How low can one go below the minimum powder recommendations on a load.

for example, if the data says min. load 55 grains and max load 61 grains is it safe to try 54 grains or say 53.5.

I am thinking of buying a 340 Weatherby Magnum and downloading to 338 Winchester Magnum levels. This should greatly reduce pressures and save brass.

Short answer is it depends on the case capacity and the powder being used. Would I worry about going a gr or 2 below min probably not.

Hodgdon for example simply lists a equation for reduced loads of H4895. Calling it the slowest powder to give uniform ignition at reduced loads. They simply state take the MAXIMUM listed H4895 for the cartridge and bullet and multiply that number by 60%. That is now the lowest load for H4895.

I would stay away from the SLOW powders for reduced loads and more in the Mid to fast. Remember H4895.

Many years ago I tried this very idea with a 300 Win Mag, bad idea.
I looked at the manuals and deducted that H4350 was a 'middle' performing powder and reduced the start loads by 2gr's, I instantly got hangfires, some were well over a second in delay.
I would not recommend reducing the start loads, they should be fine as is and produce velocity similar to max 338 Win Mag speeds.
I have both the 338 and 340, velocity is rarely any more than 100fps difference between the 2 anyway, with the same bullet weights in equal barrel lengths, it is in fact quite often much less than this.
Even a middle load in the 340 will be enough of a reduction in pressure to prolong case life, if that is your sole reason for this.

Cheers.

You should be able to play around with starting loads and get .338 speeds. A couple of things though; magnums tend to shoot best when the pressures are up there. Another is that reloading data is little more than an educated guess as to what it will do in your rifle. I've got a few rifles that won't take loads over minimum in some manuals without showing pressure, but the speed is there. It's s sure bet that that one could be loaded below manual minimums. The next one might take a grain of two over maximum to get the speed up and show no pressure signs. You might not want to back off below minimum with that one. A chronograph will help.

You don't have to back off far to gain case life. The Weatherby cases are pretty solid, if your primer pockets are holding up the body and neck last very well esp if you just size enough for reasonable bolt closing effort. I don't load the 340 but do load the .257, 270, 300 and 375 Weatherby.

Just for shits and giggles I went to Richard Lee's Modern loading second addition. Loading the .340 W down to .338 info is right there. The minimum loads as listed in the .340 are exactly what you say you want. " It's in da book" roger

You might also try using AA 8700 or WC 860 (you can get it from www.gibrass.com). They both have a burn rate about like torn up newspaper, and velocities are somewhat slower because of it.

I used it once in a 300 Wby for a guy that weighed about 130 soaking wet. His rifle was left to him by a father in law, and he wanted to use it but it beat him up. Surprisingly enough, a case full of WC 860 behind a Nosler 125 was a one-hole load at 100 yards, and it kicked about like my .270...

I was going to suggest the same as doubless, try a powder "too" slow for your cartridge, and you may get your desired results. memtb

Why not just buy a similar stocked .338-06 for light loads?

But why buy a 340WM with expensive brass and then load to 338WM level to try and save brass. Why not just buy a 338WM and enjoy cheaper, easier to obtain or make brass.

I could understand if one inherited a 340WM and found the brass expensive but why deliberately get in that situation by buying one.

Any magnum belted case can be made to last longer by sizing to head-space on the shoulder rather than the belt.

Often most belted magnums are at their best accuracy wise when loaded up, not underloaded.

When I first got a 338WM I thought it might give more power and recoil than I needed for forest hunting, so I picked the least-efficient powder (3031) for the 250 gr Hornady, and used the starting load. This gave velocities similar to the 358 Winchester.

I have made 180gr below-minimum sub-loads to practise with, using other powders, too, but always pointed the rifle up before firing to make sure there were granules on the primer.

I wouldn't try that with my residual stock of Win 785, though, as the maker seemed to want it used like shotgun powder - one exact load, to be used without working up or exceeding.

Use the minimum starting load in the reloading manual, the 340 isn't that much ahead of the 338 WIN. probably 200 FPS on average, if you own a chronograph!! Going with the 210 Nosler:

IMR-4320 67 grs. 2817 in the 340 Wby

RL-19 72 grs. 2860 in the .338 Win.

Both very mild loads..

Max loads in the .338 Win. with 76 grs. of RL-19 is 3020 FPS

Max load with the 340 and 85.5 grs of RL-19 is 3063,

Max load with IMR-4350 in the 340 is 83.5 for 3223 FPS

Get yourself a good loading manual such as the Nosler Reloading guide, its all there in black and white and you can custom load to about any combo your heart desires...