RCBS Gas Check Step still too large

[Irascible]

I'm not sure why RCBS sizes their pistol moulds to use with a lead tin mixture (from their instruction sheet), but I'm a fan of WW or Lyman #2 and the gas check area comes out too big with those alloys and with their 38-158-SWC design. It's a nice bullet and shoots well, but putting the gas check on is a chore. I sent the first mould back to RCBS as it was impossible to install the check. They replaced it and I can get the check on this one, but it's a chore. Why they insist on sizing for a soft alloy in a cartridge that is prone to leading is beyond me!
Basically I have two (well, actually 4) questions;
1. I have a good smooth bore, what is the softest alloy I can use with a gas check, in a 357 at 1300-1400 fps?
2. Is there a way to slightly enlarge or bevel the inside of the checks? A punch? what measurements?

[BABore]

You can try a couple things on the gas checks. First I would anneal a few of the ones you have (I assume they're Hornady's) Heat them up in a clean skillet until they turn dark, then let them cool or drop them in water. It don't matter which way. This sometimes relaxes them just enough. You can do a search on how to anneal them in bulk if needed later. You can also order a batch of Gator gas checks direct, (318) 387-7257. You'll get Dr. Larry Blackmon's dentist office, so don't be alarmed. Just tell the nice lady what you want. Gator's seem to run a touch bigger, and better IMO. Gas checks can also be opened slightly with a ball bearing and a light tap. Slow, but effective.

As far as your mold. How big of a boolit does it drop? If you have to size it down a thou. or two, you could also spray on a coat of mold release. It will reduce the diameter a bit for you. You may also be able to Q-tip apply a couple coats in the GC shank area. As far as softer alloys, I would try 50% WW's and 50% Pb. That will reduce size by a very small amount. The more Pb you add, the smaller it will get. Whether your gun will like it or not is up to it. I have several that shoot a soft, GC'd boolit very well at your velocity.

[S.R.Custom]

I'm not sure why RCBS sizes their pistol moulds to use with a lead tin mixture (from their instruction sheet), but I'm a fan of WW or Lyman #2 and the gas check area comes out too big with those alloys and with their 38-158-SWC design.

Actually, RCBS uses linotype as a design standard. (I've been down this road with them before.) In my experience, the problem is more likely to be with the gas checks. Checking the diameters of both, I've found Hornady gas checks to frequently run small.

To address this problem, I made a press mounted tool & punch that expands gas checks (for .44 caliber). It takes me about an hour to run through a box of 1000.

[Dale53]

I have found that the Lyman "gas check seater" to work well if the checks are merely tight. I seat all of the checks first, then size and lube them. This insures that the checks will be seated squarely (critical for best accuracy).

Dale53

[Newtire]

I did the same as Dale with a box of old Sierra checks that were tight. Funny, I scored an RCBS 32 special mould that had small gas check shanks using wheelweights and the Sierra checks fit with a little help from the Lyman tool.