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Fire forming wild cats

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Unread 07-04-2012, 09:13 AM
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Fire forming wild cats

Having fire formed many wild cats I just had an interesting outcome and wondered how
many of you have had this happen to them.

Having read about the problems of the 22 hornet while fire forming I was ready to take on all
problems but was not expecting the outcome I had.

Before fire forming, I measured the case length to see how much growth in length I had to
monitor case thinning.

The cases did not lengthen but actually shrank in length by .009 thousandths.

I can theorize why it happened but would like to hear what experiences anyone has had with
this and how/why It happened.

I am very happy with it, but it is the first time for me that a case has shortened while fire forming
in a larger chamber .

Any theories or comments are welcome (You are never to old to learn something).

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Unread 07-04-2012, 09:28 AM
Edd Edd is offline
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Re: Fire forming wild cats

Without seeing the shape of the cases it's tough to comment. But I'd guess your case surface changed shape. Did the length of the surface get shorter or just the end to end length?
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Unread 07-04-2012, 09:38 AM
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Re: Fire forming wild cats

If you improved shoulders, they pulled the neck back.
I see this all the time.
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Unread 07-04-2012, 09:42 AM
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Re: Fire forming wild cats

I've found in the past that when fire forming an improved case shape, that the over all length will shorten as much as .04" (usually in the neck area alone). Some cases are much worse about this than others, and I think it has to do with just how much brass is moving around during the fire form operation. My 30 Herretts shrink about .02" while my 6/250AI goes close to .045". I've heard guys that shoot the .243AI loosing .040" - .050" in neck length
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Unread 07-04-2012, 09:53 AM
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Re: Fire forming wild cats

Very normal for improved cases---brass has to fill somewhere...
The Truth Is Not Always Good For Business!!
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Unread 07-04-2012, 12:49 PM
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Re: Fire forming wild cats

Fire-forming 6mm cases to 6mmAI ALWAYS resulted in shortened cases. It never presented any problems though.
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Unread 07-04-2012, 01:01 PM
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Re: Fire forming wild cats

JE, about 4 yrs ago, on every rebarrel and rechamber that I did, I started recording the dimensions of unfired rounds and then the dimensions of that same case after firing. I only have 8 examples.

.375 H & H (belted) rechamber. case OAL unfired---- 2.840". Fired------ 2.854". SAAMI std. --- 2.850".

.358 Norma Mag (belted) rechamber. case OAL unfired--- 2.505". Fired---2.500".--- std.(Barnes book)--2.520".

.270 WSM rebarrel. case OAL unfired----- 2.094". Fired----2.097". ---- SAAMI ---- 2.100".

7 x 57 Mauser rebarrel. case OAL unfired----2.238". --- Fired---2.240". SAAMI---2.235". (S&B ammo).

6.5 x .284 Norma rebarrel. case OAL unfired---2.170". Fired---2.169". std (Cartridges of the World)---2.170".

6.5 x .284 Norma rebarrel. case OAL unfired--2.160. Fired---2.160". std. 2.170"

6mm BR Norma rebarrel. case OAL unfired----1.554". Fired----1.554". std. (Nosler book)--1.560".

.22 Kilbourne Hornet rechamber from Hornet. case OAL unfired----1.4" (WW brass). Fired----1.395". Std. (Hornet), 1.403".


One belted mag case OAL grew .014".-------One shrunk .005".

Two factory rimless rounds grew .002" to .003".

The two 6.5 x .284's shrunk .001" or less.

The 6mm BR was unchanged.

The K-Hornet shrunk .005".


The fireformed K-Hornet approximates your experience. For the others, I would hate to have to predict what they will do, based on the very limited data I have, but it appears that standard factory rimless rounds stretch when fired the first time. Years of reloading support this because of all the times I've had to trim brass back after repeated reloading. That experience holds true for repeated reloading of belted magnums also, but they don't grow much before the primer pockets get loose. The only belted magnum I've had to routinely trim was a .300 H & H. It would get too long before the primer pockets loosened.

Different brands of new brass and ammo were used, but since the dimensions were taken on the same hull before and after firing, the data should be accurate. In some cases, the std. dimension that I use may be a maximum case length.

JE, I got way off your original subject, but the thread caused me to go back and see if I could make any sense out of what little data I had for several cartridges.

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