My chamber cast

[UberDuper]

Just so we're all on the same page; I have no idea what I'm doing.

Here's my cast:
https://s3.amazonaws.com/uploads.hipchat.com/50112/338527/7Od00P5XOaOeCIq/upload.jpg
These are probably poorly measured

Length - 2.12
Bore - .4495
Groove - .457
Step - .089
Freebore - .175

That look right / reasonable? Anything I should go back and remeasure.

[Tom Myers]

That is a very nice chamber cast.

The casting appears to be made with Cerrosafe alloy. If you are going to remeasure, be aware that, after solidifing, cerrosafe first contracts for a short time and then begins expanding at an ever decreasing but predictable rate.

The casting expands back up to actual bore dimensions at a little over one hour after casting and continues expanding in size as time passes.

The usual instruction or recommendation for calculating the actual chamber dimensions from measurements taken from a Cerrosafe casting after the one hour time frame are quite misleading and, in my experience, and also of others, are simply not accurate. This recommentation was derived from an old data sheet that recorded the measurements of a 1/2" x 1/2" x 6" Cerrosafe casting and, although working well for that casting, the method is not proportional to castings that are smaller or larger than that sample.

Using actual measurements of many samples of various sizes, taken at different time intervals, as data points, a formula was developed that returns very acceptable results when used to calculate the actual chamber dimensions.

The Precision Cast Bullet Alloy Calculations software (http://www.tmtpages.com/Alloy/alloycalc.htm)contains a module that will return the actual chamber dimensions after entering the measurement of the casting and the hours elapsed since the casting was made.

http://www.tmtpages.com/Alloy/cerrosafe.gif

As you can see, the formula is clearly stated on the module and you can use it to manually calculate the chamber dimesions from measurements taken of the casting at any certain time.

It works best to use a micrometer to carefully measure and record the radial or diameters in 1/10000" values such as the 0.4625" value in the image above.

A set of calipers may be used to measure and record the linear or length values as closely as possible.

Simply take a measured value and multiply it by the hours since cast. Then multiply that product by 177.62. Now record and label this number as "First".

Next take the hours since cast and multiply that number by 178.39. Then subtract 1 from that product and record and label this number as "Second".

Finally, divide the "Second" number into the "First" number and the answer is the original chamber dimension.

You might also wish to measure and record the diameter at the mouth of the chamber.

If you are contemplating a bullet design based upon and fitted to the chamber dimensions, The parameters outlined in the Cast Bullet Design ~ Ultimate software (http://www.tmtpages.com/#ultimate) uses the Bullet Design Parameters (http://www.tmtpages.com/draw/UltimateHelp/hs90.htm)outlined as the initial values for a bullet designed fit to a specific chamber.

Viewing the images and diagrams on the site may also give you some insight into the critical chamber dimension values and how they may apply to your applications.

Hope this helps.

[elk hunter]

You will need to measure the diameter of the rim and the base of the case to determine what case it used. Case length and groove diameter indicate 45-70 or similar round.

[UberDuper]

I should have specified, this cast came from my Lyman 1878 45-70. It's a pedersoli '77 sharps.

Tom, thank you for all that info.

[Gunlaker]

Tom, thanks for publishing your correction formula. I've just run through a few of my old chamber castings and your corrections seem to be better than others I've seen published.

Chris.

[Hooker53]

Über, that is a nice casting. I remember one time around a year ago, I needed to cast the chamber on a Martini Henry I got in. Well. I have both Cerabend and Cerrasafe here and use both of them often. Well , guess which one I grabbed up by mistake? I like to have never got all of it out of the chamber as It would not turn loose. Had to bring the barrel in the chamber area up to just enough heat to pour, pick and scrap that stuff out of it. I keep the cera Brothers at opposite ends of the machine shop now and well marked. Lol. Have anothe Martini I need to do it too this week as well to check out the 303? Chamber.

[rfd]

i have a recent (2013) pederoli .45-70 roller that's .450 bore and .458 groove. 3 friends also have recent production pedi .45-70's, 2 sharps and a roller - all are .450 bore and .458 groove. these dayze, for the most part, i see no pressing need to chamber cast for these kinda rifles. you can mic some boolits and see how they fit the muzzle, besides making yer mandatory dummy rounds.

for the most part, if yer using 1:20 or softer alloys and either aa5744 or sr4759 smokeless, or the holy black, a .459 boolit with bp lube and loaded to trap door levels will be a good starting point for accuracy. if yer using any other kinda cartridge recipe, i'd suggest trying the components i just typed - you can pm me for smokeless load recipes. ya got yerself a good rifle, enjoy!

[Hang Fire]

I no longer trust chamber castings. I have best luck making soft lead impact slug impressions of chamber and throat.

http://www.bpcr.net/forum/viewtopic.php?t=2667

[EDG]

I would never make an impact casting of a chamber.
Why pound the locking features of your action?
Cerro safe is too easy - and after you record the dimensions you can recycle it.