Can somebody tell me where to find the table for rate of twist versus length/weight? I seen it but cannot remember where. I'm working with a 2 piece bulletin a BP rifle, 1:17twist, .504bore-.512 groove. I've been shooting a 1.6" long bullet, 774grains, over 100grffG. Gives 1100 fps.
length weight twist
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I don’t know the table you are refering to. But I sometimes use the stability calculator at the Berger bullets web site
As you’ll see, the calculator needs a bit of input data, like BC.Which is not always easy to find, but can be calculated with a ballistic calculator if you know velocity, and drop over a given distance.
It is noted in the attachment that the Greenhill formula Constant of 150 is better changed to 180 for faster, longer Berger bullets,etc.
Ross, It is noted here that the 150 constant Also needs to be changed for low bc, lower velocity rifles /bullets such as your
Big 50. I have found that A constant of around 120 to 125 should give you a better idea of stability of bullet length vs twist for your 50.
After playing with the formula, slower,not faster, is what I need. The 100gr load of ffg chronographed at 1100fps so I'll try 80grn. I tried 90 already and that was not as accurate as my first loads of 120grns. I will also try some 140 grns ffg as that is what was recommended by the previous owner.
Ross, to my calculations the projectile is stable given your figures. Just because the projectile is going fast doesn't make it stable, that isn't what Greenhills formuls (as far as I know) wasn't based on. Your barrel twist for that length projectile is stable showing a factor of 4.66, anything over 1.3 is stable. If you could change the barrel over to a 1:10 then it would bring the stability factor up to 13.48 for that given length projetile.
Tony: Yes the bullets are stable, I'm looking for some better accuracy out of this Antique target rifle. Greenhill's formula suggests that I go to a slower twist rate, but I can't. I can change bullet length or velocity. Chances are this rifle's best accuracy is already history. But trying is fun.
Also, how did you figure the stability factor?
Chances are this rifle's best accuracy is already history.
Old rifles are fun..... they also respond differently dependent on just which alloy/hardness you try them with. Some very old cartridge rifles with very worn bores can shoot groups you absolutely will not believe. With that said, you really need to try lead/tin alloy in the range of 40:1 to 50:1.
As you said, you are dealing with twist as a Constant.
You need to work the modified Greenhill "backwards" and solve for bullet length. Velocity will have a minimal effect, though
with that relatively fast twist for a 50 cal, I expect that 1.6" long bullet may be just fine. I think you can continue to tweek
the powder charge including varying compression , lead/tin alloy, and Wad column, in order to maintain and adjust compression
while keeping the bullet snug on to the leade/lands
45: The bullet is a two piece slug. The base is pure lead for bumping-expansion into the groves and the nose is 20:1 or harder to prevent distortion on loading or firing. But yes I can "play " with that.
beltfed: Diameter is what I played with today using my original bullets1.6",pure lead base, 20:1 nose, 774 grains, and only 80grains ffG. I have no idea on the velocity, but under 1000fps I assume. The bullets sized .503 before patching did much better than the bullets sized .509 before patching. The bullets are cross(strip) patched with .003 freezer wrap, slick side out and lightly greased with sizing honey. Now I will work with 75-80 grn. charge and 502-504 thou sizing. Matthews sez the bullet and ramrod should go down the barrel very easily. Some older guy wrote that these slug guns needed some where between 5:1 and 10: weight to powder. Looks like I'll be on the 10:1 end. He didn't mention twist. The very plus side is recoil is much lower. 140 grns produces surprising recoil in a 50lb gun.
Again thanks all for the help.
Ross, until your latest post above, I did not realize this is a Slug gun.
Looks like you are on the right track.
You are welcome.
Have fun and
Follow Up: Went to the range and fired ten rounds, that's alot of work with a 50lb muzzleloader. Smaller charge worked. Will try 75 grn ffg soon.
If it is a 50 cal in my I shot 80grs of FFG and I have 2 different side hammers and that what it takes . Just to give you a idea let you gun tells you what it likes.
Fifty pounds is a heavy-weight and hast to be a workout just lugging it around but it sounds like you are on the right track. My only bit of advice at this point is only change one thing at a time to see the effects. Best of luck and stay well, Squid
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