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65G HLB Project - BluntForceTrauma - 05-27-2015
Gentlemen, Barbarians, and Members of The Horde:
I want to conduct an experiment and I need your help. I want to specify and test a 6.5 Grendel barrel to see if there's any way to increase velocities while keeping pressures within SAAMI 6.5 Grendel guidelines.
The 6.8 SPC II camp, for example, claims that this can be done in their cartridge with similar case capacity to the 6.5 Grendel, and after giving it some thought, I think it's at least worth exploring. There is no magic pixie dust that can be sprinkled on a cartridge to give it more velocity; either there are universal principles of physics and ballistics and metallurgy that can be applied to any cartridge or there is only marketing hype, wishful thinking, and gambling against the odds.
The 6.5 Grendel was founded on innovation and I want to see if there's room for more. For example, in their .50 Beowulf and 6.5 Grendel bolts, Alexander Arms pioneered the use of 9310 steel, something we hear quite a bit about these days. Whodathunk? And, of course, they were one of the first, if not THE first firearms manufacturer to specify a chamber with a compound, dual-angled throat.
But let's not stop there, let's see what else we can do.
And thus we have the 6.5 Grendel Handloader's Barrel (HLB) Project. I say "handloader's" because, even if successful, a barrel with the specifications I am about to discuss will only be useful to those handloaders who understand the risks inherent in maximizing its full potential.
There is much, much more to say in this thread, and I need some heavy-duty, rocket-scientist, ballistics theory-type contributions, but let's just cut to the chase:
Typical 6.5mm barrels are first bored with a minor diameter hole of 0.256". Then metal is removed from the bore creating the grooves (the major diameter) measuring 0.264" while leaving the lands at the bore diameter of 0.256".
The difference between the major and minor diameters is 0.008". Dividing that by two gives us a typical land height of 0.004". Note that the typical land height for .223 barrels is 0.025" and for 6mm barrels is 0.003". Are they driven any less hard than the 6.5s? Than the 6.5 Grendel? I want to test a 65G barrel with a land height of 0.003", which means I need a 6.5mm barrel first bored to 0.258".
Whatever force a 0.004" land imparts to engraving the bullet and thus to pressures is effectively reduced 25% by decreasing to 0.003". Right? My math doing OK?
Now the force may, indeed, be minimal relative to the whole pressure calculus, but whatever the lands contribution may be, it is reduced by 25%.So far, so good?
Next, a typical 6.5mm barrel (also, specifically, the 6.5 Grendel SAAMI minimum specification) has six lands. Let's take away three of them. (AccurateShooter.Com reports: "P. O. Ackley believed that, all things considered, a properly-made 3-groove barrel is optimal for most common calibers.") Anyway, we've now made a further 50% reduction in engraving force. So whatever force the rifling imparted to the pressure calculus has now been reduced 50% plus the previous 25% for a total 75% reduction.
Of course, I'm gonna need 5R-style lands, with something like a 45° ramp on each side. Shall we call it "3R" rifling?
My question: You guys know anyone who can make this experimental barrel? Don't just say, "Uh, these guys kinda make barrels . . . I think." I need, "Yes. This guy can do exactly what you want and here's his website."
Finally, we need input. We need to tear this apart from every angle. We already know that reducing the size of the lands just makes the rifling erode faster. If the bullets are launched too hot and heavy they could skid and not engage properly on less-aggressive rifling. Any other problems foreseen? Don't be afraid to play devil's advocate, but expect to be respectfully cross-examined. Let's wring this idea out. Don't want to waste cold, hard cash on a non-starter, hare-brained idea.
65G HLB Project - LRRPF52 - 05-27-2015
Bartlein would be where I would turn to, but what advantages are we going to see in practical terms?
50fps more? That is almost meaningless with a 6.5mm. We're looking at .5 Mils different at 1000yds. 100fps will give you a full Mil flatter at 1000yds.
The other thing we look at is start pressure and powder selection. A micro-rifled barrel will do better with faster powders.
The problem this has created with the 6.8 is that it is so deviant from the SAAMI, that hand-loaders are crying about how low the performance is, when they have special barrels that use every little tweak to wring out more muzzle velocity, since .5 or higher BC isn't going to happen with that ogive length, not even with the 130gr Berger Classic.
While the fringe spins their wheels looking for performance, I continue to shoot affordable factory box ammo that exceeds even the tweaked performance of the competition, with lower pressures, and no need for special PRI mags that cost $46 per, etc.
Sure we could spec out a 3-groove 5R .004" grooved barrel and hand load for it to make up for the lack of start pressure. Nitride it to increase the throat and bore life.
I had a GAP-built AR10 with Obermeyer 5R rifled barrel at 22" that would shoot the same speed as 24" guns using the same load, but we can see variation of 100fps from barrel to barrel of the same length.
It would be an interesting experiment, but a one-off wouldn't give much resolution.
65G HLB Project - BluntForceTrauma - 05-27-2015
I wonder what role the half-degree angle of the first 0.154" of the 65G SAAMI throat plays in this scenario? What if the "kiss," or slight pressure, from that angle keeps the start pressure up, but then the reduced engraving from micro-rifling allows the velocity to expand unhindered? Best of both worlds?
P.S. I know I didn't address your basic point: Are any velocity gains even worth the trouble? I'll have to run some JBM charts. . . .
65G HLB Project - LRRPF52 - 05-27-2015
It averages 300psi increase from what Bill said, which is pretty negligible.
65G HLB Project - pds - 05-27-2015
Lilja makes a three land/groove barrel but the lands appear to be twice the width as a six land barrel. I've often wondered why the lands in a three groove barrel can't be the same width as a six groove barrel. I would think three narrow lands would reduce bullet fiction a fair amount and produce high velocities. Just a thought.
pds
65G HLB Project - BluntForceTrauma - 05-27-2015
Agree, PDS. I'd make them the same width as a six-groove. Downside is that smaller lands wear out or erode faster.
65G HLB Project - pds - 05-27-2015
BluntForceTrauma Wrote:Agree, PDS. I'd make them the same width as a six-groove. Downside is that smaller lands wear out or erode faster.
Shouldn't wear out any faster than a six groove barrel plus the Grendel is pretty easy on barrels.
pds
65G HLB Project - kmon - 05-28-2015
Gain twist? That does speed things up a little. 3 groove radiused lands shorter might get some more.
Boots Obermeyer has experimented in some of those directions with barrels in the past as well as a slightly undersized bore diameter barrel.
65G HLB Project - BluntForceTrauma - 05-28-2015
And? What did Boots conclude?
65G HLB Project - ricsmall - 05-28-2015
Three lands the same width as a six land barrel won't give you the same velocity cause you're gonna have less bearing surface, less pressure, less fps. That's why BHW has the polygonal rifling with wider lands, and IIRC, they(BHW) have higher,on average, fps. It's late and I've had a few brewskies, so correct me if I'm wrong.
Richard
65G HLB Project - am4966 - 05-28-2015
Broughton 5C barrels already claim to have increased velocity over other barrels. So couldn't you get a .264 blank from them. Then have extension added head spaced and gas port drilled?
65G HLB Project - Predmilitia - 05-28-2015
Following with eyebrows arched. Interesting to say the least!
65G HLB Project - BluntForceTrauma - 05-28-2015
ricsmall Wrote:Three lands the same width as a six land barrel won't give you the same velocity cause you're gonna have less bearing surface, less pressure, less fps.
Richard, yes. As Bill Alexander reminded me: "Pressure = Velocity. For a given case size, bullet, and barrel length you need to increase pressure or increase the duration to get more velocity. If you are close to maximum loading density, driving the pressure does not help as you cannot go to a more progressive powder and you cannot add to bring the pressure back up."
Others who have tried this technique claim that even though pressure drops unacceptably after one decreases the engraving force, one then must add back in and this is where we are strictly in the realm of hand loading more powder and yet faster powder to get back up to the original pressure. However, as I understand it, the point of that exercise is to replace with faster powders at the original pressure, and therein is claimed the "free" velocity gains. Correct me if I'm wrong, cuz I'm thinking out loud here. . . .
65G HLB Project - BluntForceTrauma - 05-28-2015
am4966 Wrote:Broughton 5C barrels already claim to have increased velocity over other barrels.
Did they attempt to educate us by what principle of physics these increases are gained? What case did they make to back up their claims?
As LR pointed out, on any given day any given barrel from the same batch from the same manufacturer can vary by 100 fps. They pointed this out, as well, on AccurateShooter.com some time back.
65G HLB Project - PredatorDown - 05-28-2015
Taking a step away from looking strictly at the barrel, has anyone here tried coating their bullets in Hexagonal Boron Nitride? My (very limited) understanding is that it reduces friction, thus lowers pressures (also slightly velocity with the same powder charge), which allows for more and/or a faster burning powder to be used. While maybe not as potentially innovative as rethinking the barrel, could similar expectations be achieved?
65G HLB Project - BluntForceTrauma - 05-28-2015
PD, I recently read an article where the Army tested this, but had an unexpected result. If memory serves, the coating would shave off and then build up — ironically increasing the pressures!
65G HLB Project - BluntForceTrauma - 05-28-2015
OK, was pondering this during morning commute: A smooth-bore barrel. What happens if you fire a factory 6.5 Grendel round in a smooth-bore barrel, having zero rifling?
In that case, what produces the pressure? Burn rate of powder. Neck tension. Bullet diameter interacting with barrel major diameter.
And then we add back in the rifling to spin stabilize the bullet. Seems to me that rifling — in all our guns — is a necessary evil, interfering with the "pure" expansion of burning gas and the bullet's trip down the barrel.
So it is NOT intuitive, to me, that the rifling is needed to actually add pressure to the system to produce the velocity. That should already have been taken care of with the neck tension.
65G HLB Project - stanc - 05-28-2015
BluntForceTrauma Wrote:Seems to me that rifling in all our guns is a necessary evil, interfering with the "pure" expansion of burning gas and the bullet's trip down the barrel.Sounds logical to me, John. The 120mm main gun of the Abrams tank is smoothbore, and achieves beaucoup velocity.
So it is NOT intuitive, to me, that the rifling is needed to actually add pressure to the system to produce the velocity.
You could test the idea by having a smoothbore 6.5mm barrel made...
65G HLB Project - BluntForceTrauma - 05-28-2015
stanc Wrote:You could test the idea by having a smoothbore 6.5mm barrel made...
One would think that someone in the past 50 years has actually tested this — in some caliber. Then, again, I wouldn't be surprised that such basic research is overlooked.
65G HLB Project - JASmith - 05-28-2015
I have been told that the polygonal barrel can get mildly increased velocities without the accompanying increase in peak pressure.
The idea is that the polygonal rifling decreases the engraving force which slows the initial pressure buildup. The bullet to bore seal is said to be better with the polygonal rifling compared to the relatively sharp transitions from land to groove and vice versa. The notion is that a better gas seal keeps more gas available for doing work on the bullet so the pressure does not fall off as quickly.
The net result is that powders appear to burn a bit slower, which allows a bit more charge, further increasing the effect.
NugginFutz may have more on this.
In my view, the case for this set of effects is yet to be made in a technically substantiated way, but I am hoping one of us has links to a definitive and experimentally supported paper.