67mm in a 65mm chamber

[Dibble]

I realise this has been done before but....

Am I right to presume that the overpressure issues are when the cartridge wall narrows the bore?

If we take my chambers as 20.2mm my barrels 18.2mm so the forcing cone reduces the diameter by 2mm, if my cones are 5mm long them 2mm along them they have reduced the bore by 2/5 x 2mm or 0.8mm.

So at 67mm along the barrels they are 20.2-0.8mm =19.4mm diameter.

A plastic case wall is at most 0.6mm thick so the free diameter is 19.4-(2x0.6)mm=18.2mm.

So the same diameter as my barrels and no over pressure?

 

[Old Boggy]

That sounds good in theory but I was under the impression (rightly or wrongly) that the chamber length before the forcing cone should be at least the length of the fired case.

However, I find your explanation hard to fault. Others more experienced may have differing views.

OB

[Lloyd90]

I've heard 67mm ok to fire in a 65mm chamber  

[Dibble]

The chambers measure 65mm before the cones start (I made the gauge slightly out ).

[TIGHTCHOKE]

Most boxes of cartridges will have the 67mm are okay to use in 65mm chambered guns printed on them.

[DUNKS]

I think it must depend on the forcing cones, length and shape. I have a turn of the century English hammergun and the forcing cones are an actual step. designed I am told to offer a smooth transition out of the cartridge and into the barrel. I don't shoot 67mm in it.

[JJsDad]

2 minutes ago, TIGHTCHOKE said:

67mm are okay to use in 65mm chambered guns printed on them.

And in the light that the chamber length is invariably stamped on the barrel flats during proof, I am not sure where this is all heading or what the question is ?

[Dibble]

22 minutes ago, JJsDad said:

And in the light that the chamber length is invariably stamped on the barrel flats during proof, I am not sure where this is all heading or what the question is ?

Ah ..the question was does my logic seem ok. I like to understand how things work.

If I understand I'll be more comfortable about using Eley Selects in my Churchill, they are 67mm and Eley just says they are suitable for all guns, which I don't trust because what about 2" chambers. The Eleys are easier to buy than Hull CompX  and my son shoots them.

 

[Guest cookoff013]

Cip specs mean a chamber minimum size is +2mm.

 

[Dibble]

13 hours ago, cookoff013 said:

Cip specs mean a chamber minimum size is +2mm.

 

I thought this was why they were marked 65mm, 2.5" is 63.5mm?

[Guest cookoff013]

To have a 65mm chamber, the minimum length of chamber must be 67 mm. 

[Old Boggy]

On 11 January 2020 at 19:31, Old Boggy said:

That sounds good in theory but I was under the impression (rightly or wrongly) that the chamber length before the forcing cone should be at least the length of the fired case.

However, I find your explanation hard to fault. Others more experienced may have differing views.

OB

I stand corrected by Tightchoke who rightly points out that most cartridge manufacturers state that 67mm cartridges can safely be used in 65mm chambers. It is more than their reputation's worth for it not to be true.

OB

[Dibble]

46 minutes ago, Old Boggy said:

I stand corrected by Tightchoke who rightly points out that most cartridge manufacturers state that 67mm cartridges can safely be used in 65mm chambers. It is more than their reputation's worth for it not to be true.

OB

I think Tightchoke is right as well, mainly because Plastic cases have a thinner wall than paper cases not because 2 1/2" chambers are actually 67mm, mine arn't.

[wymberley]

To have a 2&1/2" chamber, the minimum length of the chamber must be 65.02mm.

To have a 2&3/4" chamber, the minimum length of the chamber must be 69.85mm.

To have a 3" chamber the minimum length of the chamber must be 76.2mm.

Consequently, it can be seen that the Gunmakers' Association joined the EEC long before the rest of the country.