Here is what QuickLOAD models for .260 Rem up to the 6.5mm Weatherby Magnum using allowable max pressures exxept for the 6.5X55 Ackley where 60,000 psi is used rather than the standard 6.5X55mm SKAN. The bullet is the 140gn Berger Hybrid, not the absolute highest BC model available, but the highest you get by ordering from your local gunshop. It is assumed the chamber is throated for the bullet at the ideal position in the neck or close (ie a calibre depth for the shank) not SAAMI or CIP (as would be done with a custom rifle) and the barrel is 30 inches. In each case two loads are provided, that calculated for a max load of high-energy / double base powder and the highest available load for a single-based type that will reduce throat erosion somwhat, or maybe even greatly. There is no guarantee that these loads would work well. They are illustrative only and shouldn't be taken as either useable or safe.
260 Rem (55gn case water cap.) 60,000 psi
48gn N560 ................. 3,044 fps
47.4gn IMR-7828SSC... 3,016
6.5X55AI (62gn case water capacity) 60,000 psi
59.8gn N570 .............. 3,190 fps
52.8gn IMR-7828SSC .. 3,113
6.5-284 Norma (66gn case weater cap) 60,000 psi
63gn N570 ................. 3,250 fps
55.4gn IMR-7828SSC .. 3,156 fps
6.5 SAUM (72gn case water cap.) 60,000 psi
66.9gn N570 .............. 3,298 fps
66gn H. Retumbo ........ 3,249
6.5 SAUM 65,000 psi
68.3gn N570 ............. 3,375 fps
67.4gn H. Retumbo ... 3,312
6.5 WSM (81gn case water cap.) 60,000 psi
72.1gn N50 ............... 3,378 fps
71.2gn H. Retumbo .... 3,279
6.5 WSM 65,000 psi
73.7gn N570 ............. 3,414 fps
72.8gn H. Retumbo .... 3,345
6.5 WBY Mag (98gn case water cap.) 60,000 psi
91.2gn 570 ............... 3,402 fps
86.2gn H50BMG ........ 3,322
6.5 WBY Mag 65,000 psi
83.6gn N570 ............. 3,468 fps
88.3gn H50BMG ......... 3,441 fps
The rule of thumb is that if the case capacity changes by X% between two cartridges, the MV changes by X% divided by 4 all other things being equal (primarily barrel length, use of a suitable powder, and maximum chamber pressure). All other things being equal doesn't apply here as half the sextet are 60,000 psi rated and the others 65,000 psi, hence why I've taken two QuickLOAD calculations.
So the 72gn Capacity SAUM has 31% more capacity than the 260 Rem so should produce around 7.5% more velocity at 60,000 psi. It's calculated to produce an extra 8.3% at 60,000 psi, so not too far out and suggesting the SAUM with a suitably slow burning powder (N570 v N560) hasn't lost a great deal of inherent efficiency.
Going up to the 81gn capacity WSM over the 72gn SAUM adds another 12.5% case capacity. Taking both designs' rated 65,000 psi PMax now, the WSM's 3,414 fps is 39 fps higher than the SAUM is expected to produce, an increase of 1.15% compared to the rule of thumb quarter value which would be 3%. This suggests the WSM is becoming seriously over useable bore capacity compared to the SAUM. The H. Retumbo as the best of the single base powders performance is better with an extra 66 fps which works out at 2%, but still down on the formula's expected value.
Go up from the SAUM to the Weatherby and case capacity increases by 36%, so this design should produce aropund 9% higher MVs. It actually produces 5% improvement with the Viht N570 combination.
Another way of looking at it is MV and ME per grain weight powder burned. The 260 Rem hottest load produces 63.4 fps / 1.0gn and 60 ft/lb ME. The SAUM at 60,000 psi runs at 49.3 fps and 50.5 ft/lb ME. Up the charges to the allowed 65,000 psi levels and it improves marginally (as expected) - 49.4 fps per grain. The Weatherby is pretty cr*p at 37.3 fps per grain powder burned. So, the Weatherby injects around 75% more heat energy into the barrel for a 14% velocity increase and a 30% ME gain.
So what sort of MVs do you need to run a good 140gn 6.5 match bullet at over 1,000 yards? All will do it easily, even the 260 Rem in long throat chamber form. Here is where each of them drops into the transonic speed zone (Speed of sound + 200 fps or ~ 1,330 fps) at the maximum calculated MV and roughly where they risk droppingt through the spound barrier in 'standard ballistics conditions of 29.92 inches mercury pressure and 59-deg F air temperature).
260 Rem ............... 1,300 / 1,500 yards
6.5X55 AI ............. 1,400 / 1,600
6.5-284 ................ 1,450 / 1,600
SAUM ................... 1,500 / 1,700
WSM .................... 1,500 / 1,700
WBY ..................... 1,550 / 1,775
If you want to see what small 6 and 6.5mm cartridges can do from 26 inch barrel tactical rifles and with lighter bullets have a look at Scotty and Chiuck's three-round challenge video on You Tube with a 6XC / 155s and 6.5 Creedmoor with 123s at up to 1,700 yards. Well worth watching as a fun video.
) what would be the ultimate long range capable of well over 1000 yards) 6.5?
