My only two ARs are both .277 Wolverines (a 20" multi-purpose gun and a 8.5" in progress). I have been toying with the idea of having a 28" bull barrel built around the .277 Wolverine and reamed with a particularly long throat to accept very long-loaded bullets (2.5+ COAL).

I have played with my AR10s in a "straight-pull bolt-action" functionality, where I either barely opened the gas block enough to just break the bolt loose, or used pistol powders which largely have the same effect. Plus, I have gone to side-charging uppers, which make charging while prone pretty easy. So, I know I could make it work how I want, and would see some additional benefits in sticking with the AR platform like familiarity with the platform (both in operation and assembly / modification) and compatability with my existing lowers (and their well-fitting stocks and nice triggers).

I am considering a Savage build as well, but I believe the Savage .223 magazines are still limited to something like 2.44 COAL. Thus, at 2.5"+ COALs, the only real benefit the Savage offers is the leverage to consistently pull fired rounds out of the chamber. Conversely, even with the side charger, doing this with no gas on the AR might be a little tough, and could require a slight amount of gas to break over the bolt for manual cycling (Id prefer to manually eject, both to manage my brass more easily and maximize velocities).

As far as ballistics go, in this configuration I would expext a 140 SST (.495 G1 BC) could be pushed to about 2600fps or so. This gives enough velocity keep to 1800fps just past 500 yards (expansion velocity), and 1200fps at 1000 yards.

Obviously, I know there are objectively better cartridges for this kind of undertaking, even in the AR15 platform (like 6.5Gr, .224 Valk, etc). However, I wont be competing or anything, and my 20" is more than capable of taking game at any distance Id actually shoot. So, it would really just be a fun build that would still allow me to share the parts, brass, loading dies, and powder I already have--I would just need to buy a few boxes of new projectiles to get sharted. I just sold about 70lbs of gun powder and something like 15,000 projectiles in various calibers I no longer shoot (and my 700lb milling machine, to name a few other barely-used garage toys), and I would like to maintain my streamlined supply. Moving every couple of years really emphasizes the minimalist in you...

Am I missing anything?

If this is a WOLVY it sounds like a great project other than the unrealistic speeds you predict. A full sized 6.8 with about 20% greater capacity can't push a 140 that fast. Hornady shows a MAX of 2300 FPS from a 16" tube shooting a 130.

The longer barrel would give extra velocity and while you normally pick up maybe 30 FPS /inch you won't gain that over a 14" increase.   You are hobbled by the small capacity of a cut down 223 derivative case. It just doesn't have the boiler room to do what you want with available powders.

If you pursue this it will be interesting. I hope you keep us in the loop on your endeavor.

Greg

There have been recent reports of pushing 130s to nearly 2700fps out of 24" Savages at 2.26 COAL (max loads, of course). I am expecting the extra ~10-15% of powder capacity (from the long length) coupled with the longer barrel will enable the numbers I mentioned above.

Even recent developments in AR loadings are promising, with a few examples of 120 SSTs pushed to 2500fps out of 18" barrels.

Ditto what Greg said.  Many people mistakenly believe one gains 50 fps/inch no matter what the cartridge, powder capacity, or rifle system.  “That’s not how this works, That’s not how any of this works.”  Just look at the 6.8 SPC and .458 SOCOM.  Both have case capacities greater than the Wolverine yet they do not gain many fps after a certain point.  The .458 reaches that at between 12 and 16 inches, the 6.8, between 18-22 inches.  I have both the aforementioned and my own testing with both cartridges, at least with the powders I was using, that was what I found.  I have also seen the same thing with some of my 6.8 based wildcats.  One in particular, the difference between 16” and 24” is nowhere near what Quick Load and old adages say you will get.  And if going from 16 to 24 inches gain you only 160 fps (6.8 based .30 HRT) I assure you, going from 24 to 28 will gain you little to nothing.

With that small Wolverine case and powder capacity, you likely will find that 8 more inches of barrel gains you very little other than an extra pound of weight.

I get what you are saying. However, the above (and below) anecdotes, my physics background, and [not inextensive] reloading experience suggest you may be a bit pessimistic :)

I understand that 50fps / inch is very old school, and applies mostly to overbore cartridges like the .22-250. However, there is plenty of data supporting roughly 25fps per inch with the 6.8 (and even a modest amount for the .277) between 16 and 24 inches.

https://www.thehighroad.org/index.php?threads/22-barrel-vs-24-barrel.570474/

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.68spc.com/68velocity&ved=2ahUKEwjb6O6ysJnZAhXGKWMKHbZICwAQFjAAegQIDRAB&usg=AOvVaw2gTK8fK44Hhf6XOu00Wx40

On the quickload PDF, velocity per inch is dropping by a couple fps every inch as it approaches 24" (in the low-20s). Extrapolating allows an average of aboit 20fps per inch from 24 to 28, and conservatively we could call it 15fps / inch.

Going back to the original estimation, the aforementioned 130gr pill at, say, 2675fps (that user topped out with an average of 2691) roughly equates to a 140gr bullet at ~2480fps with respect to the amount of umph available in the same powder charge. That 15fps per inch takes us to about 2540, and I dont think it is at all out of the question to expect an almost 15% increase in powder capacity (from the long COAL) couldnt increase velocity by 2 or 3 percent :) .

given the case capacity being a constant I think a .223 throated and twisted for the new 95gr smk is going to be pretty hard to beat until you get a larger case. this is assuming single feeding with 2.5+ coal. even a 105 from a throated 6x45 would excel over the .277W. and you could always AI the .223 or 6x45 to get a slight gain in capacity and less brass prep

Hrmm! Your idea of custom .223 barrel might be a pretty good compromise between not having to buy all new stuff and getting the most effective ballistics. Loaded super long, and from the 28" barrel, I think it could even approach .224 VE ballistics?

You could probably only get 1/2 way to the 224V ballistics gap with a long .223 compared to mag length 224V.

The case capacity for a Lake City .223 is about 30.4g and the 224V is 34.5g, for a difference of 4.1g.  However, the 224V is 0.140 shorter, so with long bullets loaded to full mag OAL, the 224V has an additional 1.4g of capacity due to less displacement from the bullet.  This makes the total gap 5.5g.  Increasing the .223 from OAL of 2.260 to 2.500 buys 2.4g of additional case capacity.  A little less than half the 5.5g gap.

A long 2.500 224V would be interesting with an effective capacity of 36.9g, getting closer to the 43g of 22-250.

Thanks for the numbers! Very good info. To clarify, I meant that the additional 4" of barrel would also probably make up another 100fps or so compared to a 24" VE  :) . But yeah, the .224 VE loaded long in a 28" Barrel would probably be pretty impressive!