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65G HLB Project - pds - 05-29-2015
BFT,
This may be of interest http://closefocusresearch.com/calculating-barrel-pressure-and-projectile-velocity-gun-systems, but then again maybe not :-)
pds
65G HLB Project - Joseph5 - 05-29-2015
I am not 100% certain on this but I know it is close. In the Abrams smooth bore cannon they are burning by weight twice as much powder than projectile. The fin stabilized discarding sabot penetrator round projectile weighs 25 pounds and the shell has almost 50 pounds of powder in it. the closest example I can think of in firearms was the Remington 30-06 sabot round that fired a 55 grain bullet 4000 fps but they probably didn't even have an equivalent weight of powder in it let alone double the projo weight. Joseph
65G HLB Project - ricsmall - 05-29-2015
stanc Wrote:Sounds logical to me, John. The 120mm main gun of the Abrams tank is smoothbore, and achieves beaucoup velocity.
You could test the idea by having a smoothbore 6.5mm barrel made...
It uses the sabot to build pressure. Safely. A smoothbore grr would build too much too fast. IMO.
Richard
65G HLB Project - BluntForceTrauma - 05-29-2015
PDS, thanks for the link. Dug around in there and read some other articles, as well.
65G HLB Project - stanc - 05-29-2015
Joseph5 -- APFSDS is not the only type ammo used in the Abrams 120mm gun. There are also full-caliber rounds, for instance:
![[Image: ORD_M830_HEAT_120mm_lg.jpg]](http://media.defenceindustrydaily.com/images/ORD_M830_HEAT_120mm_lg.jpg)
And FYI, M829A3 total cartridge weight is 49 lbs, with the DU penetrator at 22 lbs, and 18 lbs of propellant, according to http://www.defense-update.com/products/digits/120ke.htm .
That makes the ratio of propellant to projectile weight less than 1:1, not that it matters. My point was not to suggest that 120mm muzzle velocities could be equaled, but simply to show that high velocities are possible in a smoothbore. What could be achieved in a 6.5 Grendel smoothbore (vs rifled bore) is anybody's guess.
65G HLB Project - bwaites - 05-29-2015
Regardless, you won't see significant increases. Maybe.....5%. Not 20%. At best, 10%. Not more, no way. Not enough powder capacity, and in most cases, not enough barrel length to slow powder burn speed enough to allow a significantly slower powder and thus longer acceleration time. I have one of the few 28" barrels, and I have a hard time being 200 FPS faster with it than my 16", under best circumstances with both, using the best loads shooting same weights. You are simply not going to find some magical barrel/throat combination that will override the limitations of the powder capacity.
65G HLB Project - BluntForceTrauma - 05-29-2015
I first want to understand the theory of rifling as it applies to chamber pressures.
Let's start with a hypothetical smoothbore 6.5 Grendel 16" barrel, whose bore is exactly 0.264", with zero rifling.
I load and fire a factory Hornady 6.5 Grendel 123 Amax round and get 2500 fps. Remember: Smooth bore, zero rifling.
Now I magically add SAAMI six-groove 6.5 Grendel rifling to that barrel and fire a second round.
Is the velocity going to go up or down, and why?
65G HLB Project - BluntForceTrauma - 05-29-2015
bwaites Wrote:Regardless, you won't see significant increases. Maybe.....5%. Not 20%. At best, 10%. Not more, no way.
And this may well prove the case. LR brought up a similar issue, asking if any gains are worth the effort. It's probably hard to get a consensus on what is "significant," but I'd like to hear input.
If we start with a base velocity of 2500 fps and increase it by 1%, we've added 25 fps.
By 5%, we add 125 fps. Adding 10% gives us 250 fps. You get the picture.
So is 100 fps "significant"? Is 250 fps "worth it"? What say the Horde?
65G HLB Project - ricsmall - 05-29-2015
John, my thinking may be wrong, but I would think adding rifling would decrease the pressure somewhat like a bullet with pressure bands cut into it. On the other hand the pressure may increase some due to the twist adding resistance to the projo as it travels the bore. This would be an interesting experiment to say the least. This is all theory until someone tries it, and I think we all present good points, but I'm kinda back pedaling on my comment last night about building more pressure in a smoothbore, since reading ur last two posts. Hard to say. Any takers on the smoothbore trials?
Richard
65G HLB Project - cory - 05-29-2015
There's a time and place for everything, and to often talking just gets in the way of progress.
I'd suggest getting the smooth bore and determining the actual difference. Then will be a good time to talk about whether or not the gains are worth pushing forward with this concept.
I'll add that to get good empirical results you'll need to get a SAAMI Barrel from the same lot of barrel blanks to compare the velocities with.
I'd contact Lilja, Bartlein, Krieger, Wilson, JP, Shilen, etc... and see if any of them are interested in working with you on this.
65G HLB Project - JASmith - 05-29-2015
If the ID of the smoothbore is .256 and then one cuts the rifling, the peak pressure is likely to go down, along with a likely mild decrease in velocity.
If the ID of the smoothbore is 0.264 and then one installs a fresh barrel, then the peak pressure will go up a bit with a mild increase in velocity.
In the first, case, the bullet has to be swaged down to go from the groove diameter of 0.264" to the bore diameter of 0.256" which makes the initial pressures higher.
In the second case, no plastic work is done on the bullet so the initial burn is at very low pressures but we still have a good gas seal. I don't know whether the difference between a perfect seal and the arguably imperfect seal one gets with conventional rifling is enough to make a difference though.
What would be really interesting is to extend the polygonal bore idea to one where we have rounded ridges and valleys with perhaps .003" between the diameter of the peaks and valleys with most of the circumference close to 0.264". This would have minimal plastic work done to the bullet and likely keep the better gas seal claimed by the proponents of polygonal bores.
That scalloped barrel would likely accommodate larger powders with burn rates slightly faster than the current optimum powders too.
Now that I've done the 'invention description' I seem to recall having seen a scalloped bore before but don't remember where!
65G HLB Project - BluntForceTrauma - 05-29-2015
However, Cory, I'm wanting discussion because I don't want to waste money reinventing the wheel. I can't believe this experiment hasn't been done in the past 50 years?
Richard, either adding rifling will increase velocity because you've constricted the bore and introduced more pressure. And Pressure = Velocity.
Or rifling is such a small amount of metal relative to the whole bore that the "needle" hardly moves, or it adds a bit of engraving force friction and reduces the velocity. Consider that Bryan Litz's twist rate experiment showed that one inch of twist rate only accounted for 1.33 fps average difference. So maybe rifling is hardly a factor?
Has nobody in the gun business rigorously quantified this? I'd be dumbfounded but not surprised, actually. You'd be amazed at what passes for "conventional wisdom" in the gun shops of the world.
65G HLB Project - BluntForceTrauma - 05-29-2015
JASmith Wrote:Now that I've done the 'invention description' I seem to recall having seen a scalloped bore before but don't remember where!
Yeah, I was just gonna say I've also seen that kind of rifling.
I was reading last night on some archived Sniper Country thread or some such and Frank Green of Bartlein Barrels commented that he'd seen every possible number of lands and grooves and rifling profiles and none of it really made any difference — maybe gained 25 fps with 5R rifling, well within the "noise value" — relative to a barrel simply manufactured to consistent tolerances end-to-end.
I'm still stumbling toward an answer to what decreasing the rifling bore-area aspect ratio by 75%, as described in my opening post, gives us in terms of velocity gains — or loss!
65G HLB Project - JASmith - 05-29-2015
Let's use the 120 gr GMX with a baseline velocity of 2500 fps and 500 yards for start of 'interesting target shooting' distance and 'upper-end' hunting range.
We can ignore drop because so many of us have those fancy-dandy rangefinders and palm-size mega-computers to get the number of clicks to put the bullet in the money.
That leaves wind drift for the hunter and target shooter and range to threshold velocity reliable bullet expansion. I used the 120 GMX to illustrate.
JBM gives us:
[INDENT]
range to 1800 fps 2500 2625 2750
10 mph drift (in) 22.4 20.8 19.4
range to 2100 fps 225 290 355
[/INDENT]
These numbers might suggest that even a 5% increase in muzzle velocity can be of interest to serious target shooters. The 65 yard increase in range to expansion threshold can be useful to hunters as more and more of us find ourselves in lead-free regions.
65G HLB Project - BluntForceTrauma - 05-29-2015
I ran some numbers in the JBM Ballistics Calculator. From the drop-down menu I chose Hornady's 6.5mm 123 A-Max. I input 2500 fps and a 200-yard zero. I left everything else default, including altitude at 0.0 ft. (Oh, yes, I also set target speed to 0.0 mph!)
Starting at 2500 fps, I ran the same chart five times, increasing the velocity by 50 fps each time until I reached 2700 fps, an increase of 200 fps.
You guys can easily do these five charts, will take you a total of 30 seconds. Anyway, I'll summarize:
At 2500 fps the drop at 300 yards is 9.9 inches and at 2700 is 8.3 inches. A difference of 1.6 inches.
2500 wind drift at 300 is 7.5" and 2700 is 6.6". Difference of 0.9".
That's all we get for an additional 200 fps?
2500 drop at 600 is 98.0" and 2700 is 82.3". Difference of 15.7".
(By the way, this JBM calculation seems strange: At 500 the drop is 57.3" and then it falls off a cliff to almost double at 98.0" by 600?)
2500 wind drift at 600 is 33.5" and 2700 is 29.8". Difference of 3.7".
65G HLB Project - BluntForceTrauma - 05-29-2015
Since I was on a roll with the JBM, I thought I'd compare the 6.5 Grendel to the 6.5 Creedmoor. Conveniently, Hornady offers factory loads in both and publishes their official numbers on their website.
Hornady's 6.5 Creedmoor in a 24" barrel gets 2910 fps with a 120 A-Max with a BC of 0.465. (Uh, oh!
)Hornady's 6.5 Grendel in a 24" barrel gets 2580 fps with a 123 A-Max with a BC of 0.510. (<fist pump> Yesss!!)
Note the 330 fps difference in velocity.
65G drop at 300 yards is 9.2 inches and 65CM is 7.2 inches. Difference of 2.0 inches.
65G wind drift at 300 yards is 7.1" and 65CM is 6.6". Difference of only 0.5". Yes, that's right: One. Half. Inch.
65G drop at 600 yards is 91.3" and 65CM is 72.6". Difference of 18.7".
65G drift at 600 yards is 31.9" and 65CM is 29.8". Difference of 2.1".
Not totally apples-to-apples because it's a 123 A-Max vs. a 120 A-Max, but it IS factory load vs. factory load.
I get more proud of the 6.5 Grendel every day. It is growing up to be a fine child — if you ignore the barbarism and pillaging.
65G HLB Project - JASmith - 05-29-2015
Now you know the other reason I don't look at drop figures!
65G HLB Project - cory - 05-29-2015
BluntForceTrauma Wrote:However, Cory, I'm wanting discussion because I don't want to waste money reinventing the wheel. I can't believe this experiment hasn't been done in the past 50 years?
Richard, either adding rifling will increase velocity because you've constricted the bore and introduced more pressure. And Pressure = Velocity.
Or rifling is such a small amount of metal relative to the whole bore that the "needle" hardly moves, or it adds a bit of engraving force friction and reduces the velocity. Consider that Bryan Litz's twist rate experiment showed that one inch of twist rate only accounted for 1.33 fps average difference. So maybe rifling is hardly a factor?
Has nobody in the gun business rigorously quantified this? I'd be dumbfounded but not surprised, actually. You'd be amazed at what passes for "conventional wisdom" in the gun shops of the world.
Well you see I don't mind you wasting money. BAHAHA I'm not saying that starting this discussion was a bad idea. I've been following since the beginning and it seems to me that you're not going to find anyone here with first or second hand experience on this type of experiment. I think you're going to find better input from the barrel makers when you go searching for someone to build your pipe. At least that's what I hope will happen, and hopefully you'll post the information you find, back here. haha
65G HLB Project - BluntForceTrauma - 05-29-2015
I went to Bartlein's website and they're backordered 5 to 6 months, so I figure a couple days discussion won't hurt none.
That's the trouble with you damn Marines, "Let's git 'er done! Ooorah!" All action and no wastin' time!
65G HLB Project - JASmith - 05-29-2015
God help us when the USMC becomes totally dependent on the Army for small arms.
Of course, the Navy ain't much better...