Just a guess of mine. I have always read that when fire forming brass to a specific chamber, you should start out with new factory brass. I agree with the above statement that the neck may be work hardened, whereas if you had used factory brass, it would form without the cracking.
Usually this doesn't happen until several firings.
But on the other hand I may be totally off base.
I do have several wildcat rounds that I have to fireform and have never had any of them split during the forming process. But this was also using new factory brass.
Hope you get it figured out.
HBL
I found this on google searching for annealing brass.
When brass is cold worked the crystallic structure in the brass changes,
making it hard and brittle. This can occur after reforming a case into
another caliber (wildcatting) or after repeated firing and resizing.
Annealing.
By heating the brass, the crystallic structure of the brass can be restored
to normal. Only the bullet-gripping part of the case (and perhaps the
shoulder portion on bottle-neck cases which have been or are about to be
cold formed in this area) should be annealed to restore the "springiness" of
the brass and provide a good bullet grip. The rest of the case (body) and
the web area (the "bottom" of the case) must NOT be annealed or allowed to
get soft. That could be an invitation to disaster. When manufacturing cases,
the brass is annealed several times during manufacture due to the many cold
forming steps involved in transforming brass strip into a rifle or pistol case.
The method of annealing has been described in several articles in the
litterature and elsewhere. However, they differ as to the amount of heat
applied and how rapidly the brass is cooled afterwards. Which one is
correct/best is up to you to decide. You can try squeezing with a pair of
pliers etc. and compare with a factory annealed case.
Using a propane torch.
A lot of heat, as from a propane torch, makes the process rapid, and
minimizes the risk of the heat spreading to the web area, something we
definitely do not want. At the same time it is very easy to overheat the
neck area, so you are advised to begin experimenting with some less valuable
brass, until you "get the feel" of it.
To limit the "heat zone", the cases can be placed in a pan of water so that
only the portion of the case you want annealed is above the surface. The
only easy way to monitor the temperature is by observing the color of the
brass as it is heated. One source simply says place the case in a pan of
water covering 1/2 to 2/3 of the case and heat the neck to a cherry red,
then tip the case into the water. Two other sources say that the correct
color is a light blue (660-665 degrees Fahrenheit or 350 degrees Centigrade).
The cases should be cleaned and polished to make it easier to observe the
color changes during heating. To simplify the process and get consistent
results it is a good idea to build a small platform which can be rotated by
hand or perhaps with a smal motor. On top of this platform, a small pot is
placed. The pot is filled with water up to approximately half an inch below
the shoulder. The brass case, with spent primer still in place, is placed in
the center. Now you can rotate the platform with one hand and direct the
flame from the torch to the neck-shoulder area with the other. (The hottest
part of the flame is the tip of the inner bright blue flame.) When you can
see a bluish discoloration, remove the heat and wait 10 to 15 seconds before
removing the case with a pair of pliers. Another source suggests holding the
case between your fingers and toss the case into a bucket of cold water when
the light blue color appears. If the shine disappears you are on the hot
side. Overheating will make the brass too soft to properly grip the bullet.
Using a lead pot.
A more controlled source of heat is an ordinary lead pot used for casting
bullets. The thermostat is set to 650 - 800 degrees Fahrenheit (343-427
degrees Centigrade). The cases should first be cleaned
and deprimed (to prevent forming an air pocket). To prevent the lead from
"soldering" to the brass it is first dipped in oil. Fill a shallow pan with
oil to a little deeper than what you want annealed. The case is then held
with the fingers and dipped into the oil, and then into the molten lead for
about 15 to 20 seconds. (Let most of the oil drip off first.) Then quickly
toss the case into a bucket of cold water standing behind you. It is VERY
important that not even a drop of water gets into the lead pot, something
that could result in molten lead flying about. As with the propane method, a
few simple tools can simplify matters much. A device to hold the cases, at a
predeterminated depth in the oil/molten lead, can be designed. From the
"finger-burn" method, you should know the number of seconds necessary to get
the heat up. In this way several cases can be processed simultaneously.
CAUTIONS!!!
Make sure NO LIVE PRIMERS are present in the cases when annealing. Don't let
any water get into the molten lead as that could cause hot lead flying
about. Make sure all cases are dry when you begin. Observe all the usual
precautions that applies to bullet casting when annealing cases with the
lead pot method and wear protective glasses and clothing.