Hello, I would really appreciate if anybody has any experience with this cartridge. Rifle is a pre-64 Mod 70. 26" barrel. Was thinking of trying loads for the 338/06, and working up. Any help would be appreciated, thanks in advance.
help with 340 gibbs
- Thread starter eric2381
- Start date
Yup start with 06 data and increase as you see fit. I had an improved, not a Gibbs but short neck straight sides. Very fine round you should be able to come close to factory speeds. Do to the small taper you probably won't see pressure like a normal round so pay attention. Loose primers will be your indicator. I found slow powders, loaded with a drop tube and compressed worked the best. With 225-250s think IMR 4831 heavily compressed.
The 340 Gibbs is just a 338/06 "Improved", by reducing taper, reducing neck length, and blowing out the shoulder. You are gaining at most about 10% more case capacity, so should expect no more than 2.5% more muzzle velocity over the 338/06 if you keep pressures the same, i.e. compare apples to apples.
I'd start with published max 338/06 loads and increase by 1/2 gr increments. Your starting MV will be less than for the 338/06 due to the lower pressures created within the larger case, but will eventually meet, then exceed 338/06 velocities. Once your chrony shows 2.5% more velocity than published as max for the 338/06, that is your max for the 340 Gibbs. For example;
Hodgdon says a max load for a 200gr bullet is 59.5 grs of BL-C(2) and gives a MV of 2850 fps. Start with 59.5 grs of BL-C(2) and once your MV reaches 2925 fps (2850 + 2.5%) you stop.
Of course with Gibbs cartridges, most users ignore pressure and look for "pressure signs" such as loose primers, sticking bolt, etc. That's a crap shoot I avoid.
I'd start with published max 338/06 loads and increase by 1/2 gr increments. Your starting MV will be less than for the 338/06 due to the lower pressures created within the larger case, but will eventually meet, then exceed 338/06 velocities. Once your chrony shows 2.5% more velocity than published as max for the 338/06, that is your max for the 340 Gibbs. For example;
Hodgdon says a max load for a 200gr bullet is 59.5 grs of BL-C(2) and gives a MV of 2850 fps. Start with 59.5 grs of BL-C(2) and once your MV reaches 2925 fps (2850 + 2.5%) you stop.
Of course with Gibbs cartridges, most users ignore pressure and look for "pressure signs" such as loose primers, sticking bolt, etc. That's a crap shoot I avoid.
Thanks very much for the advice guys. I too was thinking about IMR 4831, the literature the gunsmith gave me lists fast burning powders such as RL-15, W-760, and IMR 4350. For a 250 gr bullet. I do believe that is the load data that Rocky Gibbs himself worked up and that was during the 60's. Correct me if I'm wrong but I believe there wasn't powders such as IMR 4831 and IMR 7828 at that time. Thanks for your time, Eric
