Hide Bound

The Firearm Blog has an article about the new LWRC Six8, 6.8 SPC rifle.

They've gone their own way on the mag-well.  It's a bit wider, and that allows Magpul to make a 30-round PMAG for it.  The normal AR mag-well is too narrow and the plastic ends up too thin to make a PMAG for 6.8.  The new mag also allows a bit more cartridge overall length (COAL).

That's all good news, really.  I'm too cheap to convert though unless the LWRC magazine becomes the only standard for 6.8 and people stop making parts for standard ARs.  I don't think that will happen any time soon.

What I want to talk about is the comments to TFB article.

Almost immediately it turned into Grendel or .300 BLK is better or 6.8 was designed as a CQB round.

I'll start with that second one.  No.  How do we know?  The designer is still around, we can ask him.  6.8 was intended to outrange the 7.62x39mm and deliver better wound ballistics than 5.56 (both M855 and Mk.262) from a 14.5" barrel AND involve the fewest changes possible to a standard M4.  By the way, better performance at 300 yards also means better performance at CQB.  The weapons and ammo as delivered failed to do so.  The chambers were cut wrong and the rifling was spec'd for the wrong velocity plus the powder used degraded significantly when exposed to the heat in the area where it was deployed.

This cemented the performance of the round in the minds of many.

However, it is no longer 2004.

The hobbyists got involved with it and began experimenting.  The initial goal of the experiment was to get the round to perform as documented by the people who initially developed it.

The first attempts involved the time honored "more powder!" to get the velocity back up to 2,500 fps from the 2,300 with factory ammo.  This caused chamber pressure to be alarmingly, perhaps dangerously, high.

Remember when I said the original designer was still around to talk to?  The hobbyists talked to him.  They compared his notes to the SAAMI chamber specs and found a discrepancy.  The leade was too short on the SAAMI drawing.  Extending the leade lowered chamber pressure significantly and upped the velocity, but not as much as hoped.

Next the hobbyists noted that the 1:9 rifling specified by SAAMI was settled on by default because that's what .270 Winchester uses and it shares the same 0.277" diameter bullets.  The big difference is 6.8 uses the lightest bullets made for .277 and .270 Win uses bullets that cannot be practically used in an AR platform because of length.  .270 Win also pushes the lighter rounds 1,000 fps faster.  The hobbyists noted that with the slower velocities and lighter rounds than .270 Win they could probably get away with slower rifling.  So they had some .277 barrels made with 1:10 and 1:11 twists and experimented with different cuts for the lands and grooves, even some polygonal rifling.

What they got was a dramatic increase in velocity with a dramatic drop in chamber pressures, to well below normal 5.56 in fact.

Now, is it better than 6.5 Grendel?  That depends.  In general they're very hard to tell apart practically.  What length barrel do you want?  If you want a 24" barrel 6.5 is clearly superior, especially past 500 yards.  Bill Alexander designed it for longer barrels and it really shines there.  Where it starts looking poor compared to 6.8 is at the 16" barrel length that's the most popular in the AR world.  This is because of case design and powder selection.  Short, fat cases like the 6.5 just tend to do poorly at getting velocity out of shorter barrels.  To be fair, 6.5 still stomps 5.56 out of a 16" barrel; it just doesn't enjoy a clear superiority to 6.8 here.

Is .300 AAC Blackout better than 6.8?  Here I am going to say no.  The hottest .300 is ballistically 7.62x39 in a form that feeds well through an AR mag-well.  Identical to a round that 6.8 was specifically designed to beat at all ranges (and thanks to hobbyist meets that design goal).  What .300 has going for it is it doesn't need a special bolt or magazine (except some loads will hit the front rib of the magazine).