Re: WHAT CABLIER SHOOT FLATER 7MM-08 OR 6.5 .284
In my opinion, the comparison of the trajectories of the 7mm-08 and the 6.5X284 based on the 6.5mm 140-grain A-Max and the 7mm 162-grain A-Max is very misleading. Even though the bullets are technically of the same commercial type, there is a very significant difference in their Form Factors. Both bullets have the same Sectional Density of .287. However, the 7mm 162-grain A-Max has a much higher Ballistic Coefficient despite having the same Sectional Density and being of the same commercial type. The fact that the 13.6 percent difference in Ballistic Coefficient does not arise from any difference in Sectional Density suggests that one bullet is actually more aerodynamically designed, despite the fact that they are both A-Max bullets. Put simply, the fact that the 7mm-08 bullet is cutting the wind better in this instance has absolutely nothing to do with a higher sectional density and arises entirely from the fact that it is shooting a more areodynamically designed bullet.
I ran through what I believe is a much more fair comparison based on a different popular bullet type available in both calibers. I compared the 6.5mm 142-grain Sierra BTHP MatchKing to the 7mm 175-grain Sierra BTHP MatchKing (using the 168-grain BTHP MK would have been very unfair to the 7mm-08 because of its comparitively high FF). The Form Factors of the two bullets I did choose are much more similar than those of the previously mentioned A-Max bullets. This is intended to minimize any unfair advantage given to one cartridge over the other. The SD of the 7mm 175-grain BTHP MK is approximately .309958 and that of the 6.5mm 142-grain BTHP MK is approxiamtely .291060. The BC of the 7mm 175 BTHP MK is .608 and that of the 6.5mm 142 BTHP MK is .595. Their comparitive FFs are .5097999 for the 7mm and .4891764 for the 6.5mm. This results in a mere 4.2 percent advantage given to the 6.5X.284, as opposed to the whopping 13.6 percent that was being given to the 7mm-08 in the original comparison.
I ran through 1200 yard trajectories for both based on 1000 yard zeroes and a 10mph crosswind.
7mm-08 / 175-grain BTHP MatchKing vs. 6.5X.284 / 142-grain BTHP MatchKing
7mm-08 : -175.73 inches
6.5X.284 : -138.08 inches
7mm-08: 1194-1206 yards
6.5X.284 : 1193-1207 yards
7mm-08: 123.29 inches
6.5X.284: 108.33 inches
7mm-08 : 545 ft/lbs.
6.5X.284 : 540 ft.lbs.
Note: I did simplify things by using the same G1 Ballistic Coefficient throughout the entire trajectory for both bullets, as opposed to using Sierra’s multiple Ballistic Coefficient method.
I also ran another comparison based on the absolute flattest trajectories I could calculate out for each cartridge based on the highest possible velocities published in Sierra’s Reloading manual at a given weight and using Lost River Ballistics bullets.
At 1200 yards with a 1000 yard zero and a 10 mph crosswind.
6.5X.284 using 120-grain J36 with a BC of .687 launched at 3100 fps.
7mm-08 using 140-grain J36 with a BC of .664 launched at 2900 fps.
7mm-08 : -113.81 inches
6.5X.284: -93.97 inches
7mm-08: 1191-1209 yards
6.5X.284 : 1189-1211 yards
7mm-08: 89 inches
6.5X.284: 76.72 inches
7mm-08 : 682 ft/lbs.
6.5X.284 : 726 ft.lbs.
Note: The 6.5X.284 is getting an advantage in Form Factor of approximately two and a half percent.
It appears to me that the 6.5X.284 can launch a bullet of a given sectional density faster than the 7mm-08 given the same barrel length and limit of loading pressure. Actually, it can produce comparable speeds even with the same weight of bullet! In order for the 7mm to get a sectional density advantage, it would have to shoot the absolute heaviest 7mm bullets available and it doeesn’t launch them fast enough to overtake the 6.5X.284 in terms of flatness of trajectory unless you start to consider extreme distances that most likely would exceed the maximum effective range of both cartridges. I assume this is because the 6.5X.284 burns more powder and has a smaller diameter, producing a higher expansion ratio. Higher velocity at the same sectional density means a flatter trajectory given the bullets being compared have the same form factor.