What are your Thoughts On Sabot Slippage???

[mkf350]

I know some people knurled certain bullets and claimed accuracy improvements. I'm just wondering if shooting smooth bullets (SST and the like) would be sublect to slippage and reduce bullet RPM's causing erratic accuray?

 

[Anonymous]

I know some people knurled certain bullets and claimed accuracy improvements. I'm just wondering if shooting smooth bullets (SST and the like) would be sublect to slippage and reduce bullet RPM's causing erratic accuray?

If knurling improved accuracy of a particular sabot/bullet combination I would think that knurling simply brought the bullet up to a suitable diameter for accurate shooting.

For rotational slipping to occur my thoughts are that a bullet would need to be substantially undersize. It requires very little contact/force to impart rotation. I once got a handful of .446 diameter bullets and shot them in a 50 caliber Knight with our standard black sabot (50 x 451). Accuracy was on a par with bullets of the correct diameter.

The sabot obturates ( tires to expand and sets back or for-shortens) when rapidly expanding gas hits it and is capable of "filling" a few thousandths of radial space, not only in the powder seal area but the bullet cup also.

 

[sheephunterab]

I know some people knurled certain bullets and claimed accuracy improvements. I'm just wondering if shooting smooth bullets (SST and the like) would be sublect to slippage and reduce bullet RPM's causing erratic accuray?

I don't buy into the slippage theory. The fact is, that a bullet rotates less than one turn coming out of most muzzleloaders.

 

[Mountain Man]

I believe even the people who knurled them would have said that it was only in order to add diameter to them, not to make them spin better.

 

[Blue-Dot-37.5]

The only ones that I've had to knurl was the Barnes "X" .458" FB in 300 gr. It seemed to help with the accuracy. I did check them with a dial caliper during the knurling process to try and keep some uniformity. I was using MMP Orange Sabots. Now, whether the increased dia. helped, or the knurling stopped any slippage, I can't say. But, they did group better! I didn't notice any radial marks inside of the spent sabots before or after knurling... They were going sorta quick tho. 8)

But, as RW saw, it worked on the piggy just fine!

Blue-Dot-37.5

 

[sheephunterab]

I didn't notice any radial marks inside of the spent sabots before or after knurling...

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 so-called radial marks that most people saw were the base of the bullet indenting the sabot

 

[sshunter]

I tried a moly coated and uncoated bullet as a test and the coated bullets accuracy was very poor.(6 inch @ 100)vs 1 1/2 for uncoated.I knurl my 300 Barnes also with excellent results.

 

[jolop]

I know some people knurled certain bullets and claimed accuracy improvements. I'm just wondering if shooting smooth bullets (SST and the like) would be sublect to slippage and reduce bullet RPM's causing erratic accuray?

I don't buy into the slippage theory. The fact is, that a bullet rotates less than one turn coming out of most muzzleloaders.

It would be very possible to have slippage between the bullet and sabot. At 2000 fps. the bullet and sabot are in a 24 inch barrel for 1000th of a second and the sabot goes from no rpm's instantly to 60000 rpm's.
Jolop

 

[sheephunterab]

I know some people knurled certain bullets and claimed accuracy improvements. I'm just wondering if shooting smooth bullets (SST and the like) would be sublect to slippage and reduce bullet RPM's causing erratic accuray?

I don't buy into the slippage theory. The fact is, that a bullet rotates less than one turn coming out of most muzzleloaders.

It would be very possible to have slippage between the bullet and sabot. At 2000 fps. the bullet and sabot are in a 24 inch barrel for 1000th of a second and the sabot goes from no rpm's instantly to 60000 rpm's.
Jolop

Huh? The rotation of the bullet is controled by the riflings in the barrel and if the rifle has a 1:28 twist, that means the bullet rotates once in 28 inches. The bullet does one rotation in the barrel period, no matter what the velocity, and once it exits the barrel it sheds the sabot so RPMs mean nothing at that point in regards to slippage.

 

[Anonymous]

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:

 

[getonit]

Dang- this is enough to make a fella's head hurt :huh?: I knew that the bullet was supposed to be rotating for stability (and prevent the keyholing on the targets!) but 60.000rpm??? even if it falls far short of those RPM's due to sabot to bore slippage(at the instant of ignition) or bullet to sabot slippage maybe knurling DOES have a positive effect on accuracy as the amount of slippage IMO would be great between smooth copper to smooth plastic- I havent compared some that I lightly knurled to regular XTP's to know- now I'm curious :?: -Getonit-

 

[sheephunterab]

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.

 

[Mountain Man]

If you think that's fast consider a high power rifle like the 7mm Remington Magnum. That bullet will leave the barrel spinning at about 245,000 RPM. :wink:

As far as slippage, though, if the bullet is slipping horizontally/radially in the sabot, you can be SURE it is also slipping vertically/lengthwise in the sabot. The base of the sabot is NOT designed to support by itself the EXTREME force of a .452 diameter cylinder being slammed into that 1/4" thick polymer as it accellerates to 2200-2500 fps in one millisecond.

Think about it: do you think if a bullet moving at a FRACTION of the Savage's muzzle velocity impacted a 1/4" thick sheet of plastic polymer, the bullet would bounce off of the polymer instead of penetrating it? Of course not! NOR could the base of a sabot, by itself, manage to withstand the pressure exerted by the bottom of a bullet during firing.

When the powder charge ignites and the expanding gases begin creating the enormous and almost instantaneous pressure inside the barrel and against the bottom of the sabot, it "disrupts" the base/cup of the sabot, flaring the cup out and pressing it into the rifling to completely seal the bore. If it didn't do this instantaneously, the smallest amounts of superheated gas flowing around the sabot would melt and destroy it.

At the same time that the base/cup is disrupting and sealing the bore, pressure on the base is beginning to push it forward. But, the momentum of the petals and bullet is resisting this acceleration. What actually happens is that BOTH the bullet and the sabot compress during this acceleration. Believe it or not, both the bullet and sabot actually become slightly shorter for this instant of acceleration.

Obviously, the matter in the bullet and sabot has to go somewhere if their becoming shorter, and it does: the bullet and sabot each expand in diameter slightly during this acceleration. This is what Del referred to as "obturation."

So, where the combo was just thick enough for us to be able to push it, with some force, down the barrel, it is now even thicker and there is much more friction on BOTH walls of each sabot petal. On the outside wall of the petals, there is much more friction against the barrel. On the INSIDE walls of the sabot petals (inside the cup where the bullet sits), there is a great amount of friction between the petals and the bullet itself. This friction allows the petals to hold on to the bullet and THEY, at least as much as the base, push the bullet to full speed and out the barrel.

If it were not for this friction and grip between the petals and the bullet, the sabot would not work. Perhaps on some loads the bullet DOES fail to spin up to the same speed as the sabot--BUT, these are the same loads that punch a little round hole in the bottom of the sabot OR that causes you to find little pieces of shredded, melted sabot in front of the bench.

It is impossible for the bullet to fail to spin with the sabot without punching through the base of the sabot.

 

[Redclub]

60,000 RPM's seems right. If you break that down to Rpm's per second and 2000ft. per second. That would be about 1 spin per 2 feet, give or take a little.
Redclub

 

[sheephunterab]

It's definately coorrect at the muzzle but no matter what the RPM, the bullet still only rotates once in the barrel and that's all that matters when talking about sabot slippage.. Many people are under the impression that the bullet and sabot rotate numerous timees in the barrel but they don't so slippage is pretty much a non issue.

 

[sshunter]

Unless the bullet physically touches the rifling there can be slippage.

 

[Admin]

Unless the bullet physically touches the rifling there can be slippage.

There got to be some at first, but it would be very minor. If it wasn't no one would be shooting sabots.

 

[Mountain Man]

Unless the bullet physically touches the rifling there can be slippage.

Yes, there certainly can be, but if there is any to speak of, you're going to blow the sabot.

 

[Ron S]

The speed the bullet is being pushed down the bore is the issue! It has been well documented many times in reloading manuals that high velocitys in certain varmit rifle loads,with certain bullets SX,TNT,ect. will strip,rip,destroy the copper jackets if loaded above a certain velocity! Now we are dealing with a plastic sleeve,over a copper jacket! Can it slip? of course it can! If enough accelration is put on it! If we take your Grandma's AMC PACER and use the rear wheel as an example: the wheel is the bullet,and the tire is the sabot. We floor it from a stand still,and the tire rotates forward,WITH NO SLIPPAGE! Now take Granny's Pacer and put a engine in it that develops 1000hp! Floor it from a stand still, I can almost promise you,you will have slippage! Infact you won't go forward for the first few(or many rotations). My brother drag raced many years. The rear tires on his PRO-STOCK car were huge! If you looked real close at the rear wheels,they had screws that went through the rim and into the bead on the rear tire. I've got a pretty good idea what the were there for. You can make anything slip,if enough force,or acceleration is applied! Ron

 

[Mountain Man]

Certainly agree with you, Ron. To complete the scenario, if your sabot is being pushed down the barrel so fast that the rifling is cutting across the sabot, the chances of maintaining a seal and sabot integrity is pretty dang low, IMO.

Again, IMO, if you're not blowing sabots, there is no real "slippage" occurring inside or outside your sabot.