Cast about 1000 .38/.357's last week and when i sized them they didn't look right. Miked 'em and found that they are out of round right out of the mould. Diameter ranges from .353 to .363 as cast. After sizing they are about .353 to .358. The varying dimensions are for the same bullet, being rotated in the mike.

I've cast thousands of bullets with these mould blocks with no problems in the past. Any ideas as to the cause and fix?

Also, should there be any problems shooting them? Any way they can damage the barrel?

What make of mold? My concern is the .353" measurement! How it got so small?
The larger dimension could be caused by something not letting the blocks close all the way. Can you see light between the blocks? Molds are tough and even cheap Lee molds are hard to harm and I see no way a mold will start casting too small on one side. What size were boolits when they were round?
My take is it was too small to start with and something is holding the blocks open more now and causing the out of round condition.
.001" to .002" out of round is common with cherry cut molds and is no problem.
There is no danger shooting them at all but you might get gas leakage where the boolit is too small and lead the bore.
Check the block faces and locating pins to make sure the blocks close all the way.
Since you want boolits to come out a minimum of .358 and .359 is better, if a clean mold casts at .353" I would send it back and get it right.
In 55 years of casting, I found only one way to warp a mold, found when I bought some used ones. The handle pins with screws were put in so tight that they bent the blocks top to bottom. This made a gap in the center and could be seen by looking through the ends. I was able to repair them all.
NEVER crank handle screws in tight!
The best molds have floating pins to prevent this.
However, this will not cause what you have.
Give us a lot more info please.

My first question is are you sure of your mike readings?
Second why did you cast so many bullets before you checked.?

bfrshooter - The mold is an RCBS 2 cavity that I've had for quite a while - probably over 20 years. I usually cast a thousand or so .38's and another similar quantity of .45's about once every 1 to 2 years. All of the ones I cast last year have been sized and the mike out at a nominal .358 all around. The blocks are clean, close tightly, and I can't see any apparent problem. I noticed the problem when sizing since not all surfaces on the bullet were "cut" by the die. Thanks for your reply. You've given me some good info.

SR4759 - yes, I'm sure of my mike readings, and I cast so many because thats what I've done for 20+ years with no problems and no reason to suspect a problem this time. Guess in the future I'll cast a few and then mike 'em.

I am at a loss to explain it.
Check the side to side movement to make sure the pins are not worn, might need pounded out a little to tighten the blocks.
If you can't find a reason I would call RCBS, they will still take care of any problems.

Any possibility you have some zinc in your bullet alloy?

I have to assume that the mold used to cast slightly larger than 0.358 before the problem. There are several things that would cause it to cast out of round, but only bigger. These have already been mentioned above. For the mold to cast smaller than normal, and be out of round, there is only one cause as far as the mold itself. You would have to had it apart and removed metal from the block faces. Now you probably didn't do that so it has to be what your putting in the mold. Try some other known good alloy and see what it does.

you didn't use mold release did you?

The blocks don't appear to be loose and the pins don't look worn. I've cst thousands of rounds with thm, though, so they could be slightly worn without it showing. I'm using the same alloy, from the same batch of lead that I've used for the past few years so I doubt that the lead is the problem. And no, I haven't used a mold release.

Guess I'll follow your suggestion, bfrshooter, and call RCBS. Thanks for all the suggetions. I'll let you know what I find out.

Jim - this is the same alloy I've been using for years. Also, I cast some .45's at the same time from the same batch of lead and they came out fine.

Note on RCBS - very frustrating trying to call them. Called numerous times last week only to get a recording that I was caller 21 (or 18 or 23or some other number in the high teens or low 20's) and after holding for 1/2 hour or more, actually advancing to caller #8 once, the connection was terminated. Not by me. Would call back and find I was now caller 22 or so. I'll keep trying simply because I have to. Their warranty is good but their customer service leaves a bit to be desired.

I've had that - when the two halves don't lign up perfectly, the diameter will be under spec accross one face and over spec accross the other. My mould was worn and damaged so I didn't worry. My RCBS 44 mould also did the same thing but to a lesser degree and with harder alloy (which made it noticeable). The Lee ones shot just fine. Good, actually.

I said I'd give an update after the problem was resolved so here it is. I returned the mold to RCBS expecting that they would repair it. Today I received a brand new mold from them. What a great warranty!