Twist rate / slippage?


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98falstaff
December 30, 2009, 03:42 PM
I understand the velocity/twist rate formula to get bullet RPMs, but wouldn't there be some slip between bullet and rifling initially near the chamber? I just don's see how the rifling can grab the bullet and immediately start it to spin down the barrel. Does it catch up with the lands and grooves further down the barrel? What about a snub nose? Could someone please enlighten me on this? Thanks.

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rcmodel
December 30, 2009, 04:14 PM
If you recover and examine some fired bullets, you will see no slippage with jacketed bullets. They will have clean rifling lands and grooves engraved in them with no slippage evident.

Some slight amount might be found on soft lead bullets, but there can't be much at all or bore leading would be so bad you couldn't use the gun for more then a few shots.
If the bullet slipped even the width of the groove, the land engraved on the bullet would shear off completely.
And that isn't the case at all.

In short, the bullet or jacket hardness has to be matched to the expected velocity to prevent slip. And this is always done by the bullet manufactures.

Keep in mind the bullet doesn't reach max velocity in it's own length, even in a snub-nose..

And it takes very little force to rotate a bullet on it's axis compaired to accelerating it up to speed from a dead stop.

rc

98falstaff
December 30, 2009, 04:45 PM
Thanks for the quick reply. That makes a lot of sense.

Brimic
December 30, 2009, 08:57 PM
I think that at one time some manufacturers thought that bullets might slip with tight twist rates and made gain twist barrels where the rifling progressively got tighter towards the muzzle. Some carcanos are infamous for this- gain twist barrels made around 1900 or earlier were cut down to make carbines, and the rifling left in the barrel wasn't enough to stabilize the long 6.5mm bullets- giving carcanos an unfair reputation of having poor accuracy.

Sam1911
December 30, 2009, 09:50 PM
gain twist barrels where the rifling progressively got tighter towards the muzzle

The S&W X-Frame revolvers chambered in .460 are progressively rifled. I don't know the theory behind that, but I don't think it's worries about bullet slippage. After all, there are plenty of rifle cartridges at .45 + caliber that are well over 1,000 fps faster than the .460 mag and are conventionally rifled.

Curious.

-Sam

matrem
December 30, 2009, 10:12 PM
Curious.
Me too. I wonder if:
"gain twist" = marketing ploy?

Maverick223
December 30, 2009, 10:23 PM
RC got it right, the bullet begins slowly, where it can grab the rifling fairly easily, however I have heard this is not the case in the Dragunov SVDs barrel, it transitions from smoothbore into rifling about 33% down the bore, and I don't know what effect that has on bullet slippage, but apparently not a great deal as accuracy is pretty fair.

To the best of my knowledge I believe the gain twist was only used to squeeze a little more velocity out of a shorter bbl. I would think that a gain twist would create more lead or copper fouling due to the greater cut in the jacket (or lead) by changing the pitch. I think the best solution is partial rifling as seen in the Dragunov. Is it better: I dunno, but I like the idea, and it accomplishes a bit more than what a gain twist can.

:)

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