How light have you guys gone, and what is too light?

My first love is the 45-70, so I have a ton of .458/.460 lead, mostly powder coated.
I see a lot of people talking about 300 grains, I have a lot of those, 350, and 405's ready to go. But one of my favorite rounds to pop through it is a 190gr "button" that sounds like a .22 out of the 45-70.

Of course there isn't any load data for this, but it'd think a simple load could be worked up for the .458, right? Cycling will be the biggest hurdle to overcome, right after feeding, but there shouldn't be any concerns over the bullet shape and an auto, right?

I too got into the .458 SOCOM because of my love for God's Chosen Sledgehammer, the .45-70.  A semi-auto .45-70, what's not to love?

Regarding the use of "button" bullets, I have shot a ton of wide flat nose cast bullets of 300 gr. to 450 gr. weights out of the .458 and while they are not near as wad cutter shaped as what I suspect a 190 gr. Button bullet is, and if I understand the use of the word button correctly, feeding should not be a big problem.  To remove any question, would you please say exactly what you mean by a Button Bullet?

 You likely will have to cut a half moon out of the front of your magazine and in a worst case, you may have to adjust feed lips to increase the angle to get the bullet a bit higher in the front so it does not hit low on the feed ramp, but feeding is the least concern.  I have never fooled with such light bullets in the .458 SOCOM so I really cannot say what powder or load might work, but I suspect Reloader #7 would be a good place to start.  H110 tends to like heavy charges and higher pressures to burn clean but so far with my experience with bullets from 250 to 525 gr, Reloader #7 always gives good function and decent velocity.

I assume since you are shooting a cast or PC bullet, you are not really going for high velocity, just a good plinking or small game load?  In order to get your rifle to function correctly, I suspect you are going to have to use a hefty charge of powder to get pressures up high enough to function properly, and with that light bullet in a cast or PC bullet, accuracy might not be too good.  Only testing will tell.  If you have Quick Load, use it.  If not, I'd use Reloader #7 load recommendations for a 250 gr. Hornady Monoflex bullet.  Start at the mid range of the load table and work up to listed maximum.  I suspect by the time you get near max load for a 250 gr. copper bullet, you should be getting results to tell you if you are on the right track.  You likely will be able to go a bit above maximum listed for the Monoflex bullet but do so at your own risk.

When loading, make sure you use a good crimp because you do not want that short bullet to be set back into the case.  

The "button" bullet is more a descriptor for the shape of the bullet. They look like old fashion buttons that were used in the 1800 shirt collars. Kinda "I" shaped, wide base, thin middle, and wide head.

As for the accuracy, I wouldn't expect much. Just something to ring steel with, or yes, send a jackrabbit or similar. That's what I use them for in the 45-70, usually the first round when I'm out riding in the back country.

I've yet to get my 458 up and running, just got it assembled and am gathering load data for the first batches to see what's needed to make it function, but like any good reloader, I have to complicate it and see what it can really do, going harder hitting/faster is sketchy in this round from what i've found, so why not go the other way a bit?

This is similar to what I'm talking about, but it's a 195 vs the 186 listed.

NOE bullet mold

I’d agree with R7 as a good powder to work with building this load.
- suppressed or not will determine a lot on cycling issues.
- I’d start with super and work down as I’ve done ladders on 350 berrys for subs. They are really fun, increase brass life, and quiet.
- I can see if I can find all of the 458 reloading data I collected years ago. It was from pretty much everybody.

I used the lighter bullet weight dimensions but upped the weight to 195 gr.  So, this will not be exact AND USE THIS INFORMATION AT YOUR OWN RISK.  If you don't know how to wildcat safely, DO NOT USE THIS DATA.

This is from Quick Load.

First of all, I am assuming a 16 inch barrel and CAR gas.
Length of the bullet is .530"
Shank of the bullet is .320 inch.
COAL of the cartridge is 1.780 inch.
Start pressure is 1160 fps, but it may be lower since so much of the shank is grease groove.
Bullet diameter .460 inch.
Max pressure 35,000 psi.
Case fill no more than 100%

There are lots of powders that should work, but most give velocities way too high for a cast bullet, even hard cast.

One powder that did seem like it should work is IMR 4198.
With a charge of 39.9 gr., chamber pressure of 20.3 K psi with a 100% case fill, velocity should be 1880 fps with a gas port pressure of 5770 psi.  This should be enough pressure and enough gas to function the action correctly.  If you want a fast load almost certain to function properly, this one has my vote.

If you drop down to 1500 fps, a charge of 32.1 gr. of IMR 4198 will get your there at 11.5K psi and a port pressure of 3500 psi.  I doubt this will allow the rifle to function properly.  Fill on this is a safe 80.6%

Reloader #7 with a 100% case fill of 43.1 gr. and a maximum pressure of 20.3K psi and port pressure of 6000 psi, this round should function perfectly, but velocity will be high at 1946 fps and may cause leading so use a good lube.

For a 1500 fps load, 34.4 gr. of Reloader #7 with an 80% fill will yield a chamber pressure of 10.6K psi but a port pressure of only 3764 psi, probably too low again to function.  But you can keep upping the charge until it does work and with a good lube, even at max load of 1946 fps leading should not be an issue but I would want to go slower since it does not have a gas check.
 
 A charge of 38.5 gr. Reloader #7 will give you a port pressure of 5K psi, chamber pressure of 14.4 K psi, and a velocity of 1700 fps with a 90% case fill.  This load would be where I would start because I think it very well might give you proper functioning and a very mildly recoiling rifle.

Remember, use at your own risk.  

Have a couple cases if factory SBR ammo in 350gr FMJ that I bought pre covid from Target Sports USA. For reloading I have a couple thousand 405gr Remmy bullets I bought about the same time. Plan on using RL7 powder and CCI 350 primers in Starline brass.

Will be used in conjunction with a Dead Air Primal

From looking over QL, I did not find any loads that were subsonic that I thought would function with that bullet.  The only powder that MIGHT work is Trail boss but that would probably require the gas port to be wide open at .125 to .140 inch and it would be way over gassed with any other powder (read as adjustable gas block) and it likely would not function even then.  That bullet is just way too light to play well subsonic with an AR not specifically set up for that use IMO.