Hard toss-up caliber choice

From my research, and rifle ownership, i would say that without reloading it is a hands down 300blk. Especially for suppressor work. But if you handload go for the 277. I only own the 300, but i wish i had known about 277 when i bought my carbine 300 barrel. It works fine but not for anywhere near the range that 277 is capable of. My next upper will probably be a 20" 277

Why not a 6x45 or a 6x68? They'll do 3-400 yards fairly well. Not the same weight of bullets as the others but still very effective calibers.

.25-45 Sharps is a nifty looking wildcat, too. Not sure if it really offers much more than a typical .223 bullet. The Dolos is nice, if a bit spendy. It begs for a 5.56 based wildcat. But options are limited. .300 BLK, .277 Wolverine and .25-45 Sharps are the only ones that really come to mind. A locked breach 9mm could justify using the same bolt face, I think. Might need a few tweaks here and there, due to slight differences in rim thickness. But I've also heard 9mm doesn't do well in a locked breach with the DI-esque gas system.

Personally, I'd go with the .300.

unfortunately for 25-45 sharps the case mouth is so far forward you can only fit the shorter, lower weight bullets, not really the longer ones that realy allow for good performance.. as for 300 blackout, its overhyped, over-advertised ripoff of 300 whisper, if you want real performance in a 30 cal for the AR-15, look at 7.62x40WT.. 7.62x40WT has an almost 200fps edge on 300 blackout with the same bullets and barrel, its a no brainer.. you may have to use different mags, but youd have to do that with with some of the other cartridge options anyway

my opinion is though that if you can find a barrel for it, comparing to 300 washout factory ammunition the 277 not only performs significantly better with a lot more muzzle energy, but it will use bullets of a higher ballistic coefficient in the same weight class, meaning you'll get a flatter trajectory and more energy down range

comparing .277 velocities to 6.8SPC velocities using the same bullets in the same barrel length, .277 wolverine is 95% the velocity and performance of 6.8SPC in a cartridge that'll cost you about 1/3 to hand load

ballistically it seems that the BLK likes shorties

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The pressure curve for 300AAC shows continued gains up to / past 16". I don't know that I'd say that it's predominantly a SBR only sort of chambering....

My thoughts are, if your not a re-loader, you need to determine which of those choices, is the one you can find locally, without an online purchase. Sure you can plan ahead and have enough rounds on hand, but you may want some quickly and not have time for an online order, or for some reason, you many no longer be able to order online!

I would go 300 b-o. WHile I don't know much about either round, it seems the b-o is here to stay.

I would look at 6.8spc or 6.5grendel if you want a heavy hitter, or 20 practical if you want ultra-high-velocity. I don't care at all for the 300blk which is essentially a pistol round. I haven't heard much about 277 wolverine so I take it that the round hasn't "taken off" and may be hard to find brass later on. Not sure how hard it is to for from parent brass, but some forms are easy, some pretty stinking hard.

The pressure curve for 300AAC shows continued gains up to / past 16". I don't know that I'd say that it's predominantly a SBR only sort of chambering....

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The loads are generally optimized for ~10" barrels, but yes, it still gains velocity at and beyond 16". The gains, however, are more akin to a magnum pistol cartridge than a rifle cartridge. Which makes sense, as you're shooting a round that isn't far removed from a magnum handgun round in terms of case capacity vs. bullet diameter and weight, and also uses magnum pistol powders (or very fast rifle powders).

I found this graph for 300AAC velocity-vs-barrel length a while back, and thought it of interest:

So i should also look at powder capacity vs bullet size/weight?

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Those numbers tell you about efficiency, velocity potential, and ideal barrel lengths. The smaller the case capacity relative to bore size, the greater the cartridge's efficiency, and more efficient cartridges need less barrel to reach most of their velocity potential. But these small, efficient rounds do not have the velocity potential, regardless of barrel length. If you were to cut a barrel down to 4 or 5 inches, the .300 blk would probably equal the .308 win. But at 16" or 20"? Not even close. Likewise for .308 vs. some monster .300 magnum that needs 26" or 28" of tube for optimal performance.

The reason the .300 BLK is said to be an SBR round is, as I mentioned, that it has been optimized to achieve most of it's velocity in about 10". The reason it has been so optimized is for use as a suppressed round, where adding 6-8" of can to a 10" barrel still results in a carbine sized firearm.

Mad Dog Weapons Systems for all things.277 wolverine related. Check out the forum there!

if you want a 30 cal, get 7.62x40WT, 300 blackout is just overhyped, 7.62x40WT has about 200fps on the blackout, can be loaded up with subsonics just the same, function in short barrels even better, and is also made with 223 brass to fit the AR-15 rifle and bolts

300 blackout just gets more advertisement and attention but is far from even remotely as good

I wouldn't consider any of the cartridges in question viable for deer sized game at 400yds.

its no secret the 7.62x40WT is a lot more energetic than 300 blackout and able to push the same bullets at higher velocities.. the ONLY benefit to 300 blackout is it can use unmodified AR mags