RCBS 240 SWC mold? Dropping too small?

[BK7saum]

I have an RCBS 2 cavity iron mold that is dropping at approximately .4285-.429 (checked with accurate calipers, didn't have my micrometer handy). The recommended sizing die is .430. I've had the mold for about 15 years. It may have a layer of crud built up in the cavities because I didn't clean it the last time I used it.

Question is: Are RCBS molds usually right on or do some of the 15-20 year old molds drop a little small?

BTW, Alloy is 50% COWW and 50% lead with 1% tin added. Other molds with this alloy have not been dropping small.

I will clean/scrub and recast a few to see if there is a difference.

If in fact the cavities are a little small, what are my options? This is an iron mold, can it be lapped to open up to .430 or .431?


Thanks, Brad

[runfiverun]

you can lap it easily enough.
you could stop using the soft lead too that might get you close.
add some tin to those ww's and you might make the full 430.
nobody really makes molds for pure lead except for the b/p type molds,the closer you get to that the smaller your boolits will be.

[Lloyd Smale]

mine casts at 431. Send it back and they will proably send you a new one.

[Larry Gibson]

We have numerous complaints of Lyman, Lee and RCBS moulds casting undersize. Many have begun to think the quality of such moulds is such that they are made undersize. I have yet to find that to be the case. What is most often the case with each complaint is the use of WWs with too little tin and possibly incorrect casting technique. If we use an alloy different than the mould is designed for and the bullets aren't the diameter we want is it really the moulds fault? I have a standing offer to test any Lyman mould that is thought to cast undersize and if it does using a correct alloy I'll buy it for what you paid including shipping to me, I have yet to buy a mould as such. If you'd care to send the mould to me for testing I'll extend that offer to RCBS moulds. PM me if you want to.

But 1st I think you can solve the problem yourself so I suggest you clean the mould as you mention. Add 2% tin to your COWWs and then add the 50% lead. Adding 1% to your WWs after combining with 50% lead is still not enough tin, especially as the qual;ity of WWs varies greatly in tin content to begin with. The alloy you are using has a very large % of lead which alloys greater shrinkage. Also the tin % is not balanced with the %of antimony. Thus the antimony does not go into solution with the lead for the best ternary alloy which also increases shrinkage.

Cast with the alloy at 700 - 725 degrees, closer to 725. Keep the mould warm and if using a bottom pour keep the sprue plate not more than 1/2" from the spout when pouring. Open the spout adjustment so the stream fills the cavity quickly as you want the molten alloy to get into and fill the cavity as quickly as possible. The alloy will solidify in the cavity quickly, quicker than we think actually. Leave a very good sprue even if the alloy runs off. This keeps the alloy in the cavity molten as long as possible allowing all the air to get out and maximum fillout which results in the largest diameter the cavity will allow with the alloy used. Fill each cavity seperately.

If using a ladle then swirl the ladle down through the alloy when filling and use the method described in the Lyman CBH's for pouring. Again pour a generous sprue imediately after lifting the ladle off the sprue plate even if it runs over the mould. Fill each cavity seperately using a freshly filled ladle.

The RCBS Moulds are designed with a harder alloy with a higher tin content than you are using and will cast to nominal diameter with such. I find that with my suggested alloy and the proper casting technique you should get as cast at .429 - .430 at least. With COWWs + 2% tin (- the lead) the aged (7 - 10 days) bullets will probably be .430 - .431. Adding 50% lead will increase shrinkage resulting in a slightly smaller diameter bullet.

Larry Gibson

[376Steyr]

IIRC, RCBS 44 moulds used to be cataloged as dropping at .429, with a recommended 429 sizer die. I was surprised to see the latest ones are now listed at .430. My 44-250-K (which seems to have disappeared from the RCBS website) insisted on dropping right at the spec'ed .429, which would be cause for rejoicing if I wanted them to be that size. I resorted to using some Beagle tape to get them up to .430-431.

[243winxb]

Bullet Sizes & Weights – How to Vary Them


Quote:
The bullet diameters and weights presented in this list
are based on the use of Taracorp’s Lawrence Magnum
bullet alloy (2% tin, 6% antimony, 1/4% arsenic,
91.75% lead).
Bullet diameters and weights will vary considerably
depending on the lead casting alloy used. This variation
can be as much as 1/2% on the diameter, and 8% on
the weight among the most commonly used casting
alloys. For example, a .358-158 grain bullet might
show a diameter variation of .002", and a 13 grain difference
in weight.
Of the most commonly used alloys, wheel weights (.5%
tin, 4% antimony, 95% lead) will produce bullets having
the smallest diameter and heaviest weight, with
such bullets running approximately .3% smaller in
diameter and 3% heavier than bullets cast with
Taracorp's metal. Linotype will produce bullets with the
largest diameter and lightest weights. This alloy will
produce bullets approximately 1/10% larger and 3%
lighter than Taracorp. Other alloys of tin and antimony,
with antimony content above 5%, will produce bullets
with diameters and weights falling between those cast
from wheel weights and linotype.
Alloys containing little or no antimony will cast considerably
smaller than wheel weights and in some cases
will produce bullets too small for adequate sizing.
Within the limitations given above, the weight and
diameter of a cast bullet can be adjusted by varying the
alloy’s antimony content.
The size and weight of bullets of a given alloy will also
vary according to casting temperature. Higher temperatures
will result in greater shrinkage as the bullet
cools, thereby producing a slightly smaller and lighter
bullet than one cast of the same alloy at a lower temperature

[cbrick]

The best advice I give you is to re-read post #4. It is spot on.

243 is correct in his quote about antimony unless you don't want the brittleness or expense of a 5%+ Sb boolit. If you wish to go that route Roto Metals Super Hard is about the easiest way to add Sb.

As for RCBS molds casting undersize I suppose anything is possible but I have 25-30 RCBS molds and not a one of them cast undersize.

Rick

[BK7saum]

Thank You, everyone. I suspect my problem is either alloy or slight buildup of crud in the cavities.

I will address cleaning the mold first, then if that doesn't solve the problem I will add tin or change the alloy

[BK7saum]

This is a great site. I'm always impressed by the wealth of information here and above all, a willingness to help fellow casters and shooters.

I'm still trying to get all my ducks in a row here. I attempted to start casting about 20+ years ago, didn't have much luck and went to straight j-words for my reloading. Now I'm getting back into casting and have been met by numerous situations where normal reloading equipment is not set up or optimized for boolits. These include expanding dies, seating dies, chamber/bore dimensions, etc. Trying to get the optimal fit of your loaded boolit to bore/throat can be trying at best sometimes. But, for me, it is the pursuit of obtaining the "best" load that makes this interesting.

Brad