Take this for what it is worth as I have not done 30-30 to 38-55 but first I would just try graduated neck expanders with ones I have on hand or one of the universal stepped expanders. I would go slow and possibly candle anneal half way through if I split any case necks. The 30-30 cases tend to be thin and PO Ackley wrote that some batches they lost a lot of cases when forming the 25-35 AI. But I bet going to 32-40, 35, then .375 would not ruin too many cases.

With the COW method 13 grains of Red Dot has been a good starting point for 30-06 size cases so in 30-30 maybe 3/5ths of this?. I take inventory of all the faster powders I have on hand and then cross reference lead bullet loads from one of the Lyman manuals. If I have a match I start at the starting load and use tissue as an over wad if I will be transporting the loads very much. If the lower loads don't fully expand they can be recharged and fired over again. I use the Lee dippers to measure powder and COW if I am using it as it is not critical and they are fast. Especially if I am using 13 grains of powder and 13 of Cream of Wheat. Get the regular as the instant has more salt in it.

Another method is to cast a bullet with 30 caliber base and .375 diameter on the ogive. More trouble but works well and fun plinking loads.

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