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> Ackley Improved
Ackley Improved
@ 06:28 pm (GMT) |
Malcolm McHello All,Thinking about rebarrelling both a 7mm08 and a 223 and was tossing around the idea of rebarrelling them with the improved version in each calibre. I have read the pro's and cons. What i can not fully understand is the improved version capacity of the case to take more powder, therefore increasing velocity. Can you achieve the same result with the unimproved version by more powder and seating out longer, or is it that the change in shoulder angle increases the case volume. I have read that Nathan in a previous forum has stated that he has seen a lot of improved reloaders run very high pressures. In the end, are we kidding ourselves or is it a valid improvement over the original calibre. Go. |
Replies
@ 11:34 pm (GMT) |
Ed SybertRe: Ackley ImprovedThe Ackley Improved case design does several things. Among them are moving the shoulder forward, decreasing the body taper, and making the shoulder angle sharper, i.e. from say 20 degrees to 30 degrees. Combined, the changes result in more case volume. The new shoulder angle and reduced body taper lessen the case set-back or stretching when fired. This in turn makes it seem while opening the bolt that you're running less pressure. You may or may not be depending on your specific load. I'm sure others will add their comments, as there is a lot more that can be said. |
@ 07:32 am (GMT) |
Nathan FosterRe: Ackley ImprovedHi Malcolm, this is a good question and you have made a very good point regarding the possibility of throating having a similar effect.Since working with the Foster Manson Reamer designs, I have had an opportunity to compare results directly as well as time to look at the bigger picture. The FMR reamers in question include the 280 and .30-06, both common cartridges in standard and AI form. My experience with these cartridges leads me to suggest that focusing on the AI aspect alone is not altogether prudent. In real world terms, the difference in case capacity between the standard and AI forms is around 2 to 3 grains for a net gain of about 50fps. If the throat is very short, this cancels any potential gain. If the barrel is short, this also cancels gains, especially if using slow burning powders. As a contrast to the above, I have come across .280 and .30-06 rifles which have excessively long throats as a result of max tolerance reamers, designed to cope with wear during long long production runs. Although adequate for general hunting, it can prove difficult to tune such rifles to shoot sub half minute groups (for those of us interested in this level of accuracy). I would therefore suggest that while it is possible to obtain increased velocities as a result of combining the AI form with a generous throat and barrel length, one should by the same token avoid taking throating too far. You mentioned my comments regarding hand loader's tendencies toward hot loading the AI's. I still stand by this comment, the crux of the matter being hunter expectations. To put this into perspective, lets say you had a standard cartridge, in this case a 7mm-08. The rifle has a stock standard 22" barrel and you have loaded a 162gr projectile and the rifle showed a sweet spot at 2600fps, grouped about an inch through to 2680fps and then came in again at 2700fps but pressures were a little bit high (perhaps also the occasional flier), forcing you to adopt the 2600fps load. The fact is, the 2600fps is fine for your hunting so you stick with it. Later when it comes time to rebarrel, you choose a 24" barrel, AI form but keep the throat the same because the 7mm-08 is already quite long. This time around, you make up similar loads and find the sweet spot at 2675 or 2700fps without the same pressure thanks to the longer barrel and AI form. You are still within the same velocity range as before, nothing major has changed, its really just a small bonus rather than the 150fps that some try to obtain. One could of course find the same comparisons within the subject of vehicle performance and tuning. Other ways to increase power are to look towards slow burning / high bulk density powders. These can achieve very good results in some standard cartridges as well as good performance when combined with the AI form, a long barrel and good throating (that being a total of four factors to fully increase power over standard factory loads). This full combination (only where applicable regarding certain cartridge / powder combinations) may result in an increase of 150 to 200fps over traditional factory loads. The downside of the AI form is having to find appropriate dies. As for factory ammo, expect a velocity loss of 50fps and for groups to open up from e.g. .75" through to between 1 and 1.5" (compared to the standard chambering). If you want to travel with an AI rifle, a long barrel will help to offset any velocity loss if purchasing factory ammo (simply have to put up with the loss of accuracy / effective range). So as an answer to your question, yes I think sometimes we do kid ourselves. But there is (within some cartridge designs) a small gain to be had if we carefully combine several factors. In some instances, a significant gain can be obtained over traditional factory loadings if using modern powders (as per Superformance loads). But this is not always the case and so we should maintain modest expectations. If for example you want a true step up in power, you might of course instead look to the .280 Remington (versus your 7mm-08) and the .22-250 (versus your .223 which has very low energy to begin with). There is also a wildcat circulating in NZ, designed by Chris Murphy and Clive Judd, based on the .270 case, shortened slightly and necked up to 7mm, designed to better fit the medium magazine length of the Tikka rifle (fitted with .270 / .30-06 magazine). A good cartridge, optimizing the potential of the Tikka magazine design. In any case, your writing indicates that you already have a good sense of potential outcomes. All I would add to this, is that I do not believe there is anything to be gained via lengthening the throats on those particular cartridges. Hope that helps a bit. |
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