how to use CERROSAFE to nail down chamber neck dimention

[ericbc7]

I have been using Forster full length resizing dies with the factory expander ball with the understanding that i was overworking my brass. I want to hone my fl dies to minimize the neck work but am not sure how reliable any measurement i take on fired brass relates to the actual neck diameter. CatShooter suggested I cast the chamber and I fully agree. As I understand the process, I should buy some Cerrosafe, melt it on the stove, plug the bore of my rifle and pour the molten alloy into the chamber with the barrel pointed down. then knock the cooled casting back out from the bore.

Is the measurement i take with calipers on the neck of the cooled cast the actual dimention of the neck? or does the cast shrink or swell?

Also, is there some advice on how to plug the bore and do this measurement without making a mess?

Edit: I read the Brownells narrative and they explained most of the process.

 

[joshua43214]

Cerrosafe shrinks a bit then expands again. According the to chart and the brownells catalogue it is the exact same size as the cast in exactly one hour, then continues to expand at a fixed rate to +.0025" per inch in 200 hours.
http://www.csalloys.com/products-cerrosafe-alloy.html

I take my cleaning rod and wrap a piece of tape around it about an inch below where I guess the rifling starts (this is not critical, you can go 2" if you want). I roll up a small cleaning patch and slip it into the muzzle and push it a bit past the tape mark. I then push a pistol cleaning rod down the breach end and crush the patch from both directions. Cerrosafe is pretty thick and cools rapidly, it will not leak past. You can probably seal the bore with a spit soaked piece of tissue paper.

I use the pan from my RCBS 505 scale to melt the alloy in. The melting temp is so low the pan will not even discolor, and the stuff flakes right off when your done. The weighing pan spout is perfect for pouring.

You can melt the whole bar, but half the bar is more that you need for a 308 sized case - the whole bar fills most of the pan. Light the stove and warm the chamber end of the barrel over the flame until it is very warm to the touch, but not too hot to hold in your fist. Lay the barrel down next to flame close enough to just keep it warm while you melt the alloy. Melt the alloy slowly, hold it just above the flame for a few seconds and move it away, then back. It should take about 5 or so minutes to melt it. Swirl it gently a bit as you go.
Stand the barrel up (I used a big pot on the floor to raise it up and leaned it against the counter last time). Gently pour the alloy into the chamber, do not over fill it. Pour the excess into a clean cup.
Wait about 5 minutes, then pound it out with a mallet on your cleaning rod (You needed an excuse to buy a better rod anyway...)

Wait an hour and mic away.

You can wipe the splatter off the stove with a moist paper towel. Wipe out the weighing pan with a paper towel, melt it a bit more and wipe again. It is really low temp, you won't burn yourself or even discolor the pan. That last bits will flake out with no effort.

Clean the barrel with whatever you use for cleaning barrels, push a dripping jag in from the muzzle end being careful to catch the goo at the chamber end. Do this until you feel confident you have all the chunks out, then give it a good cleaning from the chamber end as normal. Oil it repeatedly, the oil will get any remaining traces out. Get into the area where the receiver meets the barrel with some dental picks, and run the picks along the bolt guides. You will always get a bit of splatter, it just comes right off. Cleaning will only take a few minutes more than normal, and all of the alloy will come out.

-Josh

 

[Scott Harris]

Measure the necks on multiple fired cases and add .0005 to .001 ....that's your chamber neck size and it's pretty reliable. You can also find a friend with pin gauges and directly measure the neck that way.

 

[XTR]

I don't think the exact neck dimension is something that you really need unless you have a tight neck chamber and are turning your brass.

You want to size your necks to about .0015 to .002 smaller than the loaded round measurement, that is referred to as neck tension. It is what holds the bullet in the case.

Take my FTR reamer, it has a .342 neck, but that measurement is irrelevant to my sizing. I have to size WW brass to .330/.331 because the measurement on a loaded round is .332, if I size less the bullets fall out. If I'm loading Lapua I size to .335, loaded rounds measure .336. (Lapua is thicker than WW)

Where your factory dies over work your brass is really in the body.

 

[harv50]

XTR are you saying you have 10 thou clearance??
reamer neck .342
loaded round .332
-----------------------
clearance... .010

 

[XTR]

XTR are you saying you have 10 thou clearance??
reamer neck .342
loaded round .332
-----------------------
clearance... .010

Yes, when I shoot with WW brass I do. When I shoot Lapua it is closer to .006.

The unturned neck walls on WW run about .012 on Lapua it's closer to .014. Pretty much every "no turn" 308 reamer I've ever seen has a .342 neck.

Remember, the SAAMI spec is actually .344 so even a generous neck like my .342 is really tighter than SAAMI spec.

(I try to anneal every loading, I've got 1500 to 2000 WW cases in my current in use inventory so when I do a batch it lasts me a few months)

 

[CatShooter]

"Remember, the SAAMI spec is actually .344... "

SAAMI is anything between 0.344" to 0.346", and sometimes factories (and gunsmiths) stray outside those limits.

 

[XTR]

"Remember, the SAAMI spec is actually .344... "

SAAMI is anything between 0.344" to 0.346", and sometimes factories (and gunsmiths) stray outside those limits.

I meant the SAAMI minimum, the point was that even though my reamer may seem a little on the loose side for WW brass it is actually slightly tighter than the SAAMI spec