chamber-case axial clearance

[lurcher]

I'm investigating building a simple wild cat by running an existing reamer in 200 thou short. The reamer print I have is .551 at the widest point (rebated rim) 200 thou up from this point it is 0.549. I have measured some pieces of new brass (as best I can using digital calipers so not as accurate as it could be) and get an average base diameter of .5445

I am thinking if I do this and then size brass with a die with 200 thou trimmed off the bottom I might run into problems.

I have read of people doing similar things and having issues getting rid of the dreaded click at the top of the bolt lift as there is not sufficient clearance at the base of the brass.

I have read that sometimes the problem can be solved by polishing the chamber and/or getting a die set up to size the brass at the base.

Another approach would be to design a new reamer to give more clearance. Can anyone advise what constitutes a suitable clearance? Is it possible to overcome this issue by polishing the chamber and getting a die set up or am I better to get a new reamer?

Are there other things I need to consider?

 

[AlNyhus]

By the figures you've given, you're probably going to be okay at the base.

But don't forget about the front end. -Al

 

[lurcher]

Thanks Al

When you say don't forget about the front end of the case are you referring to anything other making sure it is sized to fit the chamber?

My plan is to use form dies to push the shoulder down 200 thou I will then trim, neck turn, (inside ream at some point if required). The case will be sized with a redding body die with 200 thou trimmed off the base.

 

[mikecr]

I think just opposite(typical for me). :
When I was a teen, shade-tree mechanics were so sure they needed bigger clearances for higher horsepower engine builds. As an adult I learned that was just opposite of true(thanks to Smokey Yunick's testing), and that this really amounted to a fundamental shift in stock car engine building.

Anyway,,
No matter the clearance, firing will take your brass to the chamber walls. After pressure peak the chamber will spring back and so will the brass. If your brass doesn't springback as much as the chamber, you're left with an interference fit.
Now with large clearances, your brass expands past yield(it's springback potential). It still springs back some, but now it's from a larger dimension, and it's also thinner. Because the chamber is damn sure gonna spring back, you're that much closer to an interference fit.
VERY high pressure loads, or insufficient chamber wall thickness(for the case) will amplify this action because the chamber expands even more, the brass goes right with it to a new dimension, and then the chamber springs back to original, while the brass had yielded. Just as necks and shoulders yield to whatever clearance there is for them to go to(no matter how much).

With this, the shade-tree mechanics seem to be at it again..
Stuck cases obviously mean ya need more clearance AND more sizing, right? And if we have to, we'll polish out the back of the chamber till we can pull cases out long as they last anyway..
Well that brass isn't goin back to original -ever again. You can squish it all back down with a die. But brass will move to least resistance(thick toward thin) and so it will never again be like original. It'll move upward ever changing the charactor of the case, and leaving the web area thinner & thinner. The shoulders will begin to bump differently, donuts form, neck thickness changes, and you're left trimming and reaming and trimming, forced to FL size, and eventually the problem returns. Long as they last ends up being long as it's anything close to original. Or long as it still shoots, given your efforts.

Anyway,,
Holding alien-like perspectives, I went 2thou over webs for a 6.5WSSM chamber.
I also went with a large tennon BAT action(to get thicker chamber walls).
As suspected, I could run higher pressures than normal for this case, and without FL sizing -ever.
In fact, the first fireforming took the cases to fully fitted, and I did not have to pull out a sizing die to reload them until bumping was needed at the 8th reload.
I shot out one barrel with 60 prepped cases(30+ cycles) without FL sizing, and they still measure exactly as they did from 1st fireforming. I use a custom bump die, that does not size the bases.
I normally run 139Laps @ 3020fps with a moderate load, but I have pushed em to 3300 without issue.

That's just one test, one example. But it's a challenging case. You wouldn't need a larger tennon with .473 rimmed cases. And with consistent brass that diameter, I doubt you would actually need more than 1thou of base clearance. With this, the chamber would not expand so much as to cause brass yield, so your bases would just spring back like tight necks.
Just sayin..

 

[lurcher]

Ok so hopefully with little over 2 thou clearance I should be OK. I will be very tentative in load development given that it is a wildcat.

Here's a quote from the clymer website

Nothing looks worse than an unsightly bulge at the base of a case fired in a brand new wildcat-chambered gun. This can be avoided by carefully selecting a lot of brass and having the special reamer ground to match. In its published tolerances for centerfire rifle cartridges, SAAMI allows cartridge diameters to vary by as much as .008". Necks can be turned, case lengths can be trimmed to close dimensions and shoulders can be blown out and forward without much grief, but it is difficult to change the base diameter of the brass. When buying a lot of brass, a number of cases taken at random (10 % should be sufficient) should be measured to assure they fall within variances you consider reasonable. A little extra time taken at this stage can save quite a few headaches later on.

This, and other forum threads such as This one led me to believe this can certainly be an issue if you don't get it right..Three thou was quoted as a minimum here and others like 5 thou.

 

[AlNyhus]

It would wise to check the die dimensions to make sure how much sizing you're going to get (at the base) after you shorten it. -Al