What are your Thoughts On Sabot Slippage???

[Ron S]

You hit the nail,right on the head! Ron

 

[skeeter]

RPM of the bullet

Contrary to what was said the bullets rotation does not slow with a reduction in velocity. The spin was induced by the speed of the bullet at the beginning. It maintains that spin with very little reduction...mainly due to the friction of the air it travels in. The revolution speed sounds really high until you consider that the projectile is only in flight for a few hundredths of a second. at 2200 ft per second with a turn in every 38 inches the bullet makes approx 30 turns in 100 yds. As far as slippage..of course there can be some but not enough to mess up the accuracy of a load to any great degree as long as the projectile and the sabot make for a reasonably tight fit in the bore. BTW I really like my Savages...The accutrigger is great and the accuracy of the rifles is very good. BUT the thing you really have to love is NO CLEANING. And my grandson just loves to shoot :lol: Have a camo stainless and the laminated stainless. I used the muzzleloader in our regular firearms season too. One shot...one kill. or two or three

 

[FWF]

These so called radial marks had to be one of the biggest falacies in my opinion. Some folks are under the impression that the bullet is spinning at high RPMs coming out of the rifle but the truth is that it has only turned once before exiting the barrel.

The "impression" that bullets are spinning at high RPM is exactly correct.

The formula is: (Bullet R.P.M.) RPM=720xMV/TW


MV = muzzle velocity in feet per second (f.p.s.)
TW = twist of rifling in inches per turn

MV = 2000 fps
TW = 24

For a 2000 fps load coming out of a Savage 10ML-II, the bullet revolutions per minute is 720 x 2000 / 24, or:

60,000 RPM. :shock:

Ya but how many rotations does that bullet and sabot combination make before they leave the rifle? Less than one in most so the rpm is irrelevant. It's a big number that sounds impressive and confuses most people but it means little in regards to what happens inside the barrel.

You formula might work at the muzzle Randy but RPMs slow in proportion to the velocity.

OK, let's switch switch from RPM to RPS (rev/sec). The bullet is spinning at 1000 RPS. The fact is, the bullet has been rotationally accelerated from 0 RPS to 1000 RPS in the short time that it was resident in the barrel. Acceleration requires application of force; force which in this case had to be applied through the friction existing between the sabot and the bullet. Also, keep in mind that this rotational acceleration is not uniform; it will be directly proportional to the linear acceleration of the bullet, as both forms of acceleration will be proportional to the peak pressure present in the barrel as the bullet journeys down the bore.

Bottom line: I wouldn't be so quick to dismiss the importance of rotational acceleration. If using a number like 60k RPM's is being deceptive then saying something like "the bullet makes less than one revolution while in the barrel" is at least as deceptive in the other direction. I think the valuable point behind your observation is that catastrophic sabot failure is not likely the result of the bullet "drilling" the sabot. Rather, I would expect it to be the result of rapid gas-cutting of the sabot and/or the bullet punching out the base.

Now, is slippage a real issue? I really don't know. I suspect it usually isn't. I also suspect there might be some situations where the bullet slips a bit while the acceleration forces are at (or near) their maximum but then stops slipping as those forces decline (with the bullet behaving as though it's in a gain-twist bbl). Obviously, if slippage persists throughout the bore, accuracy will likely be gravely affected.

 

[Anonymous]

If using a number like 60k RPM's is being deceptive then saying something like "the bullet makes less than one revolution while in the barrel" is at least as deceptive in the other direction.

60,000 RPM is hardly "deceptive"-- that's just what it is. It is not pulled out of the air, nor designed to promote any bullet, sabot, powder, or rifle. RPM is a very commonly used measure of the rate of spin.

"Sabot slipping" would not be the exact term, but bullet slipping would be more to the point. Soft lead projectiles can strip the rifling if you push them enough-- this is well-documented in the literature. Sabots can do the same thing, of course: the generalization is that the limit for sabot reliability is a 1 turn in 22 inches barrel: 1:22. A 1:20 barrel may not shoot sabots reliably, and generally don't work well with sabots. Most White 1:20 barrels have a tough time with sabots at hunting velocities.

The lighter, shorter, ballistically challenged bullets (round ball) work best with s-l-o-w twist barrels-- a 1:60 or similiar. That is also well documented.
It is also well documented that it is better to overspin a bullet, than to underspin a bullet. There is NO such thing as "overstabilizing" a bullet. The whole idea is to make a projectile as just stable as possible, so of course it cannot be "too stable."

For most frontloaders, you have no choice-- 1:28 is the standard, and a compromise. Not enough to stabilize long, heavy bullets in some cases as Barnes 300MZ testing proved. More than you might need for a 225 gr. .45 cal. projectile.

A 1:24 twist is a good compromise for 300 gr. arena bullets particularly with spitzer points that are harder to stabilize than blunt nosed bullets at moderate velocities, even more so with boat tails that are need more spin to stabilize. More than a compromise, it is ideal for 300 gr. bullets at moderate velocities of 1900 fps or less.

The 1:22 Pac-Nor .45 that was introduced for the 10ML-II just makes no sense to me-- 1:30, 1:32, or even 1:36 is much closer to Greenhill requirements. That goes in completely the wrong way.

If anything, with improved velocities and sabots vs. 5 years ago, the minimum required twist just makes good sense-- a 1:26, 1:28, or 1:30 in a .50 seems a better compromise than the 1:24 of five years ago. It is wishful thinking to assume that a change from 1:24 to 1:26 changes things in a huge or meaningful way-- back to the basic, proven premise that bullets just cannot be overstabilized.

 

[Rifleman]

Strictly speaking, sabot slippage does occur, I had it happen to me as a young man. In the horse lot, my sabot became stuck in the mud and slipped off, the end result being horse hockey all over my sock. :shock:

 

[FWF]

Strictly speaking, sabot slippage does occur, I had it happen to me as a young man. In the horse lot, my sabot became stuck in the mud and slipped off, the end result being horse hockey all over my sock. :shock:

Well, that ought to teach you not to wear your wooden shoes out in the horse pen. You should stick to something proper, like a pair of Double-H's or Chippewa's.

 

[Ron S]

The rate of twist theory dosen't allways hold true! I have a Bushmaster Varmiter that has a 1-9 twist. It shoots 45-50gr bullets extreemly well, 1/2 moa or better. According to the rate of twist in the barrel it shouldn't. It should shoot the longer bullets best. It dose shoot them well, but for pin- point accuracy at the 100yd mark,the 45-50gr. bullets turn in the best groups! This gun (a semi-auto will also out shoot most of my bolt action guns) that's not supposed to happen either,but it dose. You can crunch all the numbers you want,but in the Real World,sometimes things work out diffrent! Ron

 

[sheephunterab]

You can crunch all the numbers you want,but in the Real World,sometimes things work out diffrent! Ron

Isn't that the truth!

 

[Anonymous]

The rate of twist theory dosen't allways hold true! I have a Bushmaster Varmiter that has a 1-9 twist. It shoots 45-50gr bullets extreemly well, 1/2 moa or better. According to the rate of twist in the barrel it shouldn't.

Ron,

It does not work that way at all. Nowhere from Greenhill, Aberdeen Proving ground data, etc., does the suggested "minimum" rate of twist say or purport that longer or heavier bullets automatically shoot better in anything. It also does not suggest that spinning a bullet faster than the bare minimum necessarily hurts anything.

It is simply a ratio of specific gravities. Nothing about exact jacket thickness, ogive, bearing surface, or other bullet design considerations are even touched upon.

 

[FWF]

The rate of twist theory dosen't allways hold true! I have a Bushmaster Varmiter that has a 1-9 twist. It shoots 45-50gr bullets extreemly well, 1/2 moa or better. According to the rate of twist in the barrel it shouldn't. It should shoot the longer bullets best. It dose shoot them well, but for pin- point accuracy at the 100yd mark,the 45-50gr. bullets turn in the best groups! This gun (a semi-auto will also out shoot most of my bolt action guns) that's not supposed to happen either,but it dose. You can crunch all the numbers you want,but in the Real World,sometimes things work out diffrent! Ron

Yes, my 9" twist AR's thrive on 55 gr Ballistic Tips. But there are other reasons (besides twist) why they don't print 100 yd groups as small with 68-75 gr bullets.

First, the chambers are designed to accomodate 55 gr ball (with its shorter ogive), which means if the 68-75 grainers are loaded with a short enough OAL to function through the mag, the bullet jump to the throat is pretty long.

Secondly, long for their caliber BTHP's (and especially their VLD descendants) were never designed to win 100 (or even 200) yd benchrest matches. The purpose they do serve is to make errors in wind drift calculations less critical at extended ranges.

Your AR may in fact shoot the lighter bullets even better if it had a 12" twist (if all else remained unchanged), but then it wouldn't handle the 68-75's at all. As stated elsewhere, insufficient twist is a bigger problem than the same degree of excessive twist, unless we're butting heads with some special problem (such as sabot failure or Speer TNT's/Hornady SX's disintegrating from too many RPM's).

 

[FWF]

A 1:20 barrel may not shoot sabots reliably, and generally don't work well with sabots. Most White 1:20 barrels have a tough time with sabots at hunting velocities.

In a similar setup (one which, unlike the White, has been designed for smokeless) could the sabot stripping behavior be attenuated ( ) by using a relatively slow burning smokeless propellant in order to soften the acceleration?

Would another accuracy limiting factor with such a setup be the sabot's ability to locate a long ogive/short shank bullet coaxial with the bore?

 

[Anonymous]

In a similar setup (one which, unlike the White, has been designed for smokeless) could the sabot stripping behavior be attenuated ( ) by using a relatively slow burning smokeless propellant in order to soften the acceleration?

Would another accuracy limiting factor with such a setup be the sabot's ability to locate a long ogive/short shank bullet coaxial with the bore?

I don't see how-- no matter what you do with a hunting load, it is still 0 - 2200 fps in under 2 milliseconds. Not exactly "soft" acceleration no matter how you slice it. 1:20 can be a problematic twist even with Pyrodex and 1500 fps muzzle velocities, so it is just too tight to be right as a generalization. The bullets I tried were long bearing surface bullets, so if the twist is tight enough to be a sabot destroyer, it just is.

The 1:20 Thompson .45 barrels were problematic, so were the 1:20 Traditions barrels.

Del Ramsey mentioned he could supply sabots that could handle not only tight twist rates, but 100,000 PSI-- as long as I was willing to put up with the very minor inconvenience of using a hydraulic press to load my gun. :shock:

 

[d94jonca]

"I don't see how-- no matter what you do with a hunting load, it is still 0 - 2200 fps in under 2 milliseconds. Not exactly "soft" acceleration no matter how you slice it."

are you saying that the even if max acceleration never exceeded the average 1100 fps/ms mentioned above (which of course it does with BP substitute - by an order of magnitude), a muzzle loading friendly sabot would still not hold up in a 1:20 barrel? i'm sure you are right, but what makes you think so?

 

[Anonymous]

I tried it.

 

[Mountain Man]

I tried it.

Hard to argue with that! :lol: