General Comment - Accuracy vs Caliber

[Splais]

I'm having a bit of a frazzle period. I've been *icking around with a 45 caliber flintlock (42" GM barrel) for a few weeks now, off and on, trying to develop a consistent load that generally goes were I point it. Been having a terrible time, and just when I think I'm getting close everything goes cafluey.

I also have a 36 caliber percussion (44" barrel maker unknown) built 30 yrs ago or so by Robert Ashe that seems to shoot quarter size groups at 50 yds no matter what I put through it. Problem is it has a 44" barrel and weighs 9.5 #'s. It's a bear to shoot off hand.

So what goes? PS: I know this is a rediculous question; but now I feel a lot better.

 

[tg]

i assume it is a straight barrel? I found that after shoting a swamped barrel that I would never go back, but that does not help here, if you can work up an accurate load off the bench then it is just a matter practice with the off hand and practice a lot...from my experience. If you cannot get a good group off the bench more combos may be needed to find the sweet spot typical GM barrels are quite accurate, I had a couple of them and they would drive tack with a bore size ball and .010/.015 patch which ever I could get down the tube, I would go to a size smaller ball for general use and hunting.If you have a buddy close by have him try some shots also,this may be revealing at times.When sighting in a gun I think the barrel should be wiped/cleaned so every shot goes thru a barrel in the same condition as the shot before.

 

[RJR]

Post up some detailed pictures of the gun, muzzle, breech, lock areas. Maybe someone will spot something you've looked at a thousand times and not seen. You know, the "second set of eyes" thing.

Rick

 

[redwing]

I would pull the barrel off, if its a long rifle. I had a 42" Poor Boy that started shooting wild groups. The old rifle had a couple of broken barrel keys and wear in the bottom of the stock. I replaced the keys and glassed the worn spots. I won a small bore shoot at state last year with that rifle. :thumbsup:

 

[Dave K]

How do your patches look after shooting. If they have burnt or cut holes in them, that will give you fliers

 

[bingo1952]

Are you shooting home cast or swaged balls? If are cast sort some by weight and give them a go. You may have a few balls that have void areas internally. :2

 

[nchawkeye]

How do your patches look after shooting. If they have burnt or cut holes in them, that will give you fliers

Agreed...We all try to make this too complicated, start with the patches first...What do they look like after shooting??? What patches are you using??? What lube???

 

[Splais]

OK, think I worded my question wrong. I'm not worried about the cafluey 45; I'm working and resolving that.

My question was more to why the 36 seems to be so inherently accurate and much more forgiving of what loads are in her. I was punching bulleyes with her off the bench, the very first time I tried shooting her. 40-50 grns, doesn't seem to matter much. Is there something inherently more consistent and accurate about the smaller calibers or did I just stumble on a very good rifle.

 

[Canerod]

I don't have the experience of some on this forum, but I love my .36. I won't own a gun that's not accurate and it holds it's own against anything else in the safe. When it comes to MLs, I only have .50s to compare it to, and it'll blow them away with almost any load you care to shoot.

 

[ryoung14]

First thing that comes to mind is twist...though I'm sure you've considered that.

I've had excellent luck with a 1:48 twist .36 (Seneca). Whether the load is light or relatively heavy, that twist seems to do the job for a .36 PRB. But normally, I shoot the .36 with a light load that's not too demanding on patch, ball fit or anything else. Because of this, the .36 seems quite forgiving.

OTOH, I've found that larger caliber slow twist roundball barrels demand a fairly heavy load for best accuracy...and then they deliver in spades. Light loads don't seem to do as well in the slow twists, but do very well in the 1:48 and faster twists.

What I'm getting at is that the .45 may be slow twist and need a heavier load to perform best. But your patch, the condition of the rifling, the size or fit of the ball may not be up to handling the higher pressures. In addition, fouling will be more severe. Heavier loads are more demanding on everything involved.
Bob

 

[nchawkeye]

I don't think accuracy is a caliber thing, I think it's a rifle thing...I also think that the deeper cut rifling shoots better (or is easier to develop a load for) than the shallow button cut rifling in some factory rifles...Of course, twist needs to be matched to caliber, faster for smaller bores and slower for larger bores...

My .40 has a Rayle rifled 1-48 and my .54 has a Rayle rifled 1-72...The .40 is accurate with
25-60grs of Goex FFF and the .54 from 50-120grs of Goex FFF...

 

[paulvallandigham]

There is an on-going debate about what is the best ROT for a round ball barrel. One school claims that if you multiply the caliber of the barrel by 1.25 you get the optimum ROT. Hence, .36 x 1.25 = 1:45 ROT. The 1:48" ROT is close enough to give optimum accuracy for that caliber. Using that thesis, and ITS ONLY a thesis:

.40 caliber = 1:50 "
.45 caliber = 1:56.25"
.50 caliber = 1:62.50"
.54 caliber = 1:67.50"
.58 caliber = 1:72.50"
.62 caliber = 1:77.50"
.65 caliber = 1:81.25"
.69 caliber = 1:86.25"
.73 caliber = 1:91.25"
.75 caliber = 1:93.75"

The reason for the debate is that there are lots of shooting matches won, and national records set with rifles that have far different ROTs than what is shown above. Most of the calibers shoot fairly well with a 1:48 ROT. That includes .40, 45. 50, and .54 caliber guns. The .58s and .62s, seem to do okay with a 1:60-to-1:66 ROT. A 1:70 ROT seems to work well with the rest of the big bores. And, if you think that sets the parameters of the debate, you are wrong there, too. There are advocates for a 1.50 multiplier over bore diameter in the debate, and RB guns in the large calibers that have ROTS as slow as 1:105"!! That shoot with amazing accuracy. More recently, people are trying to match the ROT with the velocity for a given diameter of ball to maximize rotational forces.

Given the fact that all Round Balls have the ballistics coefficient of a rock, and spinning them out the barrel is of only marginal help, the arguments seem to have missed something that explains everything, and so they go on.

So take your pick, and join in the debate. Its been going on for all the 50 years I have been reading gun magazines and listening to the experts, and I suspect it will be going on hundreds of years after I am gone. I know I don't have the answer, and I have listened to some very smart gun builders, and championship level shooters over the years talk about this very thing. Someone will come up with what appears to explain the excellent results he achieves with a given caliber of rifle, with a given Muzzle Velocity and weight of RB, but when you try to apply that to a different caliber, it doesn't work as well. Back to the drawing board.

Just don't feel like you are the lone ranger on this subject. You are not likely to get any answer that seems to work for all caliber of guns.

 

[Splais]

I really would have like to be able to figure out what barrel the Ashe rifle has. I have researched him and he had a reputation as a builder of very high quality guns. He used a lot of Large Barrels; but my barrel is unmarked, so impossible to know as Mr. Ashe passed away a few years ago. All I know is it shoots exactly were you aim it at least out to 100 yds. I haven't tested it beyond that yet.

 

[MikeChapin]

I have a .36 that I built about 30 years ago that goes exactly where it is pointed every time. It has a 42" Sharon barrel and has had heavy use over the years. Redwing beat me last year but it has taken many smallbore matches over the years. It has also taken a lot of flint matches and aggregates because it was my only flinter for a couple of years.

That being said, I also have a .32, a .53, a .58 and three .50s that will do the same thing. Each one of them was benched to find the powder/patch/ball combos that gave the tightest groups. Once that was done I had the confidence that each rifle would shoot where it was pointed. When shooting offhand I know that any blown shots are something I am doing and I need to concentrate more on what I am doing.

As far a weight goes, that gets to be less of a problem as the rifle is shot more and you get more used to it. One of the rifles I shoot the best is a Santa Fe Hawken that comes in at 9.5 pounds. I have a .58 that runs about 10.5 pounds and a shorter pull that gets a little tough to hold steady towards the end of the day. I use this rifle for a lot of practice runs because it builds those muscles I use for shooting and the other rifles don't feel heavy after horsing this thing around all day.

 

[Tom Compton]

I really would have like to be able to figure out what barrel the Ashe rifle has. I have researched him and he had a reputation as a builder of very high quality guns. He used a lot of Large Barrels; but my barrel is unmarked, so impossible to know as Mr. Ashe passed away a few years ago. All I know is it shoots exactly were you aim it at least out to 100 yds. I haven't tested it beyond that yet.

The Bill Large barrels i have seen had a ring of stars (asterics?) stamped on the muzzle.
TC

 

[FishDFly]

If it is a brand new GM barrel, you might consider lapping it a bit with fine steel wool. If not shoot, it about 200 or shots and then see what is does off the bench.

RDE

 

[rawdog]

I have a copy of Professor Alfred G. Greenhill's 1879 paper in which he derived his famous Greenhill Formula for twist. It is highly mathematical, but someone with a math/physics or engineering background can follow it. It is actually MUCH more complicated than the approximation you usually see given, ie,

Twist (inches) = C * cal*cal/L

where

Twist = one revolution in so many inches (ie, 1 turn in 66")
L = length of the bullet in inches
cal = caliber of the bullet in inches
C = a constant usually given as 150 to 180, depending on velocity.

It is derived for conical bullets where the aspect ratio (L/cal) is ALWAYS greater than one. In the paper, Greenhill gives a table of twists and the table shows that if the aspect ratio becomes equal to one, the twist rate goes to infinity, which means no twist at all!

This is because of the physical considerations that went into deriving the original equation were based on the problem of avoiding a tumbling (keyholing) conical bullet....but what does it mean for a roundball to tumble and turn sideways? In terms of the physics Greenhill considered in his derivation, it means nothing.

However, for a ball, the length is equal to the caliber and if you plug it into the Greenhill formula above you will get

Twist = C* cal*cal/cal = C*cal

This is the equation that PaulVallandigham is presenting above, where C is usually changed to 125 for MLs because of their lower muzzle velocities and the fact that they will decellerate into the trans-sonic region where there is much more turburlence, buffeting, and stability is most challenged. (this is also why modern suppository guns can use C=180 because over their useful trajectory they never drop down to near transonic region).

So, on the one hand the Greenhill Formula DOES NOT APPLY to roundballs, but on the other hand if you apply it anyway you will get twists that are strikingly close to what manufacturers often use for PRB barrels.... (hmmm, why is that?)

And on the third hand, you will often see successful results from barrels that are a very different twist (as PaulVD points out).... (hmm, why is that?)

By now, you probably don't think that should be too surprising if you have followed what I said above about Greenhill's derivation!

So, why do we have to spin a PRB and how fast does that have to be?

I am not completely sure, and I am still working on that although I do have some intermediate answers....for another time maybe, I need to close up for the night...


YHS,
rawdog

 

[paulvallandigham]

Name-dropper! :grin:

Thanks for the reference and the formula. When I wrote my response, I was too tired to pull my Greenhill formula stuff and get technical. And, because of where I knew I would end up, I didn't really think there was any good reason to spout technical formulas! :shocked2:

No one has been able to come up with a better formula than the Greenhill formula for determining the best rate of twist for shooting round balls in every caliber, at every speed. And, as we have both noted, that formula leaves much to be desired when used with Round Balls.

I do not fault Greenhill, BTW. We are all products of the times in which we live. He was living in a period of time where conical bullets were all the rage, and everyone was wanting a RIFLE that would fire such bullets accurately. Neither he, nor anyone else was at all interested in trying to shoot RBs more accurately. Enough Trial and Error testing had been done over the prior 300 years to convince people in his time that it could not be done. They may not have understood aerodynamics, or the " Rotational affect" of the Earth, etc., but they knew the capabilities of RB guns.

Today, when you ask the mathematicians, and aeronautical engineers to help find a better formula for shooting a Round Ball, they screw up their noses and ask, " What for?"

Considering the distances at which RB are effective, I think that question has far more legitimacy than we MLers sometimes are willing to acknowledge. :wink:

 

[rawdog]

Well, I happened to have Greenhill's paper in hand when I read this thread..... :grin:


Seriously, from a theoretical POV, if the ball comes out in the shape of a sphere (I will address again down below), then there is no reason to give it gyroscopic stability to prevent tumbling....

Tumbling occurs with a conical because the center of gravity and the center of aerodynamic pressure do not coincide, hence air-drag to the nose exerts an overturning moment of rotation about a sideways axis (the overturning moment = product of the aerodynamic overturning force x distance between CG and CP, and btw for other readers, a "moment" in this context is another name for a torque or "turning torque"). However, with a sphere the CG and the CP coincide, so the product = zero and there is no overturning moment.

So, with a spherical bullet (again, I will change this in a few moments down below) the only reason I can think of to spin it is to average imperfections (mass and shape defects) and/or to give it a known spin so that no random spin can cause unpredictable "knuckle ball" or "curve ball" effects (Magnus Effects).

However, spinning the ball introduces drift at extended range, so when you fix one problem(?) you introduce another.

I am not at all sure that the simplified Greenhill Formula should be applied to roundballs, even if experience has taught that it is ok. Remember what I said from Greenhill's paper, that Greenhill's ORIGINAL Formula if applied to a sphere says use NO TWIST AT ALL!!! Only the simplified approximation that everyone is familiar with gives less than infinity for the spherical bullet, and that occurs entirely by accident...it was NEVER part of Greenhill's reasoning. Furthermore, there is a lot of evidence that much less twist on a PRB would be better. As you mentioned, Lt. James Forsyth, in his 1864 book "The Sporting Rifle and its Projectiles," (another great name to drop :wink: ) suggests that twists from 104" to 144" are fine for .69 cal guns he was shooting out to between 150 and 250yards! The Greenhill number would be Twist = .69*125 = 86" which is way faster twist and would result in far more drift at ranges beyond 100yds. In stability terms this would be about 60% more stability factor, or 60% more gyroscopically stable than 144" twist. WAY MORE TWIST! And we also see 1in 48" twist shoot ok alongside 1in66" twists, where this represents a 50% difference in stability factor. But take those same two guns, one slow and one fast twist and in the same caliber and shoot PRBs at a 150-250 yard target and I think you will begin to see the diffference.

The only reason our faster twist barrels don't bother us more (for drift and perhaps for dynamic instability due to faster twist) is because hardly anyone ever shoots beyond 100yds with a PRB, and even then we have no expectation of hitting anything, having learned this from all our "modern oldtimers"....Forsyth had much higher expectations. And we have forebears who would snipe the British at any range less than 300yds with a reasonable expectation of a hit. Col. George Hanger, in his book (yes, I have a copy :rotf describes a 400yd PRB shot taken at him and another British officer by a "colonial", the shot hitting and killing the horse standing behind them. Who among us could make that shot?

Now, to delve deeper, we all know that we do not really shoot round spheres....instead to some degree or another they are obdurated (either in loading or definitely in shooting) and I think what comes out looks more like a oblate spheroid with a short flat bearing circumference. In other words, the aspect ratio is LESS than one, the ball is become slightly flattened from front to rear, no longer true sphere....and now it needs some gyro spin again..... :grin:

But the question I have not answered yet is "how much?" and "based on what principles?"

Maybe not nearly as much as we may think using (mis-using?) Greenhill as a guide.

YHS,
rawdog

BTW, it is not true that no one has come up with a better twist formula....you just don't see it in the popular gun media because gun writers may have "taken" game, but they have never taken physics, and they would rather keep their pet theories alive, sans physics.

 

[paulvallandigham]

I am aware that many target shooters these days use caliber or larger diameter RBs in their guns, making that short spheroid projectile you speak about. And there is no doubt in my mind that doing so produces better accuracy. I do think that the longer ROT is more forgiving of variations in powder loads, and burn rates, so that a long rifle with a slow twist will shoot balls at long range into smaller groups, other things being equal. In fact, I tried to get a 1:72 Green Mountain .50 cal. RB barrel on my semi-custom rifle, but the maker put a GM 1:48 ROT barrel on it instead. :cursing: As you note, the 1:48 ROT shoots okay, but when you get up to the hunting loads for longer range, its rather temperamental on how accurate a powder charge is thrown from shot to shot. I am finding that using an OP wad helps to make MV more consistent when using the heavy loads in my barrel. Velocity is increased using an OP wad together with my PRB, indicating that even in my gun, my PRB is not sealing gases completely.