If Ackley is so Improved, why do.....

[packrat]

.... new cartridges like the 6.5 Creedmoor not utilise the same feature of flatter shoulders?

I am a ballistics novice so please forgive any glaring ignorance, but I thought that one of the major advantages of the AI, for example in 243, was that the brass does not 'grow' with repeated firings from the same case, and that this is mainly on account of the shoulders being a much shallower angle from the horizontal, than the standard 243 round. Makes sense to my rudimentary understanding of the physics involved.

I've been looking at newer calibres and recently my interest's been piqued by the 6.5 Creedmoor, I know it might be a fad etc. etc. but my question is not so much about 'Should I get one' but more... Why did Hornady not design the Ackley feature of the shallower shoulders, in to the cartridge?

 

[jcampbellsmith]

Ask the folk who built it - http://bulletin.accurateshooter.com/2007/11/hornady-introduces-new-65-creedmoor-cartridge/ they're not hard to get hold of. Also read the comments at the bottom of this article.

Regards

JCS

 

[jcampbellsmith]

Creedmoor 6.5 seems to be here to stay.

Best Rifle Caliber - What The Pros Use - PrecisionRifleBlog.com

If I was starting out again I would have to consider it instead of 260 Remington, but having invested in 260 Rem for 16 years I'm not about to change.

Quality brass is now available - 6.5 Creedmoor

If you get one made, do stipulate that it will handle factory ammunition.

Regards

JCS

 

[Greener Jim]

The main area where you gain case capacity is by reducing the taper of the case. The sharp shoulders do add to the capacity but very little compared to the reduction in taper.
As this is a commercial cartridge and would have to feed well in all actions including semi auto, the sharper shoulder is undesirable as they are harder to get to feed reliably. The taper is already severly reduced so most of the 'improving' is already done thus rendering the sharper shoulder unnecessary.

 

[crimson]

Ackley rounds can be difficult to feed (not achieving a smooth transition from magazine to chamber, particularly if operating the bolt quickly) due to the shoulder profile.
Not all rifles 'suffer' from this effect but it may well be a consideration when designing a cartridge

 

[packrat]

Thanks peeps, lots of great info, I feel suitably educated

 

[Laurie]

There is a little more to it than that. The sharper the shoulder angle, the more difficulty for the manufacturer in drawing and swageing cases and the higher the reject percentage. As the neck / shoulder swage is almost the final process in making a case, a reject means not only wasted material but most of the production time and work up to that point. As a result, many manufacturers were unwilling to go beyond 25-deg angles and 30 was until recently seen as the absolute maximum for a production cartridge. (The 1960s .284 Win with its 35-deg shoulders was well ahead of its time and must have seen a substantial reject %.) Most 'wildcats' adopted by manufacturers such as the .22-250 Rem had their shoulder angles reduced to meet this criterion.

30-degrees as used in the 6.5mm Creedmoor is no issue nowadays facilitating a more 'modern' design with a shorter case-body, less taper, and sharper shoulders compared to the 1990 .260 Rem which was a simple 'productionised' version of various 6.5-08m wildcats that had been in use for getting on for 30 years before Remy adopted the concept.

 

[Muir]

Ackley rounds can be difficult to feed (not achieving a smooth transition from magazine to chamber, particularly if operating the bolt quickly) due to the shoulder profile.
Not all rifles 'suffer' from this effect but it may well be a consideration when designing a cartridge

Agreed to this and Laurie's comment.
Additionally, you are working on the premise that Ackley shoulder form is "All that." It isn't, and for many cartridges, Ackley would've been the first to say so.

In the end, added case capacity wasn't needed.~Muir

 

[packrat]

Thank you fellas.

I can feel 1 6.5 Creedmoor coming on.

 

[NigelM]

I have a 6.5 Lapua. Very impressed with it. Exceptionally accurate and shoots a 120 grain bullet very comfortably at 2950. Probably 100fps slower than the Creedmoor.

Much depends on what you want to shoot with it. The Lapua works very well on everything up to Red Hinds. Red Stags, if the distance is over 200yds, take a bit too long to fall over.

I have just invested in a 280AI which shoots a 120 grain Barnes TTSX at 3400fps. That tends to sort the problem out and is just as accurate as the Lapua. I don't have any feed problems at all with the 40 degree shoulder. Perhaps it's because the TTSX is seated well back at 60 thou off the lands.

Creedmoor looks good, but just have a think about the quarry you want to use it for.

 

[packrat]

I have a 6.5 Lapua. Very impressed with it. Exceptionally accurate and shoots a 120 grain bullet very comfortably at 2950. Probably 100fps slower than the Creedmoor.

Much depends on what you want to shoot with it. The Lapua works very well on everything up to Red Hinds. Red Stags, if the distance is over 200yds, take a bit too long to fall over.

I have just invested in a 280AI which shoots a 120 grain Barnes TTSX at 3400fps. That tends to sort the problem out and is just as accurate as the Lapua. I don't have any feed problems at all with the 40 degree shoulder. Perhaps it's because the TTSX is seated well back at 60 thou off the lands.

Creedmoor looks good, but just have a think about the quarry you want to use it for.

Thanks, I appreciate the advice. I've got a 270 for stalking, the 6.5CM will be a 'toy'

 

[NigelM]

A man can never have enough toys!