65/70/76mm

[Fargo]

Like most people I have a 3” chambered gun.

I generally only shoot game and use 67mm cartridges normally. Was thinking today about a chat I had a while ago with someone in the gun trade ( don’t remember whom) that they advised for the best patterning to use the longest cartridge you can - ie for me to use 70mm cartridges in my 76mm gun as that would help with the best patterns.

Does anyone have any experience with this or is it a load of poop? will get a pattern board out one day but seems a bit pointless as a 30g 6 supreme 65mm will pattern differently from a black gold 30g 6 70mm anyhow.

 

 

[Old farrier]

41 minutes ago, Fargo said:

Like most people I have a 3” chambered gun.

I generally only shoot game and use 67mm cartridges normally. Was thinking today about a chat I had a while ago with someone in the gun trade ( don’t remember whom) that they advised for the best patterning to use the longest cartridge you can - ie for me to use 70mm cartridges in my 76mm gun as that would help with the best patterns.

Does anyone have any experience with this or is it a load of poop? will get a pattern board out one day but seems a bit pointless as a 30g 6 supreme 65mm will pattern differently from a black gold 30g 6 70mm anyhow.

 

 

Not really pointless you would know what gave you the best pattern and then you could buy what suits your gun best 

just my thoughts 

[TOPGUN749]

I have known cartridges marked 67.5mm and others 70mm but the cases are the same length!

[enfieldspares]

There's yes, a school of thought that says that the best length cartridge is the one that matches the length of the gun's chamber.

So that there is no "jump" (as it were) between the end of the case mouth and the leed into the bore of the gun. IMHO that may be relevant if using a 65mm cartridge in a 76mm chamber or...the extreme a 50mm (aka Two Inch 12 Bore) cartridge in a 76mm or even 89mm chamber.

FWIW I think that there's some truth to it. I hear that Hull Cartridge Company are thinking of developing three inch length cartridges (for steel) that carry but the equivalent of a 1 1/8 ounce load. I have some belief hat, yes, using steel shot loaded 67.5mm cartridges in 76mm chambered guns isn't the best of ideas.

Why is this. As I think that the pressure behind the shot loading will cause it to, briefly, expand into the full diameter of the chamber space that is "spare" (if using a 65mm cartridge in a 76mm chamber) and then be constricted down as it enters the leed into the barrel.

For sure in pistol days uising .38 Special cartridges in .357 Magnum chambered cylinders you'd get leading in that "space". The same reason there is advice not to use .22 RF Short in rifles chambered for .22 RF "Long Rifle" (the standard chambering of all rimfires today) chambered firearms be they pistol, revolver of rifle.

And finally as 70mm cartridges are cheaper and unless you've inherited a shed load of 65mm cartridges  I'd use 70mm cartridges anyway if your gun will take them. The test might be to try 65mm in one barrel and only 70mm in the other and then see which leed had leaded the most. With plastic petal wads I think there will be no difference. But you'll see it with fibre wad cartridges where the lead shot is in direct contact with that "space" between cartridge mouth a leed.

 

Edited June 20, 2023 by enfieldspares

[ditchman]

slow burning powder and long barrels..............

people seem obsessed with F/Sec....why i dont know

Edited June 20, 2023 by ditchman

[wymberley]

10 hours ago, TOPGUN749 said:

I have known cartridges marked 67.5mm and others 70mm but the cases are the same length!

Simply for clarity, is that fired or not?

8 hours ago, ditchman said:

slow burning powder and long barrels..............

people seem obsessed with F/Sec....why i dont know

Possibly because they've been 'sold' on the idea that more is more.

[TOPGUN749]

31 minutes ago, wymberley said:

Simply for clarity, is that fired or not?

Fired as I recall ,will measure some again after I shoot some,but unfired wiith identical crimps the cases of 67 and 70 are the same.

Edited June 21, 2023 by TOPGUN749

[Stonepark]

The main effect of miss matched chamber and cartridge length is damage to the pellets caused by the expansion and then compression of the shot load at high pressure, resulting in less optimum (but still useful) patterns.

 

3 inch chambers (until 3.5inch came along) were a can take anything solution and allow maximum flexibility for use for both clays, game, vermin and wildfowl but sacrificing pattern density of smaller loads for being able to also fire large loadings.

3.5inch chambers have made the problem significantly worse and really shouldn't be used with anything less than 3inch hulls.

A lot of clay guns (and traditionally Baikals) are chambered in 70mm to give the best patterns with 70mm shells and not just because they are only expected to fire light loads but because it minimises the damage caused by miss matched hull length to chamber size on the shot load.

[London Best]

Back in the days when all cartridges had a rolled turnover closure, then a 65 was a 65, a 70 was a 70 after firing. 
When Eley first started loading crimp closures it was noticeable that their 65 mm cartridges were 70 after firing. Their explanation was that, obviously,  the crimp took up more of the case and so, to get the same load into the case they needed to use a longer case. In those days it was not so simple to just use a different powder as they simply did not exist.  
Eley said that the length printed on the case was the minimum chamber length in which that cartridge should be used because of the pressures generated and regardless of case length. Go figure?
Today a 65 cartridge can have it’s traditional load in a proper 65 case again because with modern powders there is room for a crimp.

[enfieldspares]

2 hours ago, Stonepark said:

The main effect of miss matched chamber and cartridge length is damage to the pellets caused by the expansion and then compression of the shot load at high pressure, resulting in less optimum (but still useful) patterns.

 

3 inch chambers (until 3.5inch came along) were a can take anything solution and allow maximum flexibility for use for both clays, game, vermin and wildfowl but sacrificing pattern density of smaller loads for being able to also fire large loadings.

3.5inch chambers have made the problem significantly worse and really shouldn't be used with anything less than 3inch hulls.

A lot of clay guns (and traditionally Baikals) are chambered in 70mm to give the best patterns with 70mm shells and not just because they are only expected to fire light loads but because it minimises the damage caused by miss matched hull length to chamber size on the shot load.

A better written and simpler explanation of what I tried to say. +1.