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Absolutely - I used to shoot multiball 'survival' loads in a .444 Marlin for fun. I used data from an old Gun Digest article - 25gr. H335 with 3 - .451" balls sized to .430" as well as the same load with 3- .440 balls sized down to .430". The .451" balls ran a combined weight of around 405gr. while the .440" balls totalled around 360gr. The 405gr. payload ran mid 1,600's and the 360gr. load was over 1,700fps. The same deal could be easily develped for the .458 - or simply go to the fluffy Trail Boss for sub-standard loads with no fillers needed.
C.E. (Ed) Harris's "The Load" of 13gr. Red dot should also work with single light weight cast bullets or probably heavy cast bullets for that matter. The load usually ran around 1,300fps and gave good accuracy in whatever round it was used in, from .308 to .375H&H. No filler was needed.

I think if you wanted to seat a bullet way down inside a .458 case, you could do this - but accuracy might not be very good (the case tapers inside) with the bullet slamming into the rifling at elevated velocities. Shoving a bullet down an inch into the .458 case would shorten the powder chamber such as to have the same length as a very short .45 case, shorter than a .45/60's 1.8" length, and that might be all that's necessary with fast powders. Start experiminting with small loads of fast powders. Stay aways from slow powders unless they're almost a full case capacity - as with the H335 loads with multi round balls. As well, one would have to have a good steel range rod just in case your starting test loads left a bullet in the bore - no damage would happen, of course if another wasn't fired on top of it, but one would want a substancial rod to be able to push it out.
I use 3' and 4' sections of 3/8" drill rod or stainless with one end grooved and firmly epoxied onto a section of birch branch or maple handle, and the other is threaded for numberous sized jags from .40 cal. to .50 cal.

By the way, those multiball laods would be good for deer or bear to about 50 yards - actually MOST effective on them, as well as being a survival self defense load for out to 100yards. Figure the gun holds 5 rounds, with 3 balls in each and each prints about an 18" pattern at 100 yards.
In the .458WM, .490" round balls sized to .458" would weigh 175gr. x 3 = 525gr, weight. You could increase the weight to perhaps 4 of them, for 700gr. - at 1,300fps, cool load. One of the slower ball powders might be best to start with, like H414, H335 and around 25gr. worth. Just a thought.

Check Lyman's handloading book "Cast Bullet Loading Manual" for ideas on cast bullet loads at low speeds. I've had mine since about 1975, so I don't know if it's still in print or perhaps something else has taken it's place.

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Daryl

"a gun without hammers is like a Spaniel without ears" King George V