quote:

Originally posted by DManson:
Hello Canadian:

Interesting project that, on its face, has several potential issues that may be difficult to overcome.
From a cartridge/chamber relationship standpoint, there are two potential problems. The first is headspacing. You wisely specified a 40* shoulder angle, which is good, but there's only .015" per side to seat on, when cartridge/ chamber clearances are taken into account. In our experience, this can be made to work.

Do you think I would be better off by reducing the taper a bit. I can go up to .6242 shoulder diameter while still maintaining a .009 per inch body taper

The second problem is case-head strength after rebating the rim and extractor groove to suit a 308-size bolt face uses standar magnum bolt face . The Gibbs case was designed for pressures not nearly as high as those common today, and you may experience case-head collapse in the pursuit of velocity. The Cheytac case is based off Gibbs dimensions, but is considerably stronger.

I am going to have RCC make brass for me (I know its expensive) but they lathe turn the brass to my specifications which should alleviate any potential brass weakness problems I would have by putzing with Gibbs or Chey tac brass.

A third possible issue is one of feeding. If you use a typical lead-core projectile, it will be quite heavy and may cause the cartridge to nose-dive after release from the mag lips and strike the edge of the chamber opening. You do have a huge hole into which it's being fed, so I bet it would work without too much trouble. If it did tend to drop, you could always use a turned brass/bronze projectile, which is lighter.

The AR design utilizes a barrel extension, into which the bolt locks up and the cartridge feeds through. I don't know if the standard hole in the extension is sufficiently large for your round to pass through. If the through-hole needs to be enlarged, will there be sufficient extension lug height to hold the pressures involved?

A related question to the above has to do with std barrel tenon diameter, and whether chamber wall thickness would be sufficient for a cartridge as large as yours.

I think this is the million dollar question with this build. I do know that the 500 jeffery has been made to work with the standard barrel extension (470 Rhino and 500 Phantom) but I don't know if by adding .021 base diameter would change that. I want to say that the .585 Nyati case had been done as well with the .510 phalanx but I cant seem to find any information.

I hate to simply raise questions without being able to provide data, but I don't have std dimensions of tenon and extension threads in front of me. It would likely be possible to make a larger extension, but it'd be much easier to use standard components.
I really appreciate you taking the time to point out potential problems (even if you dont have the answers). It is much better that its brought to light now than when I have thousands invested in parts..

A larger extension would require a larger barrel tenon, which, in turn would require that the upper receiver be re-tapped with the larger thread. This leads to the possibility of a weakened thread in the upper, as well as having to re-position the gas tube. Definitely easier to use standard components.

Other members, I'm sure, can add to these comments and likely answer the questions I raised.

E-Mail us if you have further questions about chambering your beast.

Dave Manson
support@mansonreamers.com

quote:

Originally posted by 338guy:
You might want to do a Google Search on the AR-10 Ultra Magnum. There is a good article from Shooting Times, 9-23-2010. It discusses the changes made to accommodate the 300 RSAUM and why this round was chosen over the 300 WSM. There also is an article from American Rifleman about the AR-10 Magnum.

quote:

Originally posted by MoreBS:
Do you have a bolt head?

this is the one to use

https://www.kakindustry.com/kak-industry-head-mag