I recently put together a .22LR AR-15 upper and I'm having a ball with that thing!

So, not to be outdone, my friend is getting a .450 Bushmaster upper.

Ammo for that thing is like $2 a pop!
Even the individual bullets are ridiculously priced IMO.

He's wealthy, I'm not.

So I immediately started wondering if you could put together a reload using maybe a .45 ACP or .45 Colt bullet and some type handgun powder.

I think he said the bore is .452 so maybe a cast and copper coated .45 ACP bullet would fit?

It doesn't have to cycle the action, and I'm just wondering if anything like that has been done before?

Get the Lyman AR reloading manual, It includes some cast lead loads that are exactly what you are looking for.
Most of the data seems to be around the usual magnum pistol suspects like H110, 2400, and 4227.
It looks like a 255 - 230 grain cast 45 auto tops out at 2100 FPS. Powder coated cast would have no problem with this.

I'm currently running a cast lead 55 grain 223 load with pretty respectable results. it's a good bit faster than the 450 BM can muster, and I am having no leading issues that one might expect, or fear. I don't even find gray contamination in my BCG.
If you wish to follow along with reduced loads, I do recommend the Odin Gunworks light adjustable buffer.
With it, you can restore function otherwise lost with reduced loads. But .... 223 cast is a bit extreme, though it illustrates some of what can be accomplished on the lower fringe of the platform.

In the case of the 450, however, I do not foresee you encountering any of these issues. It'll be hard to avoid making sufficient gas volume for your action with this cartridge.

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it just needs more voltage

Thank you, I'll pass the info. along.

Honestly, that seems like a worthwhile cartridge I might add to the AR collection. 350 legend has been a shining star after I worked around it's issues. 450 looks like it'll be a little easier to tame.

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it just needs more voltage

I'm a Luddite! I'll stick with 45-70 in a lever action. I don't expect to get in a fire fight with man nor beast, so the 5 round or 10 round guns will be fine. Proven brass, proven loads, and very well proven results, for 150 years.

Ivan

I have one of those too.
It's largely why I've loitered on the fence about getting involved with a 450 BM or 458 SOCOM.
I don't really NEED either.
The prospect of a cast 230 grain TC flat point at 2100 FPS is enticing though

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it just needs more voltage

My friend got his .450 Bushmaster upper.

He also has a .45-70 Martin lever gun that I have fired in the past and even that relatively heavy rifle had plenty of kick.

A round with similar ballistics in an AR?

I standing back when he touches that thing off for the first time!

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45-70 ballistics is a bit vague ...
Trapdoor ... 1895 ... or Ruger No.1 level
Honestly 458 SOCOM sits somewhere between trapdoor and 1895 level. Though it is intended to have enough flexibility to run subsonic reliably.
450 bushmaster ... I'm looking at that from a different perspective since it is not intended to launch 400 - 500 grain baseball sized chunks of lead.
seems more like a magnum handgun round strung out on a meth binge, which has some fun potential.

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it just needs more voltage

A 300 grain bullet going about 1800 FPS is about a standard load for both. It’s not really similar to any magnum handgun round unless you’re looking at 500 or 460 S&W. There are rifle rounds much more powerful but put 20+ rounds down range in a session and any normal person will be feeling it the next day.

I bought a .450 Bushmaster upper just to horse around with. I can't say I have any real use for it. It's a thumper all right, but it's not unpleasant in an AR. My No. 1 in 45-70, even with moderate loads is more "vigorous" than the .450 Bushmaster. Neither are cheap to reload for, but factory ammo is pricier. Still there's something appealing about straight wall big bore cartridges. Doesn't matter to me if they're in an AR or falling block, they still make me grin.

Here's some loads I have tried in a Ruger American Ranch and my AR.
200 Grain SWC H&G 68 clone with 28 grains IMR 4227 = 1400 FPS
300 Grain Lead FP with 11 grains Unique = 1180 FPS
350 Grain WFN with 11 grains Trail Boss = 1000 FPS
285 Grn SWC with 12 grains Unique = 1270 FPS
395 Grn Hornady Sub X with 17 grains AA#9 = 910 FPS
300 Lead FP with 23 grains 2400 = 1400 FPS

Listed in no particular order.
None of these loads will cycle my AR however the last load is close with 24 grains.

I have other loads with N110, N350, SWBO, 5744, Lil Gun and a few others if interested.

While I have been a 45-70 shooter for decades I will look at this cartridge.gun combination

I have a Marlin 1895 right now and a T/C Contender. I have not shot the contender in many years.

This year I added a Sharps Long Range which for those of you that do not know, is a single shot

Obviously I have not contemplated sending large chunks of metal downrange in a rapid fashion. A .450 Bushmaster upper could change my mind

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"Acta non verba"

The Odin gunworks light adjustable buffer should solve that.
With some of these loads.
I have an AR load running a 15 grain charge of 4227. Per the laws of conservation of matter, charge weight should closely reflect gas volume, give or take some temperature differential.
It's definitely a good idea to maintain a policy of removing the light buffer after a session. Always put it away set up for full power.
You should have the gas volume to run the gun. Just not with standard tuning.

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it just needs more voltage