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Double Rifles, Single Shots & Combinations >> Building Double Rifles & Gunsmithing

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Mauser416
.300 member

Reged: 20/06/07
Posts: 162
Loc: Corpus Christi, Texas

Shotgun Conversion Load Pressure
#208303 - 12/05/12 01:03 PM

What do y'all recommend as the maximum pressure for use in a shotgun converted to DR? My particular interest is a CZ Bobwhite 12 ga that has been converted to 45-120 3-1/4.

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There is nothing worse in this world than an enslaved man who naively believes himself free, except, perhaps, trying to explain to that same man his predicament.

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Mauser416
.300 member

Reged: 20/06/07
Posts: 162
Loc: Corpus Christi, Texas

Re: Shotgun Conversion Load Pressure [Re: Mauser416]
#208328 - 13/05/12 10:08 AM

I was considering the following:

12 ga. "rim" diameter - 0.886"
45-120 rim diameter - 0.608

pressure of 12 ga. 3-1/2" - 14,000 psi

12 ga. rim surface area - 0.618 sq in.
45-120 rim surface area - 0.291 sq in.

ratio of rim surface area = 2.124

12 ga pressure x surface area ratio
14,000 x 2.124 = 29763 psi

Do you consider 30,000 psi a good number... to low...to high?

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There is nothing worse in this world than an enslaved man who naively believes himself free, except, perhaps, trying to explain to that same man his predicament.

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DBLGN
.224 member

Reged: 16/10/07
Posts: 46
Loc: Colorado, USA

Re: Shotgun Conversion Load Pressure [Re: Mauser416]
#208361 - 14/05/12 10:13 AM

I generally won't build with any cartridge over 40K psi - less is better. Most of the large British cartridges: .450's, .465, .470, .500; are in the 34K-36K psi range. As I have pointed out in other posts, one of the limiting factors in using a shotgun action, is that the fit at the "circle" is generally not as tight as with a double rifle. See Vic Venters book "Gun Craft" and the chapter on this subject. There is a way to solve this "loose fit" at the circle (shown in Venters' book), but I have not done one yet, so hesitate to pontificate too much this subject at this junture.

"Not knowing to any degree of certainty and, not wishing to propagate an error, I hesitate to respond."

My recommendation is to keep things under 40K - less is better.

Ellis

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DBLGN

Edited by DBLGN (14/05/12 10:15 AM)

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Mauser416
.300 member

Reged: 20/06/07
Posts: 162
Loc: Corpus Christi, Texas

Re: Shotgun Conversion Load Pressure [Re: DBLGN]
#208390 - 15/05/12 03:25 AM

Sage advice Ellis. I now recall reading your previous posts about "fitting the circle". Based on some early calculations I expect to be able to do what I want well below 40K psi.

Thank you for your input.

Eric

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There is nothing worse in this world than an enslaved man who naively believes himself free, except, perhaps, trying to explain to that same man his predicament.

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hoosier
.224 member

Reged: 11/07/11
Posts: 32
Loc: Indiana,USA

Re: Shotgun Conversion Load Pressure [Re: Mauser416]
#208431 - 16/05/12 04:11 AM

If I may ask,what is fitting the circle?
Hoosier

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BigMike

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DBLGN
.224 member

Reged: 16/10/07
Posts: 46
Loc: Colorado, USA

Re: Shotgun Conversion Load Pressure [Re: hoosier]
#208437 - 16/05/12 09:18 AM

Below is a link to where this is discussed in greater depth, and I really recommend Vic Venters book "Gun Craft". He has a whole chapter on this subject and gives the best explanation of this that I have ever read. Also shows a way that this can be corrected.

Basically, the "circle" is the forward arc of the rear lump. In a double rifle, this should fit tightly against the mating portion in the action bar. This takes a lot of stress off the hinge pin and keeps the action from "stretching" open when the gun is fired. Generally, if the pressures are kept down in the 35K psi range or less, the shotgun action is usually okay, but as the pressures start go above that, the fitting of the circle is more critical. A loose fitting circle can adversely affect the cartridge cases as well, if the action flexes while shooting.

http://forums.accuratereloading.com/eve/forums/a/tpc/f/760101804/m/5861016271

Ellis

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DBLGN

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MaxGera
.275 member

Reged: 15/02/12
Posts: 98
Loc: Pennsylvania, USA

Re: Shotgun Conversion Load Pressure [Re: Mauser416]
#208438 - 16/05/12 10:16 AM

Hoosier,

If you look at one of my earlier posts, I show my set-up for machining that surface. The method that I use produces a very accurate "circle" in the receiver and it makes obtaining a good fit rather easy.

Here is a picture showing how the surface is machined by pivoting the receiver by hand against a very sharp and smooth broach. It leaves a mirror finish surface concentric to the hinge pin within less than .001 TIR.

It's a rather difficult surface to photograph clearly, but I'll try to get a good picture in the next day or so.

Edited by CptCurl (11/06/12 08:52 PM)

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hoosier
.224 member

Reged: 11/07/11
Posts: 32
Loc: Indiana,USA

Re: Shotgun Conversion Load Pressure [Re: MaxGera]
#208475 - 17/05/12 01:19 PM

Thanks for the information,looking up the other post it is clear now to me.
Hoosier

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BigMike

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Mauser416
.300 member

Reged: 20/06/07
Posts: 162
Loc: Corpus Christi, Texas

Re: Shotgun Conversion Load Pressure [Re: hoosier]
#210216 - 06/06/12 08:09 AM

I ordered Vic Venters book "Gun Craft" from Amazon today. They had it on sale for $19 and change. The preview pages include the illustration Mr. Brown is referring to. It would probably be worthwhile and interesting to go to Amazon and "preview inside the book" just for that illustration.

Eric

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There is nothing worse in this world than an enslaved man who naively believes himself free, except, perhaps, trying to explain to that same man his predicament.

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doubleriflejack
.333 member

Reged: 11/11/07
Posts: 352
Loc: Oregon, U.S.A.

Re: Shotgun Conversion Load Pressure [Re: MaxGera]
#210298 - 07/06/12 04:15 AM

Max,
When you eventually get around to mating the lump to the "circle," I hope that you shed light on that process, as to how you accomplish it?

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doubleriflejack
.333 member

Reged: 11/11/07
Posts: 352
Loc: Oregon, U.S.A.

Re: Shotgun Conversion Load Pressure [Re: MaxGera]
#210301 - 07/06/12 04:44 AM

Max,
I have noticed on some double rifles, the action doesn't have a real "circle" machined in, but instead, they have a straight line angled surface, obviously due to fact that a straight line surface is much easier to machine in. A true "circle" surface obviously gives more surface area, recoil contact, than does a straight line surface, but I am not sure if that difference is enough to merit consideration. Please comment, giving your thoughts about a straight line angled surface, as opposed to a true "circle" surface.

Also, I have noticed that some double rifles have a HARDENED steel insert dovetailed into the lump, that mates with the action "circle." What are your thoughts about this?

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Mauser416
.300 member

Reged: 20/06/07
Posts: 162
Loc: Corpus Christi, Texas

Re: Shotgun Conversion Load Pressure [Re: doubleriflejack]
#210311 - 07/06/12 07:20 AM

doubleriflejack,

That observation of a curved versus straight, angled surface had me scratching my head for a moment. The Cz Bobwhite has the straight, angled surface instead of the radiused surface. I too would like to hear the thoughts pro or con on each.

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There is nothing worse in this world than an enslaved man who naively believes himself free, except, perhaps, trying to explain to that same man his predicament.

Edited by Mauser416 (07/06/12 07:20 AM)

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Mauser416
.300 member

Reged: 20/06/07
Posts: 162
Loc: Corpus Christi, Texas

Re: Shotgun Conversion Load Pressure [Re: Mauser416]
#212413 - 10/07/12 05:19 AM

Reading Mr. Venter's book it would seem that it is difficult to quantify one as better than the other. They key as mentioned by Mr. Ellis in his posts and in Mr. Venter's book is the proper fit up of either as being of paramount importance.

I haven't been able to find anything that would indicate the concave "circle" is either inferior or superior to the "wedge" circle.

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There is nothing worse in this world than an enslaved man who naively believes himself free, except, perhaps, trying to explain to that same man his predicament.

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