Case Wall to Chamber Clearance Question

J

JerryK

Member
I'm working with the New 300 Norma Mag case (not 308 Norma Mag) for a HG project. I have a 300 Norma Mag PTG reamer print, and in comparing the print to the measured case, I have .005 clearance at the base/web, and .004 at the shoulder. This is a big 30 caliber case--103 grains water--but I pulled the neck down so it now has 98 grains. I will be buying a custom reamer, of course, so I can spec wall clearance.

My question is, for a minimum-spec chamber in a BAT action benchrest gun, how much clearance is recommended between case and chamber wall?
 
You can start an argument with that question. I'd say the PTG reamer is fine, with the case as you give the dimensions. Others might want .004 at the base and .003 at the shoulder. Al in Waugh would want more clearance.

One thing I think everyone would agree on is that the FL die damn well better match whatever you decide on. The case, coming out of the die, should be about .001-.002 smaller than the chamber in every dimension (except the neck).

How work hardened the brass is will affect springback, that is, how much smaller it is after being resized. Work hardening, like $hit, happens. However you feel about annealing the necks, you can't anneal the base. Just one of the reasons behind Al's method of dimensioning chambering reamers, fireforming, and dies.
 
you could do a search of his posts -- alinwa --

He might be tired of answering questions about this, since there have been so many posts.

But you could start a new thread, with alinwa as the subject line. I believe he is building a .300 RUM, which is on the .404 case, right? For all I know, the new 300 Norma is too.

The expert on this chambering I know of would be Dave Tooley, who I don't believe follows BR Central much anymore. If Dave wasn't involved in the development, he surely was in the testing. Not to say there aren't others.
 
Jerry.

Since you're making a reamer anyway. buy your die first and spec the reamer off of it. Your brass will cooperate since the chamber makes the brass, Kiff's print specs may be fine but you won't know till you have the resize die in hand. A tight neck program can be built off a SAAMI die. If you want a no turn neck you can have a SAAMI die neck reamed larger by the mfg for 10-20 bucks. On a case as big as your's 5 thou at the base would be good.

Greg
 
Thanks, Greg. My plan is to get a chamber reamer and a sizing reamer and make seating and sizing dies from Newlon blanks. It's not standard specs. I've reduced the neck angle from 20 to 30 degrees by pushing it into a Lazzeroni Patriot die. (Same base and shoulder diameters). It proved a simple way to reduce capacity a bit, and get a 30 degree shoulder.

Jerry
 
Jerry,

I like mass produced (cheap) resize dies that don't require a reamer and heat treating to be useful and will accept some compromise for budgetary purposes but I think 30 degrees is just right for shoulder angle as you do. Since you are making a design change to the chamber and must create a resize reamer and die I suggest you consider salt bath nitriting as an alternative to heat treating. A nitrited die is very hard, slick, rust-proof and will never be distorted dimensionally by the process as it occurs at a much lower temperature (less than 1000 F) than heat treating. Joel Kendrick is a good source for this service.

Greg
 
JerryK said:
Thanks, maybe Al will chime in. I'm curious now about his method.
Click to expand...


I've spec'd WSM, RUM (404 Jeffery) and my latest project a 338 Lapua Improved by going .007 over at the base and .010 taper/inch forward..... let the shoulder dimension be what it may.

I like 35degree shoulders.

I require that my cases be capable of being reloaded 50times, HOT, without needing any attention at all.

I won't accept case "growth."

I won't accept "click."

I don't anneal.

At this point I've no reason to change. I'm still detailing the 338L case, haven't wrung it out. I'm going to try a 7/8-14 die first and step up to a Warner if I have to. All I've made so far is the fireforming shoulder-bump die. I've learned from the 404J based cases that they're HARD, that it takes at least three, often 5 firings to tighten them up. Because of this the 338L case will take some time to come to grips with. This hawg is so bloody expensive on barrels that I'm burning up a new barrel just for fireforming.

7/8-14 has been sufficient for the new beltless American Mag cases.

My guess is that .005 would be sufficent clearance for most sane people using the new Norma case since they load 'light' IMO and most often throw cases away about the time I feel they're getting good. I am not sane, SAFE but not sane,I simply can't find reason for loading light. If I want it to go slower I'll use a smaller case.

On another note, the two 338L's I'm building are using actions (BAT 'M', Stiller TAC338) which accept the large inch-and-an-eigth tenons. I wouldn't consider using a standard tenon for either of the two large cases.

I can't offer any real experience with the Norma case but looking at it I'd definitely "Improve" it.

al
 
alinwa said:
I don't anneal.
Click to expand...

Al, I just reloaded a set of .30 BR cases for the umpteenth time. The neck wrung off, right at the shoulder. I think I should have annealed them, they'd last longer. 3-4 years just isn't long enough.

On another note, the two 338L's I'm building are using actions (BAT 'M', Stiller TAC338) which accept the large inch-and-an-eigth tenons. I wouldn't consider using a standard tenon for either of the two large cases.
Click to expand...

?? My big BAT has a 1.25-inch tenon. Where did you get the skinny one?
 
Charles E said:
Al, I just reloaded a set of .30 BR cases for the umpteenth time. The neck wrung off, right at the shoulder. I think I should have annealed them, they'd last longer. 3-4 years just isn't long enough.



?? My big BAT has a 1.25-inch tenon. Where did you get the skinny one?
Click to expand...

from BAT.

I can use 1.250 barrels if I want to.

I don't want to

al
 
Al--
This pic shows a 30/338 Lapua Mag, the production 300 Norma Mag, and my version of "improving" the 300 Norma. As far as case taper, Norma factory has already taken most of that out. I'm impressed with the case interior around the flash hole, it looks nicely drilled and beveled, almost machined. And yes, I'm using the BAT 'M' also, with a 1.375 barrel. More meat, more betta.
Jerry
 

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Hello Jerry.
Did you finish Your 300 Norma mag improved?How does it shoot?accuracy?

I want to do someting simular.Shorten a 300 Norma and make a 30 or 35 degree shoulder.
 
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