Sure - by my math the bolt thrust is the same a 5.56 at 42.5K PSI. So to make up for the missing material, 40K is reasonable.
Problem is, 6.5 Grendel under-performs both 5.56 and 6x45 once downloaded. So what's the point?
Allegedly, this is a bit of a sweet spot for accuracy when using the standard 123gr VLD bullet from a 1:9 barrel, for one. For two, you still get to use longer projectiles with higher BCs than those other two cartridges if I am not mistaken, while still tossing more mass down range, so wind resistance & momentum is superior. Lastly, if using long barrels that make most efficient use of Grendel's larger powder volume, the reduced peak pressure does not drop velocities all that much (I'm thinking it was at most like 100fps). That pressure peak ultimately forms a fairly small portion of the overall pressure curve, so dropping it down & using a long barrel with slower powder limits the trade-off. Obviously this does hurt the competitiveness of the AR15 Grendel vs. more short-barrel friendly rounds like 6.8spc, but Grendel shooters seem to typically be more accuracy-oriented vs. tactically-oriented, so it is less of an issue (unless you're claiming it to be a 'universal' solution, like 6.8 purports to be with its stubby, lower BC bullets
)
Again though, from what I have researched (not quite done with my build to where I can start experimenting on my own) the step back from magnum pressures really doesn't reduce the effectiveness of the round all that much, but makes it far more amenable to the popular AR platform. VEPR and bolt-gun owners can handload it to higher levels if they really care to, and I suppose some do. It just hasn't been shown to reap rewards significant enough to justify making the round too powerful for the AR15. As this thread has shown, the 6.5-in-an-AR niche is really crowded right now, so to say one 'outperforms' another --any other-- is kind of hyperbole. We're talking less than 100fps second differences, and usually less than 20gr deviations, chambering to chambering.
The brass tacks of what type of bullets you want a good selection in (for me, high BC's suggest 6.5 is a good place to start, vs. 6.8), the barrels you want a good selection in (6.8 is by far the cheapest for blanks at this time, but I found a 50$ chambered barrel blank), and available components (quality Lapua brass for Grendel & Hornady dies wasn't significantly different from the others)
And last but most important; I have two RPD kits and Grendel fits in RPD links like it was made for them. At 800yds, you have similar drop & higher energy (with higher sectional density) than 308, from a platform at least 10lb lighter than an M240. The 7.62x39 case head size may be a pointless commonality in the AR15 (it is), but there are a whole lot of cool guns out there designed for x39 that can be readily adapted to a FAR more effective variation of that cartridge. Many can even take advantage of the accuracy benefits, if properly prepared. A VZ58 in Grendel would be a platform lighter & smaller than an AK or most ARs, that shoots (not hits or recoils) like a 308. A VZ52 would be capable of feeding far longer/heavier than standard bullets from its 7.62x45 magazines. I am personally most excited for the Kel Tec RDB in Grendel that's been floated, since it promises to be light, compact, sturdy, simple, clean, accurate, controllable, ergonomic, inexpensive among its peers, and highly effective for a huge range of possible roles in this chambering.
TCB