There are so many things to consider, even if you specifically mention medium game at no more than 600 yards. And, there is always more than one way to accomplish it.
I don't know why I decided on the 25'06 Ackley, this being my third, but after many years of whacking coyotes to mule deer, and owning several 243s and 6mms and 25'06, I developed a marked preference for 25 caliber, based on downed animals, and percentages, runners, etc. randomly acquired.
I looked at 257 Weatherby numbers, and found that my Ackley came pretty close with much less bark, with a 28" barrel. My personal limit is actually closer to 500 yards than 600, but it does this nicely, not much effort in hold over or dialing it in, with a little range work.
Specifically, what I get is 4,000fps with a 75 VMax and 3650fps with a 100 grain Nosler. I could go a little more, but the law of diminishing returns, and thrift, prevents me. The Nosler is much flatter beyond 350. I did a little work with a 110 gr. Berger & 120 and 117 Sierra, (previous barrels) but, for my application, the 100 Ballistic Tip does everything I want, and is acceptably accurate.
Exactly how this stacks up to a 6mm'06 Ackley, I can't say? My personal yardstick on barrel life prevents me from considering anything beyond 6mm(Rem) AI as far as case capacity goes.
"Fastest, Flattest" could also mean "fastest, shortest" barrel life, and for me, that is always a prime consideration. You have to draw the line somewhere. (fixed income)
Been following this post with interest as I have been experimenting with this concept for over 4 yrs. Initially, I just kept with the plain jane '06 and 165gr SST's. Worked very well but wind drift was servere at longer ranges.
Tried the 30 Gibbs, I really like this one, but recoil crept up and wind drift was still more then I liked. No worries about on game performance though. But then, we know how well the WSM/Win mags work.
Went to a 6.5-06. What a huge improvement ballistically. I use 140gr SST and 1000m plus is no real issue for plinking. Cutting through wind is wonderful. In fact, I still hold off more then I need. Superbly accurate too but maybe that is due in large part to shooting through conditions.
Just picked up a 270 so that I would have a hunting rifle that uses factory ammo available anywhere in North America.
I shoot with a buddy who uses a 280Rem and 168gr MK. So I have covered all the bases from 6.5 to 30cal.
This is how I boil it down: For plinking, target shooting and excellent wind drift reduction, the 6.5 is hard to beat. Recoil can be moderated even more by using lighter but still very effective bullets.
At LR, initial velocity is of little relevance. High BC bullets are. You can't go fast enough with blunt bullets to matter once you go past 500yds.
For off the shelf, 6.5 gets the nod when paper/varmint is the main goal.
The 270 is a wonderful cal limited only by the bullets available. I am using the 150gr SST and this Savage shoots really small groups. However, at longer ranges, the lower BC of these bullets really shows. Shooting the same day at the same range and target, this 270 was way harder to dope the wind compared to the 6.5 and 280. I would limit this to regular range hunting and LR only when wind is low.
My friend with the 280 shoots a 168gr MK at around 2850fps. My 6.5-06 goes 2950fps with the 140gr SST. Let's call it a ballistic tie (actually the 6.5 has an edge because the BC of the 140gr SST is higher however...). Both beat the wind very well.
The very visible difference is on target. We shoot at a swinging gong and there is a significant difference when the 280 arrives vs the 6.5. Both would be more then adequate but the 280 hits harder.
The gong swings more and the impact craters are wider and deeper. This would make a difference on those less then perfect shots on larger game.
For hunting at LR, I would recommend the largest calibre and heaviest bullet. You don't arrive with alot of velocity. Bullet weight matters more at LR.
In this light, I am stepping up to a 7RM to replace my 6.5-06. With 162 or 168gr going 3000 to 3100fps, it will just hit a lot harder. I hunt in an area where shots from 250 to 900yds are possible.
Why no 30cals? Because you would like to stick to the '06 case. The 30-06 is a wonderful cartridge. A LR stick, it is not. If you step up to a Win Mag, WSM, Wby, then the 30 could be made to work. However, recoil goes up measureably and you will not enjoy target shooting with these cannons for long.
Until more bullets come out for the 270, for your uses, I would recommend the 6.5-06 AI if target shooting is way more then hunting and hunting inside 600yds is a sure thing. I would go 280AI if hunting long more often then not. Heavy bullets just do more work...
From impact testing at longer ranges, I just don't see the 6mm and 25 cals as having any real advantage with present bullets for game. They would be also tough on throats and many bullets would not enjoy going that fast.
The larger the bore the longer the throat life for a given amount of powder burnt. Muzzle velocity starts to hit a wall as bearing surface increases vs calibre. We are limited by the burn characteristics of readily available gun powders.
Wildcat and other boutique manf may change all that so this discussion can readily change. Maybe
Well here is my oppinion on it
for the ranges your shooting under 1000yds , I don't realy think that youll need the sure high BC bullets like the 105 and 107gr 6mm's , personaly I'd build a 6.5-06Ai for these reasons
1)they have a 107gr Sierra match king in that caliber which
would be a great varmint round as you'll be able to drive
it a bit faster than a 6mm bullet of the same weight due
to the round not being so overbored
2)if you do get the urge to to shoot out to the 1k mark the
6.5 bullets are hard to beat and it'll be a better round
if your varmint get to be the size of deer.
3)I'd do the Ackley inproved case just to get better case
life if nothing else.
For maximum splat factor I think that the 95gr V-max run out real had (possibley close to 4000fps)and should be realy flat shooting out to 500yds
I had a 6mm-06 once and was not reay to impressed with the velocity and the bore life sucked accuracy went from sub 1/2moa to 1.5 moa from rounds 800 to 1000. and the cases streched pretty bad to in the basic 06 slope
Well that all depends on your barrel and its length , I was able to get 3700+fps with a 107gr bullet out of a standard 6-06 out of a 28" Hart barrel , lets say you only get 100fps out of the Ackley improvement , then drop 10grs of bullet weight should yield 50-100fps then factor in the added gas flang space from the larger bore should add at least 100fps so I would say that you could get pretty darn close to the the 4k mark again depending on the barrel