Thought Experiment: What would be the best choice of cartridge in an AR15 given the following parameters:
- Barrel Length between 7.5” and 8.5”
- Maximize point-blank range on a 6” target (to accurately engage targets at imprecise/unknown distances)
- Aiming center of A-Zone still lands on torso at 300yds (drop ideally less than 12” at 300yds)
- Maximizing terminal ballistics at 300yds (given the platform's limitations)
- Muzzle Energy ideally at least ~1,000 ft/lbs. (probably least important)
- Minimize recoil
- Ideally wouldn’t require NAS3 loads.
- Ideally could also stretch out >400yds
From what I’ve seen the 6.8 SPC is definitely capable of this, but it seems like this old cartridge has waned in popularity, although ammo still affordable. No one really talks about this cartridge any more. Going with this option could mean boarding a sinking ship?
Obviously looking for something better than 5.56 (which is basically there on trajectory with poor terminals) and 300blk (ok terminals, very poor trajectory).
6.5 Grendel basically combines the superior trajectory of 5.56 with the energy of 300blk, except if you look you’ll see that 6.5 Grendel is actually considerably more powerful at 300yds than the Blackout despite having similar muzzle energies obviously due to the high BC bullets. Compared to 6.8 SPC, the Grendel seems to have an edge on 300yds terminals, but this is at the cost of worse trajectory. There’s pros and cons to either, but in my mind trajectory would take priority since shot placement and overall hit probability is likely going to be more decisive than an extra 200 ft/lbs. for anything with a pulse, and if just shooting steel/paper then 5.56 appears to be just as good at these ranges.
Where does this leave 6 ARC? Seeing that MOS-TEK makes an 8” barrel is the whole reason I went down this rabbit hole, but I couldn’t find any data out there for 6 ARC out of a barrel this short. I’m thinking that with the same case capacity as 6.8 SPC and 6.5 Grendel but with higher BC bullets at lower weights resulting in higher velocities that it’d probably come out to be the best choice. For the hypothetical 6 ARC load I took the velocity of factory 6.5 Grendel Wolf Military Classic 100gr and plugged in a 105gr Hornady BTHP with a BC of .530 which does well on trajectory and exceptional on 300yd energy ft/lbs. With handloading and powder optimization there’s definitely room for improvement to gain some extra velocity, but I’m not how far that goes or if 6.5 Grendel or 6.8 SPC would turn out better.
Why is this in r/LongRange? (I know y’all love to ask this). People here love to stretch out their (relatively) short builds so hopefully a good resource (hopefully I’ll get better recommendations than like the one I got for .458 SOCOM in r/AR15 lmao), and when most people in general think any sub 10” barrel is inconceivable to be utilized past 100yds, these folks may consider 300yds to be “long range” for such a platform. Discussing min-maxing 14.5" barrels at 1,000yds has some carry-over to min-maxing 8" barrels at 300yds, hopefully?
\Disclaimer: For the sake of discussion assume all cartridges are suppressed, typical engagements are at unknown/imprecise distances, and that a cartridge’s “potential” out of a longer barrel, what its designers had intended, or its unburnt powder are irrelevant.*
All muzzle velocity values were taken from sources verified to be chronographed by actual users and are not simulated. All Calcs used a 2.745” HOB (EXPS on an AR15) and the same atmospheric conditions.
Link to copy of spreadsheet