hours + .0022"
96 hours + .0025"
200 hours + .0025"
500 hours + .0025"

The ideal way to test the stuff for shrinkage would be to send some to a man with his cerro cast in hand, and have him repeat the test in the same chamber and compare them. Seems like a good idea if it works. 
I've used the cerro products in several different applications, and would prefer the plastic if it works. I was using it for weight in places that I couldn't cast lead- plastic wouldn't work there. But, in the rifle chamber, it'd be cheap and safe.

To unproven for the purpose unless one has $16.99 to blow but the reviews were a bit interesting all though not related. What's nice about chamber casting material after the initial chamber cast has been taken it remains accurate for future use if you use the chart for the material being used. Being one who always keeps his initial castings on hand possible expansion and contraction based on the temperature at time of use for those mentioned could be a concern. It appears there some here who are having a hard time with using the chamber casting material? I find it to be very quick, easy and accurate where the others mentioned would take up more time and unnecessarily effort but that's just me.

JLouis

Fred Boulton wrote on Nov 19^th, 2018 at 3:23pm:


BONDO!!  Who'da thunk it.

Vall thats why I posted their current sale price a pound goes a long way and it should last the average fellow a lifetime. Brownells is currently listing a pound of Cerrosafe for $29.99 and I like RotoMetals chamber casting material a bit more than I did Cerrosafe. I still have a pound or I would also jump on it the same as you are planing to do or already did. 

JLouis