Neck wall thickness

[Duper]

I bought a Cooper model 21 from an estate in 22ppc. I called Cooper to find out the neck wall diameter so I would know if I needed to turn my cases. I called twice and two different guys told me it was .243 diameter. that seemed kinda tight. I turned on case to fit that size and shot it. the fired case measures ..251. With the case coming back some after firing. What size would you think my neck wall is actually? I'm guessing probably two or three thousands over what the fired case is??? The two guys both told me they got the .243 number right from the chamber print. Thanks for your help in advance.

John

 

[tenring]

There was most likely some springback on the fired neck, maybe just .001". In selecting a bushing, I would start with the 251 neck you measured, and find a .249 or .250 bushing to begin with.

 

[gunsandgunsmithing]

Obviously they are looking at the wrong print, if your fired case measures .251. You can make makeshift pin gauges from brass by turning them to .251, .2515 etc, and coloring the case necks with a marker. When you get one that just scratches the ink off from around the neck, that's your neck diameter, or you could just turn for a .251 neck and call it good, leaving proper clearance, of course. Necks on fresh brass usually spring back a little under .001", so I'm guessing you have a .252 neck, but you're much better off too loose than too tight, so confirm.

 

[fdshuster]

Duper: I always find the fired case neck diameter to be .001" smaller than chamber neck diameter, give or take a few ten thousands (maybe, as said, depends on the age of the brass). This has been verified time and again with chambers of proven neck diameters, as verified with my reamer print drawings. Generally .001" of springback on firing.

Good suggestion to try oversize, marked cases, looking for the one that will leave scratch marks on the case neck when chambering. A chamber cast would also work.

I find it tough to believe the .243" chamber neck diameter. To get a minimum of .002" total clearance, your necks would have to be turned down to .0085", really thin. .241" - .224" bullet diameter = .017" divided by 2 = .0085". "the fired case measures .251" proves that the chamber cannot possibly be .243".

As said, your chamber is likely .252".

 

[Duper]

thanks for all of your quick replies so far. I was unsure if the spring back would be 1, 2 or 3 thousands. But from what all of you have said it appears to be around 1 thousands so that helps out there.
As far as the first reply from the factory, I thought the same thing about their answer and that's why I called the second time and was still surprised at the same answer. So I did turn on case down to about 7.5 thousands to fire it and see what it blew out to.
I didn't really want to go as far as using the cerrosafe to get the chamber dimensions. So the fired case with a better know history of normal springback helps a lot. My new lot of cases for it are measuring at .015 plus or minus a half or so, so I'll have to turn them a little.
Thanks again for all your help so far and will welcome any other suggestions also.

thanks
John

 

[fdshuster]

John: if I were in your shoes, I'd try to chamber a case that had a neck diameter of .253", won't fit (?), turn it down to .2525" & try again, won't fit (?). turn to .252" & try again, etc. etc.

I prefer a minimum of .002" ( ,001" per side) clearance, but .003" is fine also. .004" would be my maximum choice.

 

[Duper]

Thanks again. that will be easy enough to do and see when it finally slips in and determine the chamber neck wall diameter as close as one can without using some type of guage or internal mic of some type.
From every other post I've read about neck chamber clearance the .002 to .004 seems to be the range most prefer.

Will get the final dimensions determined. Turn all my cases as needed, start the load selection, load a few at a time and start getting some groups on paper.

thanks again to all.

 

[amlevin]

Maybe some Cerrosafe and make a cast of the chamber. That usually makes a good starting point rather than playing with pin gauges or fired cases.

 

[Duper]

I've looked at cerrosafe on some of the vendors sites. But didnt think I wanted to try and melt something and pour it into my chamber. Just a little unsure of how everything would release and everything. Thanks for the suggestion. i think that would be the best way to determine the dimensions everywhere within the chamber.
for anyone reading this. have any of you used cerrosafe before and how did you like it??
thanks
John

 

[gunsandgunsmithing]

It works great. Just read the instructions about measurements and time. It actually shrinks for a while. It is the best way, just not as easy as skimming a half thou off of the neck and checking with ink. Turning necks is more about consistency than actual clearance, one you determine that you have some.

 

[JohnHenry]

I shoot a Cooper model 21 VE in 22PPC. I figure my chamber neck diameter is .250 as the neck diameter of fired cases is .249. I use Norma 22PPC brass and my unturned loaded neck diameter is .246. I assume you are using Lapua brass which has a thicker neck wall. I am not sure Norma markets 22PPC brass any longer, but if you prefer not to neck turn you may be able to neck down Norma 6PPC brass. The new Norma 6PPC brass has a neck wall thickness of approx .0115. I plan to try that route when I exhaust my supply of Norma 22PPC brass. I haven't actually done it yet, but I estimate that the unturned loaded neck diameter will be approx .248 for the necked down Norma brass.

 

[amlevin]

It works great. Just read the instructions about measurements and time. It actually shrinks for a while. It is the best way, just not as easy as skimming a half thou off of the neck and checking with ink. Turning necks is more about consistency than actual clearance, one you determine that you have some.

On real benefit to using cerrosafe and casting the chamber is that you can also get a good measurement of the free-bore and throat. Cerrosafe essentially becomes a "model" of the reamer that was used. Machine marks and everyting will show up.

As stated in the quote, follow the instructions and remove as indicated. Make sure the chamber and throat area are nice and squeaky clean so you aren't having measurements altered by a bunch of carbon build-up. Make sure to not fill the chamber so full that the cerrosafe flows into the bolt lug area.

Some I know who have had it do so got it out by using a miniature butane torch and just melting the stuff, letting it dribble out of until the main "plug" was clear. Melting temp is much less than that of boiling water (158 degrees F). Stuff is also known as "Wood's Metal".

 

[John Smith]

Here is the list of various thickness measurement equipment.

http://www.semiconsoft.com/wp/products/

Hope it will be helpful for others.

 

[ireload2]

The easiest way to measure your chamber is to take a case and expand the mouth with a tapered punch or a spire point bullet.
Expand the case about .020 to .030 larger than the chamber neck. Then slowly chamber the case. The trumpet shaped case mouth will be sized down by the chamber. Pull the case out and measure the diameter of the case mouth. Repeat 3 or 4 times to gain confidence that you have the right number.

If you have a plunger ejector don't let it slam the case to one size and foul up the case mouth.

 

[Catfish]

This may be to easy, but if you have a case that was fire in that chamber and want .002 neck expansion you will need a bushing .003 smaller to allow for spring back. If your bullets are .224 and your fired cases are .251 a difference of 27, then your case necks should be .0135 thick when sized down.

 

[SheepDog]

If you stick a case in a chamber and the mouth belled when you pull out the case the mouth will be larger than the chamber because the brass springs back. A fired round is going to be smaller than your chamber for the same reason - the brass springs back. (unless you are generating enough pressure to expand the chamber by .002"+)