Ackley vs standard round

[280AI?]

New here, wondering about max loads. For conversation's sake I have a question. If a particular max load for a say 280 Remington is 57 grains for a certain powder, that would be showing pressure, by punching this out to a 280 AI why do we get so much more powder, maybe 61 grains, without pressure signs? Is it simply because the volume is higher so consequently there is less pressure because the powder has room to expand without being compressed? Just something I was curious about.

 

[Prose]

New here, wondering about max loads. For conversation's sake I have a question. If a particular max load for a say 280 Remington is 57 grains for a certain powder, that would be showing pressure, by punching this out to a 280 AI why do we get so much more powder, maybe 61 grains, without pressure signs? Is it simply because the volume is higher so consequently there is less pressure because the powder has room to expand without being compressed? Just something I was curious about.

Son it sounds like you are on a mission to know thyself? Do you not know that internal ballistics are deceitful above all things, who can know it?

Ackley Improved Cartridges | Load Data Article

<p>“Improved” cartridges have been around ever since Lisle Kilbourn cut a .22 Hornet chamber with a sharp shoulder and fired a standard Hornet case, resulting in slightly more powder capacity – and velocity. The process has since been applied to hundreds of cartridges. In fact, some factory...

loaddata.com

 

[jepp2]

why do we get so much more powder, maybe 61 grains, without pressure signs?

Yes, the additional case capacity allows a higher loading without exceeding peak pressure.

Keep in mind:
-the only reason the 30-06 can be loaded to a higher velocity than a 308 Win is because the case has greater volume capacity
- seating your bullet closer to the lands (longer) gives added case capacity, however as you get closer to the lands the starting pressure increases, so it is a balance.
- peak pressure usually occurs when the bullet is about 2 inches down the barrel. After that point, the volume is increasing so quickly as the additional propellant is burnt, the pressure is dropping.

 

[miningshawn]

For the experts, how about less bolt thrust due to less wall taper? Or peak pressure occurring further due to a more perfect pressure chamber? Or movement of the torroid by increasing the angle of the shoulder?
All considerations.

 

[Bill K]

To a degree yes the Improved shoulder would allow some less pressure, however remember each and every cartridge and rifle used will vary.
So for safe and sane loading, you always start with a start load and work up slowly for pressure signs in YOUR give RIFLE. Regardless is it is a standard or improved cartridge.

 

[Jager]

.... If a particular max load for a say 280 Remington is 57 grains for a certain powder, that would be showing pressure,

Just want to point out that that's not at all correct. "Max load," by definition, is the highest charge level that does not exhibit pressure signs. Generally, a safety buffer is also included to account for varying field conditions. And SAAMI or CIP specs are adhered to when that "max load" is determined in a ballistics lab. All that is with the specific firearm or universal receiver used by the lab. Your gun and my gun will almost certainly reach their individual "max load" at lower or higher charge levels than the one in that lab.

Bottom line is that if you're seeing pressure signs, you're already well over "max load." Hence the age-old dictum to start lower and work up.

Otherwise, aye, as pointed out by others... more powder capacity equates to greater velocity.

Welcome to the forum!

 

[NZ_Fclass]

You would do well to read P.O.Ackley's writings on the subject. In many cases he suggests that the "improvements" were made to shoot heavier bullets, using slower powders - hence the need for extra capacity or to improve poor case designs. It's also worth getting hold of the collection of Waters "Pet Loads" articles. Increased velocity? - well, P.O. did hunt with a 220 Swift!

 

[Zero333]

Don't forget some cartridges can have a huge amount of velocity increase from being AI compared to the parent case, while other cartridges are not worth AI'ing.
280 AI is worth the effort for sure while some other cartridges will get so little gain from being AI'd it's not worth the effort. Brass make can also make a big difference weather it's worth going the AI route.

 

[Twicepop]

The biggest advantage that you'll see from going with the A.I. version of any cartridge is extended case life due to not needing to trim the brass as often. Some case because of design due to large amounts of body taper and/or shallow shoulder angle will require frequent trimming when loaded to or near maximum pressures. A few that I've had that required frequent trimming because of these features are the .220 Swift, the 22-250 Remington, the .243 Winchester and the 30-06 Govt. cartridges.
The two cartridges that benefitted the most from the A.I. treatment was the 25-35 WCF and the .250 Savage.

 

[The Marshall]

I have used a 280 Remington since 2012 with standard brass cases and nickel plated cases. The chamber on that rifle is tight. Heavy loads have not resulted in cases stretching but I still check every time I reload them. For the consistent velocities and accuracy I get there is nothing a 280 AI could do that my 280 won’t do just as well. Somebody wants a 280AI for its high velocities——do it and don’t regret it. I’m OK with the 280.

 

[Steel Mover]

What the 280 AI will do, is meet the 7mm Rem Mag stats without a belted case or magnum bolt face. Makes it possible to build an easy switch barrel rifle with standard .473 bolt face cartridges.
And for a 6mm shooter, the 240 Laser, based on 280 brass is a real accurate VLD smoker. This cartridge designed by John Bass tied the group size world record for 1,000 meters.

 

[Ned Ludd]

As has been noted, increasing case volume by removing some case taper and changing the shoulder angle means less pressure at a given charge weight. In other words, the larger the "pressure cell" volume, the lower the pressure at a given charge weight with a specific bullet. However, the AI modification does not change the primer pockets. So, yes, you can get more powder in the AI case and make some modest velocity gains, but if you use enough powder to meet or exceed MAX pressure in the AI case, the primer pockets will still expand and ultimately render the case un-usable.

 

[Clark]

I built a number of 257 Roberts Ackley rifles in 2002, and thought "no more Ackley chambers. Forming brass is too much trouble."

But in 2018, I built a 280AI and loaded ammo with Nosler 280AI factory brass.
It worked well and now I have a number of 280ai rifles.

 

[TrapperBR549]

I built a number of 257 Roberts Ackley rifles in 2002, and thought "no more Ackley chambers. Forming brass is too much trouble."

But in 2018, I built a 280AI and loaded ammo with Nosler 280AI factory brass.
It worked well and now I have a number of 280ai rifles.

Love the old model 70. What barrel is that?
I had a very similar rifle built for my wife in 280AI. Pre 64 action, Brux barrel, McMillan stock.
I did not have good luck with the Nosler brass though. I fireformed some decades old Remington brass that seems like it may last forever.

 

[Clark]

Love the old model 70. What barrel is that?
I had a very similar rifle built for my wife in 280AI. Pre 64 action, Brux barrel, McMillan stock.
I did not have good luck with the Nosler brass though. I fireformed some decades old Remington brass that seems like it may last forever.

1949 Win M70 280AI Bartlein barrel, McMillan stock
1" (3) shot group @ 200y on sight in 10-1-2018
Later that month I got a buck.

 

[Dusty Stevens]

What the 280 AI will do, is meet the 7mm Rem Mag stats without a belted case or magnum bolt face. Makes it possible to build an easy switch barrel rifle with standard .473 bolt face cartridges.
And for a 6mm shooter, the 240 Laser, based on 280 brass is a real accurate VLD smoker. This cartridge designed by John Bass tied the group size world record for 1,000 meters.

Which record we talkin about and what year was that?

 

[Clark]

The 7mmRM is listed at 3.29" OAL, 61,000 psi, capacity of 82 gr H2O, and a headspace of 0.220 to 0.227".

The 280ai is listed at 3.33" OAL, 65,000 psi, capacity of 74 gr H2O, and a headspace of 2.1437-.006".

I load the 7mmRM at 3.34" OAL, 75,000 psi, and a headspace of 0.215".

I load the 280ai at 70kpsi and 3.34" OAL.
I use a PTG 280ai GO gauge to headspace.
PTG says the 280AI go gauge is 0.004" shorter than the parent cartridges'. PTG says the parent go gauge is at the min of the chamber range. SAAMI says the 280R min chamber is 2.100". That would make the 280ai go gauge 2.096".

With IMR4166 this pushes the 120 gr NBT 3280 fps in the 7mmRM and 3150 in the 280ai

With a 200 yard zero and personal limit of 8moa correction, this limits the 7mmRM to 575 yards and the 280ai limited to 550 yards.