This is what I use to do a chamber cast, in an AR barrel (277 WLV) but should be close, everything except a light hammer and a lead block I use to tap the chamber cast out.
In order to block the bore, I use a gas check, but some use a patch, which works but then you have to get it off the metal to reuse the Cerrosafe later. I select a gas check that will be about the same as the bore, so it seals well.
I mark the rod so I know the check is just past the ogive,
I spray some release into the chamber, to make it easier to take out, then I put the gas check in, right up to the rifleing to get the entire throat.
I melt the Cerrosafe in a laddle, because it hardens quick, and makes it easier to pour in time.
I pour the metal into the chamber (a little fuzzy), with the AR you have to make sure to stop before the locking lugs or it will be a PITA to take out due to the large mushroom of the cerrosafe.
I put the end of the barrel, chamber side on a block of pure lead so it doesn't mar the barrel, in your case it shouldn't matter since its installed on the rifle.
I then insert the rod into the bore and tap the chamber cast out. As you can see, the top one is what it should look like, the bottom one was a PITA to get out.
Ok, so take the chamber cast out just a few minutes after pour, because it initially shrinks, then expands to about full size. I started using an aluminum rod, bent the heck out of it, so used a steel one with an aluminum gas check on the end to protect the bore. That was the mushroom one, it wouldn't move, either due to too long a time, or the extra large sprue
