A more detailed account of my brief description earlier…
Cartridge conversion is viewed by some as difficult and hardly worth the time. I think it is worthwhile, especially for someone fairly experienced with reloading, because it is really just an extra step or two added to the reloading process. In this case two steps, cutting down and annealing, but some conversions require neither. Some Wildcats offer definite performance or logistic advantages, and of course there is a sense of pride and accomplishment that accompanies the process.
The first step in converting is obtaining suitable parent cases. This is not too difficult with 357 Herrett since its parent case, the 30-30, is among the most common of rifle cartridges. Give the cases a good examination looking for defects like splits or dents in the rim, etc. Being careful here can make subsequent steps more predictable.
The second step is to cut the case to length since 357 Herrett is shorter than a 30-30. Take off about 3/10ths of an inch from the end of the case. The best tool, by far, that I have found for doing this is a small, cheap, ultra-fine narrow-kerf Zona “razor saw.” These are about $7. They are way neater than a hacksaw, and much faster than a Dremel (and don’t send abrasive dust into the air). If the edge doesn’t come out great or got crushed a little, don’t worry. As long as it isn’t too short, those imperfections will be corrected.
The next step is to anneal the case neck/mouth area. Annealing is a metal treatment process that softens the metal. This makes the brass more ductile and malleable, which will be necessary to prevent splitting as the case mouth is expanded from .308 to .358 inside diameter. The way I do this is with a small propane torch. Just run the torch over each side of the brass lined up for a few seconds until it visibly discolors. Make sure not to hold it too long or you will anneal the brass body and weaken the case. If discoloration goes below the neck area then it has gone too far. I usually err on the side of under-annealing than over-annealing. If you get splits in the sizing step, then you are under-annealing, but if you don’t get at least one split, you may be over-annealing. I split one in this batch of fifty-five (another three I cut too short leaving me with 51).
Now that the brass has been annealed apply a quality case sizing lubricant as directed. Make sure to get some on the inside of the case mouth, too.
Now flare the case mouth. There are dies that do this, but I use a steel pin punch with a flared shoulder. The ¼” or 5/16ths” punch works fine. Just a couple light raps with a light hammer is all that is needed to put a slight flare on the end.
Now you will deprime, expand, and size the case. This requires a little more attention than usual. Since 357 Herrett is supposed to headspace on the rim, you have to make sure you set back the shoulder just far enough that the case isn’t headspacing on the shoulder.
However, you do not want to set back the shoulder more than necessary since this will cause case stretching and wear out the brass prematurely. Since I am loading for a T/C G2 Contender, this is very easy. Screw the full-length resizing die into the press until it touches the shellholder when the ram is at top of stroke. Then back out the die a couple turns. Size a case.This will be hard at first since the expander button has to expand the neck up to diameter (it will never be this difficult again). Now trim the case (see subsequent trimming instructions). Insert the case into the chamber. You shouldn’t be able to close the action because the shoulder is too far forward and the rim isn’t seating into its groove. Now turn the die in an eight of a turn or so and size the case again and then check to see if the action will close. Keep doing this until the action just closes. Then give it another 1/16th to 1/8th of a turn for good measure. Now you may size all your other cases based on this die setting.
The next step is trimming. I use the Lee-style lock-stud and pin type trimming system. This will require a custom pin from Lee, which costs $16. I’ve made these things on drill press with a file using a grade eight bolt in the past, but this takes a long time. I really think Lee does a better job and it is worth buying the custom pin. What is great about the Lee system is that it keeps the neck concentric with the case, the mouth perpendicular to the case, and it adapts to a cordless drill. If you only cut off around 3/10ths of an inch, then you only should have to trim 30-80 thousandths to arrive at the max case length of 1.750 inches.
Then I file off any dents or burs that are on rims, and chamfer the inside and outside of the case mouth with a chamfering tool. I give the case a good inspection. Then I wash then in hot water with ample dish detergent. I use a little nylon brush to clean the inside of the cases and rinse thoroughly in hot water and let it all dry out by the wood stove overnight. These cases are ready for loading.
It is best to fireform the case with a starting load. The case will expand to fit the chamber well, and now you may only neck size the case. The case will have a slightly larger capacity (so max loads will be safe) and last longer if you neck size only. Neck sizing dies for the 357 Herrett may be purchased, but I prefer to have a custom Lee collet neck sizing die made. It’s worth the wait in my opinion. Eventually, after a few reloadings, you may find that the brass has expanded to the point where the shoulder is bumping against the chamber and the case and the action wont close because the case has gone to headspacing on the shoulder rather than the rim. The solution here is to full-length size it again as described above to set back the shoulder slightly. After many reloadings this full-length sizing may thin the brass in the critical web area of the case, usually on the inside, where you can’t see it. This is why you should periodically run a bent paper clip or dentist pick inside the case and “feel” if the web area is thinning. The web should be thicker than the brass in the middle part of the body. If it isn’t thicker or is thinner stop using that brass. It has become worn out. All good things must come to an end sometime, but at least with 357 Herrett, which operates at moderate pressures and is minimally sized with each reloading, should long time before you will have to repeat the conversion process.