Maximum chamber dimension for wildcat?

J

jkpq45

New member
Here's the plan:

.30-06 cartridge as parent case, necked down to .243. Kinda like 240 Gibbs.

I have a 6mm-284 reamer I'd like to run into my 243 Win barrel about 2.500". This will create a chamber diameter of about 0.5042" at the breech face.

What will happen to the .30-06 case when I try to fireform it in this chamber, knowing the brass head diameter is a minimum of 0.4630" with a base OD of ~0.4968"? So we're talking of a diameter difference of 0.0412" at the rim and 0.0074" at the base.

Thanks for the expert advice,
jkpq45
 
You are getting ready for bad things to happen. You need 284 Win brass plain and simple.

Do not shoot 30.06 brass in a chamber you describe! ! ! ! !
 
I cant say for sure what will happen because it defies all the rules I have ever "wildcatted" by. Having said that, why not build a 6x284? It has potential to be bench rest accurate. Its standard and you dont have to make anything.
 
IMO you need to decide on either .240 Gibbs/6mm-06 or 6x284 and not somewhere in between. Sounds like a good way to ruin some brass and possibly a barrel and/or action to me. There's no advantage that I can see to be gained by trying to "hybrid" the two anyway. There's a little more case capacity with the '06 but in a .243 caliber round it's a waste IMO. The .284 case is also a more proven target round and seems to be more inherently accurate. If I were in your shoes I'd just ream the barrel for a standard 6x284 and not think twice about it.
 
So, in theory, this situation would fireform the brass to the chamber up to a certain point, then, when the brass got thicker, the case would not fireform to the chamber.

So what happens past that point (closer to the base of the cartridge)? If the pressure of fireforming doesn't distort the case body, it wouldn't rupture, would it?

I envision the body between the case web and the taper of the case body being supported by the chamber would look pretty weird.

Looking at SAAMI drawings, most cases (with the notable exception being 7mm Weatherby Mag) have a clearance to the chamber wall of about 0.001" diameter. I wonder how much is too much?
 
Looking at SAAMI drawings, most cases (with the notable exception being 7mm Weatherby Mag) have a clearance to the chamber wall of about 0.001" diameter. I wonder how much is too much?
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How much is "too much" depends on what cartridge you're dealing with, what part of the case you're talking about, what kind of pressures you're talking about, and what the pressure profile is (quick and violent, or slow and smooth).

Regardless...
.30-06 cases in a 6mm-284 chamber will not work out for you. Even if the cases don't rupture during the first firing (which is highly likely), they're going to be severely stretched and distorted. Any hope of future consistency and safety from those cases will gone.
 
I am the voice of experience. One day at the range I was shooting a couple different guns. Two of the guns were identical 6.5 Arisakas. One I had re-cut to 6.5x55. It was a slow, hot day and I stupidly had both types of ammo within reach. I was shooting one shot at a time and just reached over without looking and stuck a 6.5x50 round into the Swedish chambered rifle. I felt a light recoil and I extracted the case it rolled across the table. At first I thought I lost part of the neck because it was so short. The brass had stretched to the limit and formed to the chamber size, EXCEPT right where the head started. The OAL was about .200 longer and the area by the head was paper thin. I was extremely lucky that day. The head will not expand unless under extreme pressure in a chamber tight enough to contain the brass flow of the rest of the case. Your case will probably shear right off in front of the head. I have worked on many guns over the years that had the wrong ammo in them. Some of the lighter loads you can get away with a jammed action or nothing happens. Some really intense rounds (As you described) usually ruin the gun and somewhat injure the operator. Even if the shooter gets away unharmed, he usually has a bad case of "Flinchitus" for a long time afterwards.
 
jkpq45 says:
Looking at SAAMI drawings, most cases (with the notable exception being 7mm Weatherby Mag) have a clearance to the chamber wall of about 0.001" diameter.
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Looking at SAAMI chamber and case specs, the clearance around the cases are closer to .003" to .005"; a diameter difference equals twice the clearance; .006" to .010".

For the .284 Win.

pressure ring diameter point in chamber; .5010" +.0020"
pressure ring diameter on the case; .5000" +.0020"

shoulder diameter point in chamber; .4759" +.0020"
shoulder diameter point on case; .4748" +.0020"
 
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I misread that. I was not looking at the .007 spec. Not as bad as I thought. The standard for body dimensions is .008 on the ammo specs. If you did the measuring and stuck to the ammo with a high case diameter? Still, if somebody else got a hold of it and fired really small diameter brass, it could be bad. If you mic new brass from "The big 3" you can see quite a difference in the body diameters.
 
To put everyone's mind at ease, I'm now looking into a toolpost grinder for my lathe to reduce the body diameter of the reamer. That way the chamber I cut can accommodate 30-06 brass as the parent.

I just can't afford 6.5-284 brass, so something has to give.

Thanks, all, for your advice!
 
I misread that. I was not looking at the .007 spec. Not as bad as I thought. The standard for body dimensions is .008 on the ammo specs. If you did the measuring and stuck to the ammo with a high case diameter? Still, if somebody else got a hold of it and fired really small diameter brass, it could be bad. If you mic new brass from "The big 3" you can see quite a difference in the body diameters.
Click to expand...

A lot of commercial ammo and component brass is sized smaller than minimum SAAMI spec.

I used that fact to my advantage for my 6mm wildcat. It's a ".243 Winchester" chamber, but under SAAMI minimum spec by 0.002" to 0.003" in nearly every dimension except neck diameter.
Even with that "ultra-tight" chamber, Remington ammo and brass chambers like butter. Winchester is snug - not tight, just snug. ...because it's all under SAAMI minimums. Federal is a crap shoot, but that's because they don't understand how to control head space. - In the same box of ammo, you might have 3 cases 0.008" under minimum spec, 12 cases in tolerance, and 5 cases 0.003" to 0.030" OVER maximum. ...but I don't like their brass, anyway.
(I haven't tried other brands in the wildcat, because I either don't like them, or don't care. But the measurements I've taken show that under-spec cases are the norm for every rifle cartridge I reload.)
 
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