Barrel Wizardy .... Making a "Reverse" smooth twist type barrel

[Motorhead]

Let me start with the thought had many years ago when the FX "Smooth Twist" came to market. That being of a pellet accelerated down a good length of smooth bore barrel, them slamming it into the rifling at near full speed was an abrupt thing and likely to create some issues with pellet skid and deformation

Well time has shown it did indeed have shortcoming and over these past few years the design has indeed been improved upon to a point of them being excellent shooting barrels with the correct choice of lead.

Over the years of re-barreling many actions have accrued a pretty decent stash of take off barrels and shorts from those cut off.
Which takes me to the crux of this post ..... MARRIAGE of a Smooth Bore to that of a Rifled Bore but in Reverse configuration

I had in my stash a .177 caliber EVANIX smooth bore barrel of @ 19" usable. Half dozen of so .177 cut off stubs from various barrel manufactures or rifles just shortened.
What needed to be done was push a New pellet threw the rifled barrel shorts, make sure rifling impression was uniform .... Then taking the same pellet and pushing it into the Smooth bore barrel SEEING how much more diameter crush there was / or not ( If it came out of rifled barrel and fell threw the smooth ) that was not going to work

What I found was an @ 6 1/2" section of 12 groove LW conventional rifled, that when a pellet came out and was placed into the smooth bore it was just snug enough to remove @ 80% of the rifling marks when exiting the smooth bore. PERFECT FIT of what was wanted for this R&D experiment.

** IDEA HERE IS THIS ...... SPIN the pellet first and get it moving to 50% or better of it's potential speed in a rifled barrel, Then transition it into a smooth bore where the head & skirt get a minor resize & would seal up near 100% to the smooth bore surface reducing compression losses and nearly eliminating any further bore drag. The spin created would continue having pellet exit with a smooth and non distorted head or skirt tail.

Painstakingly indexed the bore center line of the rifled stub to an @ .0015 TIR, then bored it out @ .700" deep to receive the Smooth bore barrel.
Same was done to get the Smooth barrel within the same TIR and turned the O.D. to fit the rifled stub with light interference fit.
Tad bit of feathered leade on the Smooth barrel and a good crown job on the exit of the rifled short, Some loc-tite 680 sleeve retainer and they get pressed together to made 1 barrel of @ 24 3/4"

( Pics 1 & 2 )


What a JSB 16.2 grain Beast Exact looks like Exited from Rifled then Exit from Smooth ( Pics 3 & 4 )

As you can note the rifling is impressed well and upon exiting the smooth is nearly gone having been smoothed back out ..... Pretty cool

Going to post this and will follow up on how it shoots

So .....
This barrel was made to fit the JSAR Raptor mini where it has been returned to balanced valve operation, custom valve guts, light hammer, SSG, tapered transfer yada yada yada

As set and with this barrel fitted, it is SMOKEN a JSB 10.3 at 1202 FPS !!! for 33 FPE, Shoots a JSB Monster 13.4 at 1085 FPS !!! for 35 FPE, with the JSB Beast 16.2 at 990 FPS !!! for 35.4 FPE

Shooting it today on @ 80 yards play range ( No actual targets outside small stones etc ... ) Was at 80 yards hitting a large softball size rock over and over again in what looked @ 1/2 to 5/8" spread. * This with the 16.2 Beasts.

So for a first test of concept at what is IMO pretty substantial power

the barrel works quite well indeed. This was a bare barrel not having a shroud or being tensioned. Doing so should stablize harmonics further ? and can hope better accuracy overall

* Got the air stripper made and installed so it can get a shroud and be tensioned.

That's it for another "Thinking outside the box" episode

Scott S

 

[AncientSword]

Very cool bit of wizardry...

 

[GeneT]

I'll be interested in accuracy results. It seems like the smooth bore would slow the rotation considerably. The velocity might improve, but the rotation has nothing to keep it going, and the smooth bore adds a lot of drag to the angular velocity. Could be a way to stabilize an over-stabilized projectile, but might be hard to predict. Time will tell - I look forward to seeing your results!

GsT

 

[qball]

Scott, that’s is some barrel wizardry for sure! Amazing work no matter how you look at it, can’t wait to see the final product. 


although this go me thinking, the original ST barrel’s intent is to use the smooth section to accelerate the pallet with minimum drag and give it a spin at the end. The results were pretty darn impressive but had some draw backs as you pointed out. What if the 6 inch section of the rifled barrel is 3/4 of the way towards the end of the barrel? Or add a section of smooth bore at the end of “ST” barrel or even a normal full length rifles barrel. Which I guess would act like an extended smooth choke. would a spinning projectile be optimum inside smooth bore? Would it slow down the pellet and the spin? if the goal is to smooth out the pellet for better aero dynamics then a 6 inch section of smooth bore in front of a fully rifles battle so the trick with minimum list of spin? The RD or slug like pellets need more spin than the original ST barrel’s design which is based on the traditional Diablo pellet or JSB extract to be exact. 




VERY interesting experiment!!!! 

 

[Motorhead]

I'll be interested in accuracy results. It seems like the smooth bore would slow the rotation considerably. The velocity might improve, but the rotation has nothing to keep it going, and the smooth bore adds a lot of drag to the angular velocity. Could be a way to stabilize an over-stabilized projectile, but might be hard to predict. Time will tell - I look forward to seeing your results!

GsT

Scott, that’s is some barrel wizardry for sure! Amazing work no matter how you look at it, can’t wait to see the final product. 


although this go me thinking, the original ST barrel’s intent is to use the smooth section to accelerate the pallet with minimum drag and give it a spin at the end. The results were pretty darn impressive but had some draw backs as you pointed out. What if the 6 inch section of the rifled barrel is 3/4 of the way towards the end of the barrel? Or add a section of smooth bore at the end of “ST” barrel or even a normal full length rifles barrel. Which I guess would act like an extended smooth choke. VERY interesting experiment!!!!

With the @ 19" of smooth bore, and knowing the LW is a 17.7-1 twist, the pellet is only needing to do just a tad over 1 rotation upon exit. The bore drag on pellet drops to near zero once it is resized to the uniform diameter. Rotation as well forward motion is largely unencumbered. Inertia of the spin created is in no way going to scrub off in one rotation IMO.

Another benefit is going to be a lack of leading up being there is no rifling once pellets at high speed should not be skidding against the rifling helix or forced threw a choke shaving lead further. The pellet exits perfectly smooth on the bearing surfaces and should fly w/o odd profile disruptions as we generally get.





As too long rifled barrel and short smooth bore .... see no reason why not other than as pellets speed up there leading & rifling fouling tendency increases exponentially faster they fly.



We shall see ... just sharing thoughts here.

 

[thumper]

Interesting project. Seems to me the smooth portion would really slow down any spin. But, it may be what some projectiles need, and make them less particular about twist rate. Or varying velocity may have a bigger or lesser effect. 

But man, I ain't gotta clue obviously. Be cool to see what you come up with. Rock on with it! 

 

[Long_Gun_Dallas]

Very cool project! I'm honestly not sure if I would have ever thought of this, but pretty awesome you have the tools to make it happen! Looks like promising results as well 

Here's hoping it pays off 

 

[Guest]

Cool beans. Bravo on exploring

 

[stoti]

This is a really cool project Scott. I can’t wait to hear how the accuracy testing turns out. Even if there is a bit of friction after the pellet has been sized to the bore diameter, my guess is that there won’t be nearly enough friction to slow the angular velocity enough to degrade accuracy. Just a total guess though. The bummer thing is...If accuracy turns out to be phenomenal, we will never know how much the angular velocity has been slowed or if it was slowed at all. Curious minds want to know but I guess if accuracy turns out to be great, that’s all that matters! 

I had to read and re-read how you attached the pieces a couple of times to wrap my head around it. Bad Ass. Having the tools and especially the knowledge to do something like this is so awesome. My hat is off to you. I love this kind of thing and am super glad you’re sharing with us.

Have a great weekend.

Stoti

 

[airgunmike56]

I guess we can call this the Scott Barrel, 

I wished i had the know how and the tools to do all this . My hats off to you.

Me, I would just take a FX barrel and mount it backwards , Maybe use a lot of JB wield and duck tape,

I have come full turn and have started shooting the 177 caliber again, Super accurate and often over looked caliber.

Mike

 

[Michigander]

My thoughts went to rotation as well. For two reasons. First the rifled section is short so velocity and spin rate are both still ramping up at the time the pellet hits the longer smooth section. Does the smooth section need to be so long in order to smooth out the pellet rifling? And then drag like was already mentioned.

But the answer as to effect on spin rate should be easy to obtain. When developing loads for centerfire, I would sometimes make a mark on the side of a bullet as a load identifier. I was surprised to see that mark show up on the edge of the bullet hole in the target. These are bullets travelling 3000+ ft/s. I just tried the same thing with a pellet, and yes the mark can still be seen. In my experiment, I stapled a couple of pieces of paper to the ends of a scrap of wood 7 1/4" long, marked one side of a pellet with a permanent marker, then shot through the two pieces of paper. The blue mark didn't show up super well against the lead smear, but it was there. Maybe red would work better. Anyway, I measured 213 degrees of rotation in that 7 1/4", which figures to 1 turn in 12 1/4". So just do that to compare spin rates of a standard barrel to your new design.

Hope it works out!

 

[Motorhead]

Me, I would just take a FX barrel and mount it backwards , Maybe use a lot of JB wield and duck tape,
Mike

Where this differs from a FX is where the FX barrel is a smooth bore having crushed in rifling near muzzle, pellet fits the smooth bore up until such time as it reaches rifling where it is then forced to shape shift as it passes threw the rifling's helix.
Now think about simply flipping an FX ST barrel around and shooting it backwards

Would the same thing be happening as I am experimenting with ?
NO IT WOULD NOT and here is why .....
If pellet started out within the rifling it would get uniformly SIZED DOWN where ever the rifling LAND was in contact. If the groove was also now smaller ( This starting at the same bore as the rest of barrel BTW ) then the area between the grooves makes the entire pellet smaller. ONCE PAST the rifling released back into the smooth bore it wound NOT BE IN CONTACT with the bores surface, due to now having land grooves in it suffer compression blow by and loose potential YAW stability afforded by the contact with bore

DOING SUCH A BARREL as a 1 piece affair as in rifling first then smooth bore is mechanically near impossible IMO.


Guys I'm just thinking here having had this idea for many years and now applying it all those justifying conversations in my own head are reemerging and simply going into text.

Nothing ventured nothing gained ...

Scott S 

 

[Guest]

What about a reverse gain twist polygonal rifling rather than lands and groves? Not smooth enough six sides? Go to 12.

Probably been done...

 

[Motorhead]

My thoughts went to rotation as well. For two reasons. First the rifled section is short so velocity and spin rate are both still ramping up at the time the pellet hits the longer smooth section. Does the smooth section need to be so long in order to smooth out the pellet rifling? And then drag like was already mentioned.

So what I am left to ponder is the actual RPM is directly tied to the SPEED said pellet achieves before leaving the rifling. While forward velocity increases exponentially the RPM rate or any increases stop upon exiting rifling & entering the smooth bore.


So my guess is the only way to know what speed is being created in the 6" of rifled barrel it to just shoot a 6" barrel over a chrony. Run the numbers on RPM then see what the speed is out of a full length rifled barrel and run those RPM figures.

This is going to be interesting simply because my testing with these long and heavy .177's showed crazy good accuracy at really high speed that in wrapping my mind around it would equate to a full length twist rate of a barrel substantially slower !! .... a LOT slower.

 

[Motorhead]

Over on another forum where we're having a similar conversation RSTERNE ( Bob ) made an intersting post on the subject.... THIS:



Interesting idea.... What is the twist rate of the rifled stub you used?.... You said LW, so probably 17.7"?.... I wonder how much RPM the pellet loses going through the 19" long smooth bore portion.... ie what the effective twist rate is at the muzzle?....

Pretty hard to determine the answer, but my guess is a lot less than the twist rate of the rifled stub, for two reasons.... 1, the pellet continues to accelerate but there is no rifling to increase the RPM (if it doubles in speed, the twist rate is halved at the same RPM).... and 2, there must be some bore friction slowing down the RPM in the smooth bore portion....

It would probably take marking the pellet nose with a pencil, and then shooting it through two paper screens to measure the rotation between the screens, like was done to measure the EFFECTIVE twist of the original smooth twist barrels.... Their answer varied with different pellets (due to fit) but in some tests the degrees of rotation between the screens worked out to an effective twist rate of 1 turn in 13 FEET.... Yeah, that's right a 156" twist rate.... and yet those barrels worked well with pellets.... It shows how little RPM is really needed for pellets.... 

In any case, Scott, congrats for thinking outside the box.... AGAIN....

Bob







Doing some digging also found a very interesting post from 2015 by YYRAH

**

Fredrik told me the .22 ST barrels were 1:16″ to that point in time a couple of years ago. I don’t know if the Bobcats are/were any different.

However the question you didn’t ask was what the pellet spin rate is when it emerges from that couple of inches of “rifling” after stripping through the “rifling”.
Fredrik and FX have processed many iterations of the ST barrels.
Dan Brown measured spin rate at 1:160″, (that is not a typo error) , early on..
I measured my .22 cal at 1:64″ ; and a .25 cal at approx 1:75″ from later barrels. But it also depended upon pellet weight and velocity as reflected in the momentum of the pellet when it impacted the couple of inches of “rifling” at the muzzle..
Fredrik then measured a .22 cal as 1:44″ spin rate from memory. That was from an 18th iteration barrel ( mine was a 16th).
Some two years ago after Fredrik and I had discussions about this (it had been news to him then) Fredrik was thinking of producing some for us to test in 1:30″ to 1:40 “rifling” twist . However his ST barrels were shooting as well as any other barrels, for their purposes, and as the machines that produce them are complex and expensive, I don’t know, but don’t think, he experimented further with the idea of trying to match the spin rate -to-rifling rate such that stripping of the pellet did not happen. I could conjecture that such a change may have just as likely been detrimental to the proven accuracy of the system.

But back to your actual question; 1:16″ was the standard ST rifling rate. …. Harry.

 

[Airgun-hobbyist]

I like where this experiment is going. Good on you for thinking outside of the box. 

As far as the pellet spin rate slowing down substantially on the smooth bore, I don't think it does. It probably does slow the spin rate a relatively small percentage is all. Reason being, the pellet is imprinted with rifling and is moving at a given spin rate when it transitions into the smooth bore. As it travels through the bore the rifling is smoothed out through a choke of sorts but when doing so it will still be wanting to travel down the bore in a path of least resistance from said grooves. However the longer it is inside of the smooth bore the more it will slow in rotational speed.

If you do determine that the pellet is slowing in rotation too much, maybe shorten the smooth bore and lengthen the rifled bore proportionately. 

 

[JohnnyPDX]

...Was at 80 yards hitting a large softball size rock over and over again in what looked @ 1/2 to 5/8" spread. * This with the 16.2 Beasts.

VERY good start and can't wait to see the paper with different projectiles. I won't pretend that I understand the technical discussion but I do understand results.

The successful 'marriage' of those two barrels is a mix of genius, hard work, skills, and God. Nice work!

 

[JW652]

Fwiw. the last few barrel runs that John Whiscombe used on his dedicated FT rifles are 19"polygon with a 1:1M twist. His theory was that there was no reason for a fast twist as a waisted, diabolo pellet is not spin stabilized. Plus there was no engraving damage to the pellet, thus increasing aerodynamic efficiency. It certainly works well for its intended application. As his rifles are designed to change barrels in mere minutes, it was an easy swap, Another cool thing is that the swap was the only variable.

John used to send me various design changes to bench test and also shoot in actual, two day, 100 - 120 shot matches to see how a change would perform in real world. competition. The barrel change was so dramatic that I bought several in .177. Even have a few left for projects in the future.

Thanks for sharing your own project. It will be interesting to follow.

 

[Michigander]

To calculate actual spin rate you also need to know the velocity at the target(s). Then you can apply this formula:

Velocity x (12/twist rate in inches) x 60 = Bullet RPM

I'm not saying I know what the spin rate needs to be, but you could at least compare designs with regard to that characteristic.

 

[Michigander]

Measuring my barrel twist rate at 18", I assume it is actually 17.7" as per LW.

With a MV of 750 fps I get 30,508 RPM at the muzzle using the above formula.

After marking a pellet with a RED Sharpie, I got nice clear marks on the two targets and could accurately measure the rotation at one turn in 16.3". Assuming a velocity of 700 fps at the targets, I get 30,920 RPM at the targets. Which is well within experimental variation, as the two numbers should be the same. So this very simple method for measuring spin rate actually seems to work.