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dia of drive band on cast

In powder burning rifles, it has been found that the best precision/accuracy is obtained when the nose portion is the size of the lands to a couple of ten-thousanths over: +.0002 / -.0000. The driving band(s) at the base should be as close to the groove diameter as possible. However, again you can go oversize a bit but not under size.

In your case I'd try to find a mold and alloy that gets the nose to 0.2491 and the base to 0.2552. If the base is bigger when you get the nose to size you can bring the base down by sizing. 
 
I design molds for air gun slugs, check out some of my designs at higharchunter.com. What I_Like_Irons says is correct. on the sizing. I have a couple questions? what is the barrel twist rate? Are you looking For a hollow point? What weight bullet are you looking to make? How far are you looking to shoot? hunting or target bullet? And most of all is the barrel choked? Choked barrels do not shoot cast slugs well.
 
This is for my escape from airforce .25 cal.. I will have to call and ask about choke and rate of twist. The tech there did tell me that lands were .249 and groove was .255. thanks for the response. This will be most for hunting at up to 75 yards. I live in wood that most shot are at 35-50 yards. I have been shooting 380 acp slugs out of my evanix 9mm seems the bore on that is larger then 9mm. They shoot really quite well . But I have not come across the right slug for the 30 yet. I have a daystate 303 an evanix 30 rainstorm and a flex which I am looking for a slug at this time. I would like to make 25 cal and 30 cal for future use. I like to cast and have done it for PB for over 50 years and even swaged some but my swage press is an old herters press , hard to come by the right dies. I will take a look at your site. thanks again.