I noticed the same thing when reading up on the cartridge prior to buying mine.
None of my manuals agree on the maximum powder charge. They vary by about 5 grains. So, I went off the 284 Winchester data with the same bullet weight, and the results were much more comparable. Most of my manuals don't even list the 6.5-284 cartidge. Alot of the guys that are shooting competition are shooting more powder than the books say they should.
I am shooting way more powder than the books say to get 2975'/sec with the same load out of a 26" tube..............54.5 grns H4831sc. Didn't start seeing any ejector flow until 56 grns powder! 54.5 was the best group of anything from 51 to 56 grns, and I can neck size the brass.......it's not so hot that it has to be FL sized every firing. Differences in bores and chambers and throats apparently.???
3000'/sec is expected out of this cartridge with 26" barrels or longer as far as I've been able to research. When I watched David Tubb shoot it at a mile (on video) he mentioned he was getting 3000' / sec. The Hodgdons data for the 140 says max is only 2700 something. ??
Guess I should atleast try to answer OP's question!
~2730fps and only 49.1Kpsi
Your barrel is too short for this cartridge, and even with 3.5gr less powder than SBruce, your muzzle pressures are higher than his.
Your case fill is only ~90%
With this, the powder and the short barrel, I would expect high extreme spreads.
You did not say if your bullets are touching the lands. This would bump up the velocity from 2730.
Same load shot today at 700 yds for the first time. Wind 0-2 mph from the right.
OP, do you have a chronograph? I just use a cheap Chrony and verify velocity is about where I want it once I find the most accurate and consistant powder charge. You can probably get one for about 100 bucks. This load I am shooting was only chrono'd - 5 shots worth, but drops are really close to right on with the ballistic program, so it's gotta be accurate enough.
Mike. i am off the lands approx .1 . I do have an inexpensive chrony and need to get back to the range. Just was curious. there seems to be alot of knowlege here. Will do.
Also, I tried the retumbo. didn't find it shot well in my rifle and noticed compressed loads in recipes listed on LRH.
I will chrono the original 51 gr. I will also load some with different seating depth( to the lands)
Out of a 24" barrel you should be running about 2875 FPS with 51 gr. of H4831sc. My current loads for both my rifles is 57.5 gr of Retumbo in my 26" Cooper for 2965FPS, and 53gr.of H4831sc in my 24" Savage at 2950FPS. These loads have been chronographed and drop tested at long range. I use Berger hunting VLD's seated .075 off the lands, Lapua brass, Fed 210's with H4831sc, 215's for Retumbo.
I just shot some 140g Nosler Custom Competitions in my 111LRH with 51g H4831sc and a BR2 primer. The average was just shy of 2800fps. The same powder charge with Accubonds and WLR's went in the mid-2800's. The different loads were shot minutes apart.