doublegunshop.com - home Forums General Discussion Classic & Custom Single Barreled Sporting Rifles @ doublegunshop.com Minimum breech diameter?

How is the minimum safe breech diameter determined? I know that there has to be math involved.
With all the metalurgists patrolling the board I am hoping to get an answer for a project I am working on.

The example I am working with would be minimum breech diameter for a .470 base diameter in the 50K cup range.

Thanks,

Tom

I'm not a metallurgist, but it seems to me that this would depend on both the barrel steel and on what you consider an acceptable overload margin. What you really need is for an experienced firearm designer to answer the question. I'm sure there are well established rules of thumb and that sort of thing rarely makes it into textbooks. There is a discussion and example of the math involved in a book on building a single shot rifle published by the Home Shop Machinist magazine. I don't remember the exact title offhand and my gun books are all packed away at the moment.

If I understand your question, the cartridge base is going to be inside a receiver ring as well, so the barrel diameter around the chamber going forward a bit is likely to be critical as well.

Thanks 1878.

The reciever section is min .950 x 1.15. The steel is 4140 for both barrel and reciever.
BTW, I understand that this is off topic but I have seen many excellent replies (and a great deal fo knowledge) on this board.

Tom

T. Hall, moulder of young minds! (look that up in the dictionary).

The current issue of Rifle Magazine has an interesting article titled THE CRUCIBLE about J Rigby's attempts to barrel the first .450 Nitro Express. They "wasted" (Rigby's term for failing proof) four sets of barrels before they got it right.

Not as technical as you need but an inspiring read nonetheless. It's available for download here, the entire article starts on the 15th page of the .pdf.

http://www.riflemagazine.com/magazine/PDF/ri257partial.pdf

I can give you several empirical examples, successful and otherwise. Raw data, as always, is subject to individual interpretation and opinion. The following info assumes a modern steel high-strength barrel of 4140 or similar.

According to urban legend, Ackley said that the minimum wall thickness should be at least 0.125". IMO this is too little for the chamber area and I personally use a 0.175" minimum.

The Martini Cadet (0.750"-14tpi) has proven safe with 50K loads and the 0.375" case head of the 223 Rem. It's also safe with 40K loads and the 0.420" 30-30 or 225 Win case head, but the 50K factory loads of the 225 Win will cause problems.

The Win high wall (0.935"-16tpi) and the Sharps 1878 Borchardt (0.945"-16tpi) are both quite safe with 0.470" case heads & 50K loads, and FWIW I myself have built & shot a belted magnum on this size barrel shank. But I'm a hot-rodder at heart(grin).
HTH, regards, Joe

Ahhh Robert, nearly another year over with the urchins. Um, I mean budding minds.

Thanks J.D. for the personal experience. I was actually trying to decide between the .420 and .470 diameter as a point to work from.
The diameter of the reciever works for both, but...
As far as the Martini, is the problem with the barrel or with the breech? Is it a bolt thrust issue?

I am actually looking to completely unsupported barrels like Contenders and Encores as baselines. The barrel will be threaded into the reciever but I still wanted to have a minimum to work from. Knowing the hoop stress info would let me know how much insurance I was working with.

Tom

Here is the book I mentioned with the calculations you need detailed:

http://www.midwayusa.com/viewProduct/default.aspx?productNumber=947189

I would scan the relevant pages from my copy, but everything is packed away at the moment for some major house work.

Tom, the Cadet problem is strictly in the barrel shank diameter or rather lack of it. The hoop strength just isn't there. The Martini design is one of the strongest in the area of breech thrust support and if the little Cadet had a much larger shank diameter then there would be virtually no limit (except length to 'turn the corner' into the chamber). The large Martini has long since proven itself VERY sturdy with quite a few being chambered for the 30-06 and similar.
Regards, Joe

Hey JD.

What type of problem came up with the .225?

?

Tom

Bulged chamber under the threads.
Regards, Joe