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Jason_G
May 23, 2007, 11:22 AM
Any secrets to expediate the matching of a good bullet weight to a particular rifling? I've always assumed that the faster the twist, the bigger the bullet needs to be, but is there a way to find the perfect match without going through alot of ammo (read: money ;) ), in a "trial and error" fashion?

Jason

mikejonestkd
May 23, 2007, 11:26 AM
Here's some reading material:

http://www.riflebarrels.com/products/caliber_twist_rates.htm

http://www.chuckhawks.com/rifle_twist_list.htm

http://www.eabco.com/TwistRate.htm

http://www.gunnersden.com/index.htm.rifle-barrel-twist-rates.html

http://www.chuckhawks.com/rifle_barrel.htm

http://www.accuratereloading.com/twist.html

hope this helps.

Red Tornado
May 23, 2007, 11:32 AM
I think in general, the closer you get to the maximum weight bullet for your twist rate, the more likely you are to achieve great accuracy. I'm sure there are exceptions, but it did seem to hold true for my .223. Other calibers I can't offer opinions. (Well, I could, but that would require snapping into keyboard commando mode. :p )

I'm sure the answer is somewhere in the list Mike left you.
RT

Jason_G
May 23, 2007, 11:50 AM
Thanks for the links, mikejonestkd. It looks from the chart on the first link that the 200 gr. .308 would be the best bet out of a 1:11. I'm thinking about getting a Springfield M1A, and was curious about which weight ammo would be best out of it. Thanks.

Jason

mikejonestkd
May 23, 2007, 11:58 AM
Some of the more knowledgeable M1A owners here will probably chime in but the 200 gr bullets are not ideal for any .308 win. You are probably better off sticking with 150-168 grainers for a .308 and IIRC the MIA is set up for 168 grainers, or was it 173 grainers....dang alzheimers....

here we go thanks to wikipedia on the 7.62 x 51 .308 win

Cartridge, Caliber 7.62mm, NATO, Ball, M59 (United States): 150.5-grain 7.62x51mm NATO ball cartridge. A further development of the initial T65 cartridge.
Cartridge, Caliber 7.62mm, NATO, Armor Piercing, M61 (United States): 150.5-grain 7.62x51mm NATO armor piercing round, black cartridge tip.
Cartridge, Caliber 7.62mm, NATO, Tracer, M62 (United States): 142-grain tracer cartridge, orange cartridge tip.
Cartridge, Caliber 7.62mm, NATO, Grenade, M64 (United States): 7.62x51mm NATO grenade launching blank.
Cartridge, Caliber 7.62mm, NATO, Ball, M80 (United States): 146-grain 7.62x51mm NATO ball cartridge.
Cartridge, Caliber 7.62mm, NATO, Match, M118 (United States): 173-grain 7.62x51mm NATO Full Metal Jacket Boat Tail round specifically designed for Match purposes. Introduced in 1968 as XM118, standardized in 1970 as M118. Produced at Lake City Arsenal.
Cartridge, Caliber 7.62mm, NATO, Ball, Special, M118 (United States): 173-grain 7.62x51mm NATO Full Metal Jacket Boat Tail round specifically designed for match purposes. Produced by Lake City Arsenal. This is a interim match round which utilized M80 ball brass with the 173-grain FMJBT bullet. During this period in the early to late 80's the performance of the round declined. Powder, primer, brass, bullets were no longer produced in matching lots.
Cartridge, Caliber 7.62mm, NATO, Ball, Special, M118LR (United States): 175-grain 7.62x51mm NATO Hollow Point Boat Tail round specifically designed for long-range sniping. Most commonly made by Lake City and American Ballistics.
Cartridge, Caliber 7.62mm, NATO, Duplex, M198 (United States): 7.62x51mm NATO duplex round with two 84-grain bullets. The developmental designation was T314E3.
Cartridge, Caliber 7.62mm, NATO, Tracer, M276 (United States): 7.62x51mm NATO so-called "Dim Tracer" with reduced effect primarily for use with night vision devices, green cartridge tip with pink ring.
Cartridge, Caliber 7.62mm, NATO, Match, M852 (United States): 168-grain 7.62x51mm NATO Hollow-Point Boat-Tail cartridge, specifically designed for use in National Match competitions, later approved by US Army JAG for combat use by snipers.
Cartridge, Caliber 7.62mm, NATO, Saboted Light Armor Penetrator, M948 (United States): 7.62x51mm NATO Saboted Light Armor Penetrator cartridge.
Cartridge, Caliber 7.62mm, NATO, Armor Piercing, M993 (United States): 126.6-grain 7.62x51mm NATO armor piercing round, black cartridge tip.

Jason_G
May 23, 2007, 01:00 PM
Some of the more knowledgeable M1A owners here will probably chime in but the 200 gr bullets are not ideal for any .308 win. You are probably better off sticking with 150-168 grainers for a .308 and IIRC the MIA is set up for 168 grainers, or was it 173 grainers....dang alzheimers....
Ok. That makes my assumption incorrect then. The 2nd chart in the first link basically states that 150 to 200 is suitable for .308/7.62mm out of 1/11. I thought it was best to pick the heaviest bullet within this range for maximum accuracy, but I guess that's not the case. Maybe the rifling is too slow for that weight. The 168 or 173 grainers would be sort of right in the middle of that range and would probably be a good starting point for trial and error if it comes down to that. I guess I could start in that weight range and look for keyholing, but I wouldn't know whether to go up or down in weight if there is instability :confused: .

Jason

mc223
May 23, 2007, 01:18 PM
http://www.netrifle.com/shortmags/ref_data/TwistRateCalc.asp

Jim Watson
May 23, 2007, 01:23 PM
The 11 twist M1A barrel was originally meant for target shooting with 168 gr commercial bullets like Sierra Matchking or the old 173 gr military match FMJ boattail.

Yes, an 11 twist will stabilize a heavier bullet but that rifle, barrel, and bullet package are known successful.

Magnum Wheel Man
May 23, 2007, 02:24 PM
without delving into the links... I was always taught that the longer the bullet ( & thus normally the heavier ), the more twist was needed to stabilize the bullet...

I don't think the weight ( in of it'self ) is relavant... but of course if you are talking the same bullet material, & the same bore diameter, & bullet design, then a 180 grain 30 cal bullet is going to be longer than a 120 grain 30 cal bullet

if you start getting into different bullet materials ( comparing a solid copper & a copper jacketed lead bullet of same bullet style for example )... the molecular weights of the copper & lead are different, with the copper being lighter, & thus weight for weight, the copper bullet would be longer, & need more rifling twist to reach best stabilization, when compared to the lead cored bullet...

...again, my understanding, that there is a range of bullet rotation that is optimum, & any deviation faster or slower than that, results in increased bullet unstability, the further from the optimum twist, that the twist rate is changed...

... I would have thought that the bullets velocity would also have to be calculated into the equasion... IMO...the developement of the whisper series of cartridges would be some interesting reading.

FirstFreedom
May 23, 2007, 02:44 PM
The answer to the question is NO, you cannot find the *perfect* match without a lot of time / effort / money spent to find the best load that works best in your particular rifle.

But you CAN get close, and find out a starting point, and likely the general *range* of bullet weights that increase your chances of finding that perfect match. And it's not just bullet weight - it's the weight & length/shape of the bullet as well as how much of the bullet's length is the full diameter's width, making contact with the rifling...e.g. a boattail bullet needs more twist rate to stabilize than a non-boattail (flat-based) of the same weight, because it's longer and less bullet surface area is contacting the rifling.

Scorch
May 23, 2007, 03:19 PM
I disagree in principle with FF, but agree in all practicality. You can calculate the ideal twist rate for a barrel/bullet combination, and if you have enough money, you can match it perfectly. Benchrest shooters do this routinely, as do some firearm manufacturers, but most people choose a twist to stabilize the heaviest common bullet weight for that particular caliber. For example, for a .224 caliber, 52 gr bullet, the ideal twist rate would be about 1:14.5" or so. Since this is not readily available, people use the next readily available twist rate, 1:14", for most applications requiring that particular bullet to be stabilized. You could pay for a barrel to be made with the exact twist you want, but it becomes cost prohibitive.

For a twist rate calculator, go to
http://www.z-hat.com/twistrate.htm

FirstFreedom
May 23, 2007, 04:20 PM
I disagree in principle with FF, but agree in all practicality. You can calculate the ideal twist rate for a barrel/bullet combination, and if you have enough money, you can match it perfectly. Benchrest shooters do this routinely, as do some firearm manufacturers

Scorch thanks - well, ok, you're probably right...to an extent...since benchrest shooters l almost uniformly use long, pointed, boattail bullets, then whatever charts they use can be tailor-made to the corresponding bullet weight, since the other wild card factor, shape, is held constant. I would like to see those charts, however - do you have a link? In any event, when the wild card of bullet shape is NOT held constant, then you can't merely correlate a perfect twist to a perfect weight, without accounting for the wild card factor somehow...but you can get pretty close with most small and mid-bore-sized calibers. Flat-nose and/or flat-based bullets, or even pointed bullets with a very short, round ogive are gonna need less twist to stabilize, even at the same weight. This is why frontstuffers that use very heavy (round balls, or tin-can shaped) bullets over 300 or 400 grains, still successfully stabilize the bullet even with very very slow twist rates like 28, 32, 40, 48, 50, etc. So no chart is gonna tell you that - a sophisticated *software program*, maybe, but not a mere chart (as I say, unless the chart applies to uniform bullet shapes - all the the long slender ones).

OK, now I see your link to the twist rate calculator program - that's about what I thought.... lemme study that and get back with you. But the bullet shape is terribly important as a general matter when you've got frontstuffers stabilizing 500 grain bullets at low speeds with 1 in 40 twists.

I'll give you another example of what I'm talking about.

With .25 cal rounds (.25-'06, .257 robts etc), with *ordinary* spitzer-type pointed bullets, it is generally accepted that 1 in 10 will stabilize up to 117 grainers, and sometimes 120 grainers. But with Lost River, high-BC bullets, with boattails and very long tapering noses, Lost River will tell you that you need a 1 in 10" for their 90 grain bullet, yet you need a 1 in 9" (not common) for their 100 grainer (still 20 grains less than what's normally stabilized with 1 in 10"). Bullet shape. It's the length-to width, or length-to-weight ratios (or an amalgam of both).

Then of course there's ALSO bullet velocity to contend with, and how that affects, and is counter-affected by, everything else. I'm not entirely clear on how speed interacts as a factor, but I'm under the impression that it's *relatively* unimportant (weighted quite lightly) in the overall formula.

And of course, let's not forget that physics aside, even if you could get the exact twist rate for the bullet you wanted to shoot, in a custom barrel, there's still that Je ne se qua (sp?) of little variances of one gun to another; one barrel to another - you never know (in my limited experience anyway), exactly what your particular rifle is going to just happen to "like".

Jim Watson
May 23, 2007, 04:34 PM
Hmm. Most of the benchrest stuff in my Sinclair catalog is for 14 twist 6mm barrels and 68 gr flatbase bullets. That is for standard 100-200-300 yard group benchrest. The thousand yard benchresters have to go with the high BC bullets as used for bullseyes.

If you are calculating rifling twist based on the Greenhill Formula, you are way behind the curve, it is based on velocities and bullet shapes not often used now. And fudging the constant does not help much.

I think rifling twist is overanalyzed these days. People think they can get great accuracy by ordering just the right twist so they don't have to pay attention to ammunition quality. I recall what P.O. Ackley had to say about that once. "I rebored a rifle to .35 Whelen and the guy said that I had ruined his barrel by using a 14 twist instead of 15. Which must make Winchester wrong because they use 12."

Scorch
May 23, 2007, 05:10 PM
It's the length-to width, or length-to-weight ratios (or an amalgam of both
Actually,I think it's the length in calibers that matters. So a .5o cal muzzleloader shoots a bullet that is 1-2 calibers long (.5"-1"), whereas a .50 BMG shoots a 720 gr bullet that is 4 calibers long (2"), so it needs a tighter twist. Also, short, stubby profile bullets, like pistols use, are easier to stabilize than long needle-shaped spitzer boat-tails, so they go with a slower twist. Not that a faster twist wouldn't stabilize it also, but tighter twist raises the pressure needed to propel the bullet down the bore.

JohnKSa
May 23, 2007, 10:31 PM
I think rifling twist is overanalyzed these days.Yup.

jlmurphy
May 23, 2007, 10:41 PM
Back to the original thread, he wanted to find a good weight for an M1A. Heavy bullets are probably better for a heavy benchrest rifle, but the M1A is a relatively thin barreled service rifle, and the powders used for heavy bullets are too slow for the gas system on that rifle, and the weight of the rifle makes extra recoil a problem.