I have been shooting cast bullets in revolvers for 30 yrs. Now I've been asked a question that I cannot answer. ? If I am shooting a .357 barrel what is the maximum size my cast bullet can be and still function ? We generally size .001 over bore diameter or.358 ? but can it go .359,.360 and with a cast BHN around 12 will there be pressure problems? Can .360 cast bullets be fired safely in a .357 bore? Your experience?

I'm shooting .358" in my 9mm's if that means anything.

With cast bullets you want to size to throat diameter.

Shooting lead bullets of slightly larger than normal diameter is usually no big deal.

Presuming these are standard loads or target leads you will not be able to elevate pressure form a 12BHN projectile to achieve a catastrophic revolver failure.

If you are shooting gas checked hunting loads that are already at the bleeding edge of the cartridges SAAMI pressure limit, then perhaps it would be possible for an undersized barrel and an oversized projectile to achieve firearm failure, however it is highly unlikely.

Like novalty, I have been shooting .358 plated projectiles in my 9MM semi-autos and SMGs for many years now. About 10 years back my buddies and I got an unbelievable deal on a pallet of these 158 grain Truncated Cone .358 projectiles. We split it between us and I have been using it as my sub-conic load ever since.

It doesn't matter how big your bullets are, as long as they will chamber, as the cylinder throats will swage them to throat diameter as the bullet passes through...

Along with slugging your barrel, slug/measure the cylinder throats. Use the cylinder throat diameter as a starting point for bullet diameter (my 38/357s run real close to .358" throats, so I size all my bullets to .358" and it's a good fit for the .357" groove diameters barrels)...

As long as you aren't running pressures right near maximum, bullet diameter can be sized as large as will still chamber in your gun, which is usually around .362" or so. This doesn't mean it should be, as anything larger than .001" larger than the throats will be sized to throat diameter when fired! Short answer, slug your throats and size to that dimension to +.001". This will usually be .358-.359"

Assuming cyl throat dia is at least bore dia. Undersized cyl throats will size any bullet to that size & if it is smaller than bore dia, you have a problem. How big is too big, I wouldn't want to push bullets much more than 0.002" larger at midrange or higher pressures. Low pressure target type loads, not an issue, probably to 0.004" over, but I have to think it plays havoc on the forcing cone.
I've seen M25 with huge throats, like 0.456", most won't shoot well with cast lead, too much bullet deformation.

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The OP asked about *safe* , not desireable or best accuraccy.

With a 12BHN plain base internal ( vs heel based ) , the ctg wouldn't chamber before signifigent pressure issues.

( Presuming chamber/ throat dimentions at least somewhat resembling 38spl. Now if you were to assemble parts of ( somthing ) to combine a 5 ( 357 ) bbl with a m .38LC cylnder with no shoulder intended for heeled bullets @ .380 or so ? Well that would be interesting. )