308 Ackley improved

[warth0g]

- how much improvment in velocity can be expected?
I am currently getting 2 860 fps from my 308.
I am getting good accuracy from this riflle.
Can I expect the same acuracy if I rechamber to 308 AI?

If I can get an 10 % gain, i will only need 90 klicks (1 cm at 100meter) to reach 1000 meters, compared to 113 now, and also giving me a better first time hit probability. We are talking competition shooting, tactical and maybe f-class or similar if we can get that running.

warthog

 

[Vern Humphrey]

- how much improvment in velocity can be expected?
I am currently getting 2 860 fps from my 308.

Very little. Jim Barsness says you get 1% increase in velocity for every 4% increase in case capacity -- assuming you load to the same pressure levels. The Ackley Improved cases add less than 4%, so you probably would not see a statistically significant increase -- and certainly not one you could consider a noticeable gain.

I have shot a few wildcats and Jim's rule comes out right on the button. The dramatic increases in velocity claimed by wildcatters in days gone by came from three factors:

1. They didn't have affordable chronographs in those days.
2. They had no way of measuring chamber pressures at home in those days.
3. They lied.

I am getting good accuracy from this riflle.
Can I expect the same acuracy if I rechamber to 308 AI?

If it's done right and carefully, the sharp shoulder of the AI round may have a slight positive effect -- but again, probably too small to measure.

If I can get an 10 % gain, i will only need 90 klicks (1 cm at 100meter) to reach 1000 meters, compared to 113 now, and also giving me a better first time hit probability. We are talking competition shooting, tactical and maybe f-class or similar if we can get that running.

A 10% gain would require a 40% increase in case capacity.

You're talking turning a .308 into a .300 Winchester Magnum -- it ain't gonna happen.

 

[warth0g]

OK!

Thanks for a very informative answer!

warthog

 

[dfaugh]

Yeah, I've toyed with rechambering various guns to different AI chamberings. Read alot, and listened to many of the wildcatters. And what seems to be the case is that the AI chamberings will give a worthwhile improvement on smaller cases (and some older case designs like the 30-30), but diminishing returns on larger cases. For example a .22-250 AI will probably give you 200 fps. more velocity, but the .308 would be more like 100 fps. The only other frequently mentioned advantage for the AIs is longer case life, with less or no trimming

I'm currently building an 8mm-06 AI, but the only reason I'm going with AI is, since I have to ream it anyway, I might as well go for the AI. And, the only reason I'm building such an "odd" caliber is because I have all the bits and pieces laying around, including a good VZ-24 action and military barrel (that needs to be shortened and recrowned, and the throat cleaned up (hence the rechambering)), a decent stock and a couple spare scopes. Based on what I've seen/read I should get performance between .300 Win Mag and .338 Win Mag. But, I bet its gonna hurt....