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NitroX
.700 member

Reged: 25/12/02
Posts: 40432
Loc: Barossa Valley, South Australi...

The 75 % Rule
#390180 - 05/03/25 12:18 AM

The 75% Rule, is the alleged regulation result that if you have a regulating bullet load, if you reduce the bullet weight tom75%, using the same powder charge, the lighter bullet will still regulate.

So:
.375 300 gr and 235 gr;
.400 .416, .423 etc, 400 gr and 300 gr;
.450 480 gr and 350 gr;
.470 500 gr and 350 gr.
etc.

I've never tried it.

Have you tried the 75% rule? Please report your results?

--------------------
John aka NitroX

...
Govt get out of our lives NOW!
"I love the smell of cordite in the morning."
"A Sharp spear needs no polish"

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DarylS
.700 member

Reged: 10/08/05
Posts: 27480
Loc: Beautiful British Columbia, Ca...

Re: The 75 % Rule [Re: NitroX]
#390190 - 05/03/25 04:53 AM

Interesting, but seems to me it worked in my .458 2".
I used the same powder charge for 350gr. .400gr. and 510gr. and all loads shot into the same group. Single barreled rifle,
nicely bedded.
500gr. 2,059fps
400gr. 2,150fps
350gr. 2,296fps

--------------------
Daryl

"a gun without hammers is like a Spaniel without ears" King George V

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93x64mm
.416 member

Reged: 07/12/11
Posts: 4313
Loc: Nth QLD Australia

Re: The 75 % Rule [Re: DarylS]
#390196 - 05/03/25 06:54 AM

I didn't have any luck with my 450/400 using 300gn .412"projectiles - it was rather woeful!
I haven't pursued that course, my cast loads shoot better than my Woodleighs, using a different powder so I've stuck with those loads as my 'go to' loads.

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Rule303
.450 member

Reged: 05/07/09
Posts: 5191
Loc: Woodford Qld

Re: The 75 % Rule [Re: 93x64mm]
#390201 - 05/03/25 10:26 AM

This did not work for me with the 416Rigby in a CZ550. It does work for me in my 375H&H (Rem700), 35 Whelen(Rem700), 35 Sambar (Ruger M77) and 358RUM and 270 both Rem700 and now Steyr Prohunter. In all but the 416 and 358RUM it seems to work with different brands of bullets as well.

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jvw
.300 member

Reged: 15/05/08
Posts: 104
Loc: Australia

Re: The 75 % Rule [Re: Rule303]
#390202 - 05/03/25 11:27 AM

This so-called "rule" was dreamt up by the late Cal Pappas after one of his rifles regulated with a lighter bullet and a reduced charge of propellant. It may or may not work in a given rifle. It certainly doesn't work in my .450 NE or in any of the double rifles owned by friends who have tried it. A rule it aint, not by any stretch of the imagination.

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Huvius
.416 member

Reged: 04/11/07
Posts: 3596
Loc: Colorado

Re: The 75 % Rule [Re: jvw]
#390222 - 06/03/25 06:41 AM

Quote:

This so-called "rule" was dreamt up by the late Cal Pappas after one of his rifles regulated with a lighter bullet and a reduced charge of propellant. It may or may not work in a given rifle. It certainly doesn't work in my .450 NE or in any of the double rifles owned by friends who have tried it. A rule it aint, not by any stretch of the imagination.

Cal didn't really dream it up.
What we call the 75% rule today was employed by Holland & Holland in their 500/450 magnum.

We discussed this topic a little in the past:

http://forums.nitroexpress.com/showflat....true#Post245663

On another thread I said:

"...when Holland & Holland touted the accuracy of their 500/450 nitro all of the superior groups were shot using their 365gr bullet in rifles very likely regulated for the 480gr bullet.
And that, gentlemen, is where the much lauded (and believed to be modern) idea of the 75% rule came from."

--------------------
He who lives in the past is doomed to enjoy it.

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93x64mm
.416 member

Reged: 07/12/11
Posts: 4313
Loc: Nth QLD Australia

Re: The 75 % Rule [Re: Huvius]
#390224 - 06/03/25 07:08 AM

It's not to say that the 75% load is a fallacy - especially in doubles!
I have a 'light' load for my 500NE or rather its Graeme Wright's 440gn NFB load, one of my mates used it in his Sabatti double - it shot exceptionally well in it!
That load does wonders on pigs by the way.

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mauserand9mm
.400 member

Reged: 03/09/09
Posts: 1064
Loc: Queensland, Australia

Re: The 75 % Rule [Re: 93x64mm]
#390232 - 06/03/25 12:42 PM

Works in my Merkel 500NE - both 570gn and 450gn in front of 106gn AR2209 regulate.

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