Diameter Tolerance for cast bullets vs actual bore?

[fiddler]

What is the diameter Tolerance for cast bullets vs actual bore diameter?

Looking at 38-55 mold with diameter of .379 but the available lee sizing dies are listed as .377, .378, and .379.

What size die is correct? I have not slugged the bore yet. Do I need to?

 

[yomomma]

They always shoot as cast and see how it works.

Idealy you want to be 1-2 thou larger for good fit. So .379 it would be for me.

 

[Janeau]

Start by slugging the bore. Thats the only way to know what your barrel need. Then +.001 over.
No need for more.

 

[fiddler]

Excellent. Thanks guys.

 

[gunlaker]

Ideally you want the bullet to fit the throat more so than the grooves for max accuracy. One trick that works pretty well is to use a fireformed case and then use the biggest bullet that slips into the case mouth. If you are going to get a custom mold made then you won't need a sizer die. I almost never need to use one.

Chris.

 

[Helka]

for my 9mm it leaded at 1 thou over and stopped at 2. So now I do 2 in everything

 

[fingers284]

If you are going to custom size your slugs to a bore, slugging the bore is always the starting point if custom ordering a mold...if just picking a mold size off the shelf then trial & error shooting is about all you can do

 

[Potashminer]

I had read that you can alter - slightly - the diameter of the cast bullet from a mold by altering the alloy that you use - more or less tin, etc. Or was also old school "beagling" the mold to get fatter cast bullets. More recently, I have read that one can powder coat to build up thickness of bullet. Your rifle might shoot better or worse with different lube on the bullet - I am not real sure if both lube and powder coat can be used at same time? I think the 38-55 was especially varied - is another CGN posting about a 38-55 that has 0.383" bore - and I am pretty sure I read of a USA guy had a very snug one with 0.376" bore (but I can not find that article right now)? So a slug is likely the first step, to have a clue what you might be up against.

 

[fiddler]

I had read that you can alter - slightly - the diameter of the cast bullet from a mold by altering the alloy that you use - more or less tin, etc. Or was also old school "beagling" the mold to get fatter cast bullets. More recently, I have read that one can powder coat to build up thickness of bullet. Your rifle might shoot better or worse with different lube on the bullet - I am not real sure if both lube and powder coat can be used at same time? I think the 38-55 was especially varied - is another CGN posting about a 38-55 that has 0.383" bore - and I am pretty sure I read of a USA guy had a very snug one with 0.376" bore (but I can not find that article right now)? So a slug is likely the first step, to have a clue what you might be up against.

I guess I had two questions instead of one.

So now I know what tolerances I can work with if I order a mold or sizer off the shelf. I'll slug the rifle bore this weekend.

All good info.

 

[Potashminer]

Go back up to Post #5 about sizing to fit the throat - I think that is what happens when you go 0.001" or 0.002" over the bore size - that gets you very close to the throat diameter, often. I am sure anything that you can get to chamber will fire fine - the barrel leades are going to "swage down" the cast bullet to whatever size that it needs to be - it is that step that might be tricky - to fill the throat (to be larger than bore size) - and still have a round that will go into that chamber.

Was not on a rifle, but I had read of some revolvers that would have smaller throat in some chambers, than the bore size - pretty much have to get those reamed out to appropriate size, or use very soft lead to get the bullets to "swage up" in diameter when they hit the leade, since they may have got "swaged down" as they left the smaller throat chambers in the cylinders. I think it was an article by Brian Pearce using plug gauges to show a particular revolver had several diameters of throats, among the six chambers in that cylinder. His solution, I think, was to get them all reamed to be the same, suitable diameter.

 

[fiddler]

I had read that you can alter - slightly - the diameter of the cast bullet from a mold by altering the alloy that you use - more or less tin, etc. Or was also old school "beagling" the mold to get fatter cast bullets. More recently, I have read that one can powder coat to build up thickness of bullet. Your rifle might shoot better or worse with different lube on the bullet - I am not real sure if both lube and powder coat can be used at same time? I think the 38-55 was especially varied - is another CGN posting about a 38-55 that has 0.383" bore - and I am pretty sure I read of a USA guy had a very snug one with 0.376" bore (but I can not find that article right now)? So a slug is likely the first step, to have a clue what you might be up against.

Bore Slug is .379. Which I suppose is good because the mold is .379 and I cannot find a .379 sizer die.

 

[Potashminer]

Sounds like you figured out the slug part. FYI - some fussy people will get slug inch or two into muzzle, than inch or two into throat - to compare - if your slug was pushed all the way through the barrel, is going to give you the smallest diameter that it was subject to. Your throat area, chamber area or muzzle might be larger than that - up to you if that is important information or not. Is some amount of things to play with, for cast bullets - is not just the sizing / fit to bore - also lube, loading (velocity), etc. to get results. But in the end is what you are after - where do holes show up on target - might be good enough on the first go at it.

Rough rule of thumb, for many, would usually be that if barrel slugged to .379", then you would want a .380" or .381" cast lead bullet. Various ways to get that - including powder coating, I think.

Was also, about forever, thought to scrub out all possible copper jacket fouling from a barrel before trying lead cast bullets - other posts on CGN, recently, have suggested that has been shown to not be such an issue - but I still think is one of those things to consider, if you are trying to figure out why you are getting 6 inch 5 shot groups at 25 yards - some "thing" is not as it should be - sometimes is hard to always pin down what that "thing" is.

 

[fiddler]

Sounds like you figured out the slug part. FYI - some fussy people will get slug inch or two into muzzle, than inch or two into throat - to compare - if your slug was pushed all the way through the barrel, is going to give you the smallest diameter that it was subject to. Your throat area, chamber area or muzzle might be larger than that - up to you if that is important information or not. Is some amount of things to play with, for cast bullets - is not just the sizing / fit to bore - also lube, loading (velocity), etc. to get results. But in the end is what you are after - where do holes show up on target - might be good enough on the first go at it.

Rough rule of thumb, for many, would usually be that if barrel slugged to .379", then you would want a .380" or .381" cast lead bullet. Various ways to get that - including powder coating, I think.

Was also, about forever, thought to scrub out all possible copper jacket fouling from a barrel before trying lead cast bullets - other posts on CGN, recently, have suggested that has been shown to not be such an issue - but I still think is one of those things to consider, if you are trying to figure out why you are getting 6 inch 5 shot groups at 25 yards - some "thing" is not as it should be - sometimes is hard to always pin down what that "thing" is.

I've owned and had hundreds of milsurp rifles come across my bench in 45 years. Slugging is not new. However lead casting is new.

 

[yomomma]

Bore Slug is .379. Which I suppose is good because the mold is .379 and I cannot find a .379 sizer die.

See post 2

 

[EhGun]

Powder coating will probably add .003". I would just powdercoat your bullets straight from the mold and you're good to go.