30-40 Krag in a 303 SMLE action - max pressures?

Correct me if I'm wrong, but am I safe to assume that I should be able to use load charts for 303 British (up to 45,000 CUP), rather than for 30-40 Krag (up to 40,000 CUP), in a rifle built on an SMLE III action with 303 brass necked down, in a 30 calibre barrel? Not that I am planning on hot loads, but I've realised that with a 1:10 twist barrel I've got a pretty wide range of projectiles available, and it potentially won't be far off what I can shoot in my 308. In fact even the 30-40 Krag charts on the Hodgdon reloading website show a max. of 43.5gr Varget with 150gr Nosler Partition, which is pretty close to what I load for the 308.

I'm using 46gr 2208 behind 125gr Sierra Pro-hunter in 308, which although is the max. charge in my old ADI loading guide, is 2 grains below the starting charge of 48gr on the Hodgdon website.

But then in the same old ADI guide, its says to reduce max loads by 2 grains for SMLE actions (against the P14 actions).

Anyone got any experiences/wisdom they care to share?

Look at it from other end of the horse.... How hot do you really need it to be??? A mild , accurate and easy to shoot load will serve you much better. If you want 30/06 performance buy a 30/06.

Personally I think, yes. If you're using reformed .303 brass in the same action it was designed for, in a configuration that's practically the same, then there should be no reason you can't load to the same pressure it was designed for.
I would say that the lower pressure of the Krag, like many other older designs, is based upon the rifles limitations and not on the cartridge design.
Obviously standard safe load development practises apply.

Greetings @bjp,
Do you know for sure you have a 1 in 10" twist? A lot of these rifles were built using .308 take out barrels from target rifles that had a 1 in 12 or even 1 in 14" twists. The chambers are cut for heavy projectiles so your 125 grain will have significant jump to the rifling. My very limited chronographing has shown velocities much lower than expected in my 24" barrel with AR2208 and AR2206H so chronographing your loads should be a priority. This will give you a better understanding of your pressures.
Regards Grandpamac.
PS Just shuffling through my notes and I see that I have chronographed the 40 grain start load of AR2208 and the 150 grain projectile in my 24" barrel Krag rifle at 2,220 fps. Hodgdon got 2,567 fps. Also 38 grains of AR2206H with the same projectile in the Krag produced 2,192 fps compared with 2,350 fps in my .308. The Krag cases, even when formed form .303 seem to have more capacity than the .308.

Thanks @grandpamac. Rifle bought from Akaroa1, was re-barrelled by Din Collings using .30 cal Browning MG barrel, stamped with 1:10 twist. Good point re jump to rifling, I'll check that.

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Originally Posted by bjp

Thanks @grandpamac. Rifle bought from Akaroa1, was re-barrelled by Din Collings using .30 cal Browning MG barrel, stamped with 1:10 twist. Good point re jump to rifling, I'll check that.

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Greetings @bjp,
My rifle, originally a Martini Henry, was converted to .303 in 1896. The barrel is a take out Sportco with a 1 in 12" twist originally .308. Din cut the chamber in the late 1990's so our rifles may be from the same reamer or at least similar. My loads with the 150 grain Hornady flat base have around 5mm of jump to the rifling with reformed .303 cases and 3mm with .30-40 cases. This with the projectile seated 7mm into the case. 165 grain flat base projectiles could be seated close to the rifling.
Regards Grandpamac.

Yes.
I would not worry about it, just keep an eye on the velocities when you start or check if your brass is growing a lot... If Grandpamac is right about the long throat then you will actually get slower then expected velocities and can put some more powder in.

Greetings again @bjp,
Thinking about the question posed in your OP I realised that I had chronographed data for both the .30-40 and .308 with the same load. 38 grains of AR2206H in the .308 and with 150 grain Hornady Spire point produced 2,380 fps in a 22" barrel. The same load in the 24" barrel of my Martini Krag produced 2,190 fps which indicates that the Krag has a larger net case capacity than the .308. This is bourn out by the case capacity for the two cases quoted in the Nosler data for the 150 grain projectiles. I did not have the same data for the .303 and the .308 but after some number crunching from the data I did have the .308 and the .303 seem to have close to the same net capacity. The .308 loads run to much higher pressure so don't use them. To be clear here I am talking of the case capacity with the projectile seated not to the case mouth.
This indicates that the .30-40 case is larger than the .303 so .303 data will be safe to use. The data for the .30-40 is sparse and old where there is better data for the .303. Using the .303 data will likely produce lower velocities and pressures than stated and it would be best to chronograph as you go to make sure velocity and pressure are kept low.
Regards Grandpamac.

Generally or the 30-40 I use 303 data
Cross check it against 308 starting loads and 307 mid point loads

But personally can't see any reason to be anywhere near the maximum pressure for the LE action

I've just done load work on my new old 30-40 and gone with a 100% case filling load of H1000 and 150 grain bullet.
GRT indicates about 2350 fps and only 25k psi
And that in a single shot action that is good for 42k psi.
But why push it that hard when it's 126 years old

Thanks all, appreciate the advice. Searched up GRT now going down the rabbit hole of simulations - being an engineer I love a good simulation tool! I put in a couple of loads I have chrono data for, GRT was pretty close, within 10 fps, so that's encouraging.

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Originally Posted by bjp

Thanks all, appreciate the advice. Searched up GRT now going down the rabbit hole of simulations - being an engineer I love a good simulation tool! I put in a couple of loads I have chrono data for, GRT was pretty close, within 10 fps, so that's encouraging.

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Yeah it’s bloody good eh
Like any simulation it’s only as accurate as the information you give it
You can get it really accurate if you muck around with case capacity testing etc

I done a black powder load the other day GRT said 1399fps my real average over the lab radar was 1406