Grendel and 5R Rifling (Haters gonna Hate)

[bwaites]

So your saying you get more velocity out of a 6.8 with the same powder and same weight bullet than a 6.5? Or the 6.5 needs less powder to compare the two rounds safely??? I do not understand why the 6.5 needs limited load to compare it to the 6.8. That makes no sense. If they both have a 120gr bullet and the 6.8 can only handle X amount of powder and the Grendel can handle X+1 safely, then the Grendel has better velocity period.

Sort of like if you put a 100 hours power engine, 4:10 gears and the same transmission in a VW Bug or a Heavy Chevy, the VW will be faster, there's no magic in that either..

Maybe I'm missing something but why are we comparing the 6.5 and 6.8? I thought the topic was about 6.5 with 5R rifling and maybe another 6.5 type of rifling. That would be nice to discuss.

Tedward,

Think of the bullet as a piston. If the surface of the piston is larger, there is greater displacement as the piston moves, allowing more work to be done by the piston (the same principle of larger displacement car engines), since the "work" of a bullet is unidirectional (and nothing is moved by the gas expansion except the bullet), then that larger piston translates into greater velocity, all else being equal. Modifications of the throat, grooves, etc. all impact that velocity somewhat, but if ALL the things are equal otherwise, the larger diameter bullet will be faster.

However, the drawback is that the larger diameter bullet (of the same weight) will be less aerodynamic, because it has a larger frontal surface area exposed to air, and because it is shorter overall. Thus the BC is lower, and the slightly slower bullet at the barrel will catch and surpass it soon. (Sometimes by 100 yards). That's why the Grendel is such a good cartridge. It allows the use of high BC bullets in AR15's. No other cartridge allows the use of such aerodynamic bullets at these weights.

Finally, velocity is a direct function of pressure. Increase pressure increase velocity. Increase pressure enough....go BOOM!

I have two 5R Grendel barrels, both cut rifled. Their velocities with factory Hornady are within FPS of expected. I suspect that the Chronographs being used on the rifles that seem really fast might be a tad bit optimistic. Thats been discussed elsewhere, since Chrono builders can do that. People don't like to see that their pet loads aren't really as fast as they thought they were, so the calibration at the factory is a bit high.