Thoughts on Deep Knurling for Sabots

[ET]

Prelude
When we view what is happening as to the role a sabot plays we see the sabot grabbing the bullet, transferring a rotational spin to the bullet and the petals in coordinated unison releasing the bullet to send it on its accurate way. At least this is what I interpret is occurring.
Now it’s felt that obturation also plays a role here and for monolithic bullets that do not really obturate we add a process identified as knurling to assist in the above process.

Main Body of Topic
The term we most commonly use when the sabot is pressed against the bullet is GRABBING. For this application I’m going to use the term SURFACE LOCK instead of GRABBING.
On a smooth body bullet the surface lock of the sabot to the bullet appears to be fairly consistent between the petals and creates a unison release when leaving the bore.
Introducing a shallow depth knurl may still produce a fairly unison surface lock between sabot petals.
Now my focus of interest is on deeper knurling that may offset this unison of surface lock between the petals thereby affecting the sabot release. If even 1-sabot petal is minutely delayed from its release on the bullet then the bullet is thrown off course or even a longer release occurs then the bullet is sent into a tumbling action.

For the moment I’m not going to get into why I suspect uneven surface lock but want to stay focused just on the surface lock condition with a deeper knurled bullet.
So I plan on trying the application of a release agent onto the bullet first to see what affect it will have and if an improvement is observed. If there is an improvement then this would add credence to what I suspect about an uneven surface lock condition with a deeper knurl.

Purpose of this post.
If anyone believes that I’m trying to display any level of intelligence here you are mistaken because I’m displaying the exact opposite. Quite often I find my thinking is restricted to a unilateral approach instead of a multilateral approach missing other factors and I’m looking for comments or thoughts from others that would reveal other factors I’m overlooking because of my restricted focus of one line directional thinking.
I will eventually explore this when I get a chance to return to using a deeper knurl for a bullet but for now need to quench some current irons in the fire first.
Again any comments or thoughts shared would be appreciated.

Ed

[Deleted]

Ed, in my thinking when the bullet exits the barrel the sabot petals are under extreme pressure @ the crown creating a peeling effect at this juncture. the sabot would have to be almost welded to the bullet to create a sticking situation. while this might be possible i dont think it is very likely.... but then again you theory could be correct.....Bill

[dannoboone]

One of the cameras they use on "Top Shot" could answer your
questions in just a few shots. ;D ;D

In the case of my rifle (PN.45 10MLII), it will not shoot the
monolithic Barnes bullets accurately unless they are knurled.
Some are less than .400, and get knurled out to as much as
.4015. Is this considered deep knurling?

Ed, you're giving this a lot deeper thought than I ever have!
Didn't give it any more thought after finding "what works" in
my rifle.

[Deleted]

Et
IMO i'm thinking the sabot under pressure leaving the constriction
of the barrel starts to open up like a blooming flower.This effect combined with air getting caught between the petals and bullet also is another releasing force involved.
I'm not so sure about the release agent being introduced because ,IMO, it will act like a lubricant ,and I think it would offset
the benefits that the knurling produces.
I am picturing your deeper knurling question,Is there a maximum
beneficial knurling depth? I think that there is, at some point the lock will be TOO good for smooth sabot release.
The situation of un-uniform locking and bullet tumbling,Iam thinking thats more of an alignment problem or accurate alignment should keep that problem to a minimum.
Rangeball's new bullet has some pretty deep knurling combined with the deep cannaluring . It will be interesting to see what the
next step will be in that development.
Just a few thoughts ,I will be interested to see what others here
think of your knurling question.
Greenhorn

[ET]

Okay let me expand on this a little.
Let’s take a .451” bullet and knurl it to say .454”.
First thought is the surface on 1-side has only been raised .0015” but the depth of the knurl is more like .003”. Remember you have to take say .0015” from the original surface height to use to build the outside by .0015”.

With the file method the teeth are not uniformly the same producing the same indention of roughness to every indentation to the knurl. This in turn could possibly alter the resulting surface tension or surface lock when the plastic sabot is forced into the indentations. Now that is also dependent on how much force is applied to compress the sabot.

I know there are many “What Ifs” here to go on so I’ll add one more. If the roughness of the knurl indentation can be smoothened out thereby reducing the surface tension or surface lock this in theory should allow a smoother release from the sabot. A smoother release should also produce a more uniform sabot petal release. So the direction this hypothesis is going.

In order not to detract what the knurl does for rotational spin and sabot fit the release agent needs to be applied with only a film coating and needs to dry & harden. Maybe something as simple as say a car wax brushed on.

As to the question of what determines a deep knurl I don’t have a readily answer.

Guy’s I appreciate the responses and found some additional food for thought.
As to the eventual end result again if accuracy is in anyway improved then I would say the effort was worth it. If not a good lesson learned.


Ed

[DHinMN]

I have to knurl my bullets to get a tight fit. It seems that you can increase the diameter of the bullet up to .003 of an inch if need be. If you are trying to get up to an exact size it can be somewhat difficult. It can get bigger than wanted and then you roll it with a flat surface and reduce the diameter. I have often thought that it would be nice if I had a die to run them through after knurling to get the perfect diameter. The other thing is, is how far do the petals of the sabot stick out over the ogive (the place where the bullet starts to taper to the point). Some of the sabots I use have to be trimmed. So if they are trimmed so that they are out of the way of the short starter and ramrod but still long enough to stick out over the ogive then the petals ought to blossom out the millisecond that it leaves the muzzle and catches some air thus peeling it right off the bullet. Another thing is how perfectly balanced is the bullet. How perfect are they to begin with and how well are they in balance after we get done knurling them or what ever else we do with them. So I'm thinking if you think you have a problem with the sabot coming off the bullet you may have to look at the length of the sabot. Just my thoughts. DH

[Deleted]

ET
You might want to see if the corbin drum roller knurling machine,
Part # HCT-2 or the hardened drum HCT-2C. would put a more desirable knurl than the basic file method does
Greenhorn

[ET]

DHinMN

Now that is something I didn’t even consider because of my 1-line directional thinking.
Normally I trim my sabots only 1/16” past the straight wall on the 250FTX . It appears to work perfectly with this length past the straight wall.
With a large knurl being introduced maybe the ticket here is trimming my sabot say 1/8” past the straight wall that would produce a stronger affect on the sabot release.

Thanks for your reply that added a new picture on this concept being explored.

Greenhorn

The tool mentioned might produce a better knurl but way too rich for my blood to find out. I already have the Corbin canneluring tool and that was pricey enough. Everyone can't afford such a tool and what I'm seeking here is a possibilty of improving a condition that may exist with a cheaper method.


Ed

[artjr338wm]

Greenhorn,
where does one get a look (internet wise) at the Corbin machine as well as buy one?

Arthur.

[10ga]

This presents some physics and a lot of opinions to sort through. I'll try to throw a few other parameters into play.

Centrifugal force! Some years ago did a bunch of varmit hunting and whilst trying some "explosive" type boolits couldn't hit a backstop frame. What gives? Well at closer gange I was getting a "pattern"? Well, the centrifugal force was tearing the bullets apart upon exiting the barrel. Also haven't you seen those videos of the guys shooting the prarie dogs and they go spinning into the air? Centrifugal force transfered from that bullet at 1X8" revolution and 3500 FPS etc... I would think that the centrifugal force such as with the conversion barrels with 1 in 20" twist would have a lot of force throwing that sabot off. Question for you physics guys, at a given velocity what is the centrifugal force from a 1 X 20" twist vs 1 X 24" vs 1 X 28" etc... etc... for say the most common twists that are used in SML.

Then the adhesion factor. Sorta like the grip of a "rock crawler" tire and a NASCAR or drag slick. What kind of adhesion to get. I know the racetracks are sticky and the tires are sticky and that is how the cars go around curves and stick to the track. They also have about 800 horsepower ripping those stickey tires up too. On reason the tires only last a short time at the track is they are soft and adhesive and come apart. Long mileage tires are known to be "hard" and do not have as much adhesion. The ones used now in SML are hard compared to 10 years ago. Does the same apply to sabots?

Then, what if you only knurl the bottom 1/2 of the bullet? Or only with the edge of the file? Or a single vs a double cut file? Or a heavy knurl and then size down to desired diameter and you're taking the "points or edge" off the knurling?

So many variables, so many opinions, so much physics and chemistry. This will be a very interesting and ongoing post for a while. I guess mine is just $.01 worth. 10 ga

[gunny]

corbons web sight 139.00 i believe.gunny

[Jon]

Et unless I'm mistaken if you already have the tool all you would need is the rollers. It looks like you are 75% of the way to having the tool you need.
Jon

[spoonover]

Sept 11, 2011 17:01:33 GMT -5 10ga said:

This presents some physics and a lot of opinions to sort through. I'll try to throw a few other parameters into play.

This is crazy, @ 2700 fps the sabot is coming off. Plus the centrifugal force. Knurl and be content you will have perfect separation. It is not sticking, go for it!

I think Richard one of are experts has found no reason to knurl? I have found no benefit?

[shoot2reload]

Has anyone actually noticed a decrease in accuracy by knurling?

[pposey]

sure helps my Encore,,, helped with looser bullets in my savage

[artjr338wm]

Went to Corbon's website found their knurling tools. I am concerned though as they dont seem to have one for knurling all copper bullets like my Barnes TMZs and Spit Fires, only all lead bullets.

Am I missing something?

Arthur.

[youp50]

An observation;

In my 50

I get usable accuracy with a Barnes 250 TMZ. If I use the original smooth sabot and I lean on the files while I knurl.

I often notice one or two petals missing from the spent sabot. This could indicate a petal sticking to the bullet.

I still manage to maintain near 1 moa.

I had thought that the missing petal was from the stress a heavy dose of H4198 and IMR 4759 creates.

[olegburn]

If one will spin the sabot as fast as it spins when leaving barrel the centrifugal force will spread the petals like a propeller.
I'm convinced of that.
Adding the resistance of air it is guaranteed it will open fast.
I think the valid question is not sticking but how uniformly this disengagement or separation happens,though I tend to think it should have very insignificant effect on bullets flight.

[10ga]

If the sabot or any part sticks to the bullet your accuracy is shot. Just check on what happens, with BP or smokeless PPing, if the patch sticks accuracy is bad. I am certain if a sabot sticks, even for a short distance, the accuracy is way gone. With PP the patch is usually cut or heavily engraved by the rifleing and comes off at the exit and looks like a very small puff of confetti. As we all know the sabot is in relatively large pieces or, we hope, intact and goes down range a bit but not attached to the bullet. IMO the centrifugal force literally flings the sabot petals open or even off the base, with the differential ballistic coefficient of the two pieces, bullet and sabot, causing bullet to seperate from the sabot PDQ. But that is just IMO, but I think it's a pretty good theory. Anybody have a kid that needs a science/physics project for school? 10

[Dave W]

If this happens with our application, then yes I can concede you may be on to something Ed.
www.thefirearmblog.com/blog/2008/01/04/remington-accutip-bonded-sabot-slug/

[gunny]

In the spark photography pictures sure looks like it is taking awhile to seperate. To deep a knurl and a potential for pore accuracy? Maybe, gunny

[shooter]

Sept 11, 2011 20:34:50 GMT -5 artjr338wm said:

Went to Corbon's website found their knurling tools. I am concerned though as they dont seem to have one for knurling all copper bullets like my Barnes TMZs and Spit Fires, only all lead bullets.

Am I missing something?

Arthur.

Arthur,I talk to corbin last week about making me one for the copper bullet knulling machine he said he could but he said it about a month wait he told me that it will require a special wheel. I think he told me it would be around 250.00 he said he only made about 4 of this kind knulling machine for a all copper bullets.

[ccchunter]

How exactly does a knurlng machine work?

Nate

[ET]

DW

Really enjoyed the link displaying what an ideal sabot release actually looks like.
Surprisingly it triggered another question as to would a straight knurl like an oversized cannelure be better than say an angled knurl produced by a file for releasing the bullet from the sabot?
I know the plot thickens with another maybe.
Anyway my attention is being momentarily diverted for hopefully quenching another iron in the fire tomorrow.


Ed

[artjr338wm]

Shooter,
Thanks for the reply and info. Before I would be willing to invest $250, I would need to be certain of one thing.

My 10ML-II will shoot all copper Barnes bullets superbly, but only if I knurl them to within a specific OD + or - .002. Is the Corbin tool adjustable to produce two different specific knurled OD's? for example produce a finished OD of .454" from a .451" bullet, or .453-.4535" finished OD from a .451" bullet?

My guess here is I should most likely contact Corbin with these questions.

I have enough of 290 grain TMZs and 245 grain Spit Fires to make investing in a knurling machine that can give me consistently the same finished OD worth the money, but only if the tool can give me a adjustable consistent knurled OD.

I can at present accomplish this finished OD with files, but I have some difficulty producing the same finished OD bullet after bullet and have been looking for a method to make the knurling process "Arthur Proof" so to speak.

My only other alternative is buying two sizing dies and a arbor press, all of which the combined cost is over $250. So you can understand my interest in a knurling tool that MIGHT be able to solve my issues and do so simply and more cost efficiently.

Thank,
Arthur.