mold casts undersize boolits

[bosbill]

I recently acquired a RCBS 38 150 KT mold and it throws undersize boolits. I have tried 16 to 1 and WW and both measure approx. the same, front band is .356, base or rear of boolit measures .357. I can start a .359 pin in rear cavity .358 goes in easy in both. I have cast at 650 and 700 degrees with no measurable difference. Any thoughts. I read where guys say their molds throw .358 .359. I guess I'll send it to RCBS!

[bhn22]

I'd have to find my old RCBS manual, but I believe the old molds were proofed on linotype, you may need more antimony to get your diameter up. It's also common to find undersized front driving bands in molds to allow the loaded bullet to enter the throats of tight chambered revolvers. Lymans Keith designs were also like that for decades.

[paul h]

Take a couple of the bullets you cast, drill and tap the bases for a 10-32 screw. Screw a ~1" long 10-32 into the base and cut off the head of a screw. Chuck the screw into the chuck of a drill press. Put a dab of 180gr valve grinding compound on the offending band. Remove sprue plate from mold. Clamp mold around bullet. Turn on drill press. With hand pressure closing the mold run lap for 1 minute. Repeat in other cavity with a fresh dab of lapping compound. Clean mold, replace sprue plate, start casting.

I had several .357" wadcutter molds (for 38 supers) that I lapped out to .359". Well worth the effort.

[bosbill]

The print for the mold is on the internet and the dimension for the forward or front band is called out as .359 all of the bands are suppose to be .359

[M Hicks]

as bhn22 eluded to; alloys can change the dimensions of bullets. The dimensions in the prints are nominal and can be changed by many factors. Mold temp, alloy temp and alloy composition are a few.

[randyrat]

I have had molds that just cast too skinny....Lap them out, sell, give away, or take a big 2 lb maul and pound it to little bits...

When they cast way under alloy just is not going to cut it. I gave you the options.

[Maven]

Post #2 is correct (bhn22). If you attempt to lap the mold, you will also void RCBS' warranty. It would be much simpler to return it to them with an explanation and let RCBS either replace it or lap it for you.

[detox]

See if the boolit will slip thru cylinder forcing cone after sizing. I size all my 357 magnum boolits using a .357 Lee sizer...boolits will spring back larger .3575-.3579. My forcing cone on my S&W 686 measures .3570" using .3570 pin.

What are you using for measuring? A micrometer is more accurate

[bosbill]

I'm using Starret mikes on the OD and .0002 - pins for the ID.

[Boolseye]

This puppy may be a candidate for lapping if you're up for it.
I use valve grinding compound followed by comet, but I do it by hand. no power drill for me.
Since it's a steel mold, however, the power may be needed.

that said, RCBS will probably replace it no questions asked.

[Garyshome]

You can also use Cleanser to lap your mold. I use it to polish molds that give me trouble.

[gwpercle]

Try shooting some at .357 and see how they perform. For years I have been casting and shooting them at .357 and never had any problems. I was going to buy a .358 sizer die when I started, but the Lyman Cast Bullet Handbook stated the 38 spcl/357 mag. boolits in there were sized to .357...so thats what I got. Never saw any difference in groups when shooting some store bought Valiant cast and sized .358....try em, you'll never know if you don't.
And don't believe that talk about they GOT TO BE .358 or .359 to shoot well, I got some trophies that prove that .357 can be quite accurate.
Gary

[theperfessor]

I'm surprised nobody has suggested "beagling" the mold. There are several stickies here about how to do it, basically you put a layer of thin metal foil tape on one mold block face to add a couple thou' to the diameter. Surprisingly, when I've tried it it really didn't create much out-of-roundness beyond what was present in the original mold. And it is reversible, unlike lapping, etc.

Just a thought.

[Iron Mike Golf]

... I can start a .359 pin in rear cavity .358 goes in easy in both. I have cast at 650 and 700 degrees with no measurable difference. Any thoughts...

Did you measure that when the mold was cold? I'd expect the cavity to expand about .001 when up to casting temp.

I'd make a batch of lino and run the mold a little on the hot side and see what you get for dimensions.

You might try beagling, like Perfesser said, before lapping.

If you use cleansers, check the label. I'd avoid Bar Keeper's Friend, as it contains oxalic acid.

If you go the lapping route, I recommend leaving the head on the screw and using a screwdriver to lap by hand. I don't think you need to take much metal, if any, off.

[seanhagerty]

I agree, beagling before lapping. Do not damage your mold by lapping, just put a few small pieces of aluminium tape on the mold face and cast a few to see what happens. If it doesnt work, take them off. No harm to the mold.

Before doing any lapping, call the mold manuf. and see what they say.

[Buckshot]

..............RCBS has the best warranty in the business. I'm with Maven and whoever else suggested sending it back to them. If you do, be sure to include a slug cast in each cavity with the blocks. Lots of people have no issue in sending a $20 mould back to Lee, so sending a $75 mould back makes sense to me.

.............Buckshot

[cbrick]

My experience with RCBS is that if you send it back they won't question anything, simply ship a new mold.

Heed the advice already given about alloy making variations in casting diameter.

Anyway, 10 days since the OP posted, let us know what ya decided to do and how it turns out.

Rick