Split: BC discussion Pellets

[Franklink]

Over the years I have machined around 500 barrels I suspect. Most of my barrels are custom dimensioned. I used to sell some of the custom blanks to shooters without machining. The majority of these barrels were duds according to the guys that bought them. Some of them came up with the idea that I would get in a large batch of barrels and had some magic way of determining which ones were good and bad. I would then keep the good ones for myself and sell off the duds. Nobody ever came up with an explanation of how a guy could tell if an un machined barrel was good or bad....but nonetheless, the idea is still commonly heard today. Every once in a while one of these guys would send the dud barrel back to me and I would chop of their leade and crown and redo the machining to use the barrel on my personal guns. Never found one to be a dud, yet. Strange, no?

Mike

I suspect you're one of few people that have seen such a large sample size in regards to barrels. The competitive airgun shooting crowd is fully aware of the kind of accuracy your guns are capable of. With those two thoughts in mind, what say ye about what some of us have seen where certain barrel/pellet (likely even pellet lot dependent) combos seem to defy science and what every one else shooting that day is seeing? Have you seen this phenomenon in all those barrels youve touched?

 

[zx10wall]

Good point Derrick. And I think the STX liners have about the same effect as the poly barrels when it comes to BC. 
Good point with the .22 RDMs compared to the .25 Heavy. BCs are close enough that the one with the best precision is the choice. I’m trying to decide now, .22 RDMs with Bleu or .25 Heavy with Delta Wolf. More testing needed.

You should bore scope Bleu. There is a reason for those errant rare fliers and it could be hiding around that leade and TP. 

 

[Centercut]

So Derrick, you've decided that the .22 RDMs are better for precision shooting than the .25 Heavy? Hard to argue with the success you've had with the .22 Prophet shooting RDMs at high speed. I'd be interested in a shoot off between that gun and the .25 Pulsar with .25 Heavy at 100 yards shooting targets, not just groups. 

 

[zx10wall]

So Derrick, you've decided that the .22 RDMs are better for precision shooting than the .25 Heavy? Hard to argue with the success you've had with the .22 Prophet shooting RDMs at high speed. I'd be interested in a shoot off between that gun and the .25 Pulsar with .25 Heavy at 100 yards shooting targets, not just groups.

I’ll try to do that. I’ve done my own unshared comparison and well........I’m shouting the Prophet. 

 

[Franklink]

From a similar discussion over on Gateway to Airguns....I asked Bob Sterne for some input on that Reynolds number idea and he humored me/us. 

From Mr. Sterne:

"Reynolds number certainly has an effect, but I have no idea how to quantify it.... Basically it is a representation of how many molecules are flowing along the length of the slug per unit of time.... The variables are length in the direction of travel, velocity, and air density (actually viscosity).... When an object is travelling slowly, or is short, and is smooth, the flow is laminar.... The lower the Reynolds number, the thicker the boundary layer is.... If the surface roughness exceeds the thickness of the boundary layer, the flow "trips" to turbulent.... The Reynolds number increases as the length of flow increases, starting at virtually zero at the leading edge, and increasing as the flow moves further aft.... Therefore, the boundary layer starts out thick, and gets thinner as it moves aft.... At some point, either the surface roughness poke through and trips the flow to turbulent, or we ask the flow to do something it doesn't like, and it separates, or does something else, and becomes turbulent.... Once the flow tranistions to turbulent, it doesn't want to go back to laminar, although sometimes under exactly the right conditions, the flow can detach and reattach, and become laminar again after it reattaches.... You can actually get "bubbles or eddies" where the air is circling, and some of it going against the flow.... an example may be in the waist area of a Diabolo pellet....

In practical terms, the flow over a smooth nose of a bullet should be laminar.... Ask it to go around a corner (like where the Ogive meets a Meplat).... or have it run into a groove (like a lube groove or step in the body).... and it trips to turbulent.... Most ballistic models assume the flow over the nose is laminar, and over the rest of the body it is turbulent.... Right or wrong, you may as well flip a coin, IMO, but the longer the slug and the faster it is moving, the more likely that is the case.... In addition, although surface drag is lower for laminar flow, turbulent flow tends to cling better to complex surfaces, particularly those dropping away from the flow, such as a boattail.... How much can the flow turn and stay attached, or reattach.... According to Miles Morris, balistician, about 8 degrees is probably the limit for Subsonic flow over airgun slugs.... Miles has just released a proposed new drag model for airgun slugs, which I wrote an article on, you can read about it here....

https://hardairmagazine.com/ham-columns/some-new-airgun-slug-drag-models-by-miles-morris/

The biggest problem we have is that when we calculate the BC, we are relying on a drag model to compare our slug or pellet to.... and the comparison is never 100%.... Lapua do it differently, they measure the Cd for their bullets at different velocities, and then use the actual drag curve to predict the flight.... This is a far better way than using BC, but it means a different drag model for each pellet or slug.... Do you have the time to figure out the Cd curve for your own slug?.... especially with the knowledge that it can't be used accurately for a different slug?.... Yeah, neither to I....

I have been beating the bushes about us needing a change in the way we calculate trajectories and wind drift for a long time now.... I had hoped my Labradar would give me the tool I need, but it turns out what I need the most is time.... and I'm now 72.... I needed to start half a century ago....

Bob

 

[Franklink]

So Derrick, you've decided that the .22 RDMs are better for precision shooting than the .25 Heavy? Hard to argue with the success you've had with the .22 Prophet shooting RDMs at high speed. I'd be interested in a shoot off between that gun and the .25 Pulsar with .25 Heavy at 100 yards shooting targets, not just groups.

I’ll try to do that. I’ve done my own unshared comparison and well........I’m shouting the Prophet.

I was very impressed with that .22 Prophet I had for brief review, there's something to that combo of slower twist barrel and MRDs for sure. I was bummed when all the life changes required me to cut the review short. 

 

[zx10wall]

So Derrick, you've decided that the .22 RDMs are better for precision shooting than the .25 Heavy? Hard to argue with the success you've had with the .22 Prophet shooting RDMs at high speed. I'd be interested in a shoot off between that gun and the .25 Pulsar with .25 Heavy at 100 yards shooting targets, not just groups.

I’ll try to do that. I’ve done my own unshared comparison and well........I’m shouting the Prophet.

I meant shooting the Prophet but maybe shouting, "the Prophet is coming", would be more appropriate 

 

[Franklink]

From a similar discussion over on Gateway to Airguns....I asked Bob Sterne for some input on that Reynolds number idea and he humored me/us. 

From Mr. Sterne:

... You can actually get "bubbles or eddies" where the air is circling, and some of it going against the flow.... an example may be in the waist area of a Diabolo pellet....

Bob

So, perhaps the fat-waisted profile of the MRD (vs the narrow wasp waist of say an 18.13gr, .22) might help in minimizing turbulent air around the waist of the pellet, thereby helping it fly straighter for better precision, accuracy, wind resistance, etc........................Maybe. 

 

[Arzrover]

What Mike said...

I've only found 1 bad barrel in many less than Mike's experience and it had a measurably loose bore. Possibly it could be salvaged with larger pellets... i don't know. There have also been many posts on AGN that claim unchoked barrels won't shoot pellets well which is complete crap. I can't say I'm up to Mike's experience and attention to detail , but I'm learning and trying.

On the 30 vs anything else... MRDs and 25 King Heavy Mk2's didn't exist until recently which definitely affected the outcome of early EBR matches plus add in the seeming poorer performance of small calibers in FX barrels and dominance by FX in earlier events means that mostly you would see 25 and 30 cals in the winner's circle . This has been changing more recently with many brands starting to challenge FX at longer ranges and better projectiles that are proving out the bc superiority. 

Most recently, my experiments in comparing my top 3.... 22 MRDs, 25 King Heavy Mk2's, and 30 cal 51 gn, have yielded pretty much equal groups at 100 yds. Shooting for scores shows the real differences , though. The 30 cal isn't in the same league at all, blowing at least 25% more for the 51 gn and really bad for the 44 gn. The MRDs and Mk2's seem pretty close but I find the MRDs EASIER to shoot. Can't say the holdoff is less, but the recoil difference makes it less sensitive to small pressures on the rifle for my 22.

Sorry for the ramblings but in my experience, the JSB chart has a pretty good correlation to my experience. The only reason I would ever consider a 30 is just to hit something HARD... not accuracy or ballistics.

I HAVE run into a couple of oddities, though. I have a 13 ft lb rifle that will shoot right with any 20 ft lb one that I've run across in wind drift and uses 8.4s. Also, the current barrel on my USFT blows significantly less that a pile of others I've tried. Is it bore dimensions, leade, or crown.... not sure but I've prepped a fair pile of barrels with great care and these stand out. Also, when we were testing barrels for the ART project, one of the things I documented was the measured drag (to compute bc) and did so for each barrel at a couple of velocities for each pellet. These barrels were prepped to a spec by LW and it was pretty interesting to see correlations between the choke and bore profile and the bc.

Well... not sure what all that was worth... just trying to relate a little bit of experience.

Bob

 

[zx10wall]

From a similar discussion over on Gateway to Airguns....I asked Bob Sterne for some input on that Reynolds number idea and he humored me/us. 

From Mr. Sterne:

... You can actually get "bubbles or eddies" where the air is circling, and some of it going against the flow.... an example may be in the waist area of a Diabolo pellet....

Bob

So, perhaps the fat-waisted profile of the MRD (vs the narrow wasp waist of say an 18.13gr, .22) might help in minimizing turbulent air around the waist of the pellet, thereby helping it fly straighter for better precision, accuracy, wind resistance, etc........................Maybe.

That's what my brain says. The MRDs have the benefit of being drag stabilized with the attribute of being successful at high speeds for diabolo pellet. Just don't spin them so fast that the spin takes over as the speed degrades down range!

 

[Arzrover]

Interesting that you say that, Derrick. We can't change the spin RATE easily but can change the velocity and that changes actual RPM. I've puzzled over the 16.1s in 177 in that respect. Several barrels shoot them extraordinarily well at 50 but they decay badly by 100 and can only be extended a little bit by higher velocities. 10.3s and 13.4s do not seem to suffer like that. Barrels are 1 in 17 now. Would you go slower twist, or faster?

Bob

 

[zx10wall]

What Mike said...

I've only found 1 bad barrel in many less than Mike's experience and it had a measurably loose bore. Possibly it could be salvaged with larger pellets... i don't know. There have also been many posts on AGN that claim unchoked barrels won't shoot pellets well which is complete crap. I can't say I'm up to Mike's experience and attention to detail , but I'm learning and trying.

On the 30 vs anything else... MRDs and 25 King Heavy Mk2's didn't exist until recently which definitely affected the outcome of early EBR matches plus add in the seeming poorer performance of small calibers in FX barrels and dominance by FX in earlier events means that mostly you would see 25 and 30 cals in the winner's circle . This has been changing more recently with many brands starting to challenge FX at longer ranges and better projectiles that are proving out the bc superiority. 

Most recently, my experiments in comparing my top 3.... 22 MRDs, 25 King Heavy Mk2's, and 30 cal 51 gn, have yielded pretty much equal groups at 100 yds. Shooting for scores shows the real differences , though. The 30 cal isn't in the same league at all, blowing at least 25% more for the 51 gn and really bad for the 44 gn. The MRDs and Mk2's seem pretty close but I find the MRDs EASIER to shoot. Can't say the holdoff is less, but the recoil difference makes it less sensitive to small pressures on the rifle for my 22.

Sorry for the ramblings but in my experience, the JSB chart has a pretty good correlation to my experience. The only reason I would ever consider a 30 is just to hit something HARD... not accuracy or ballistics.

I HAVE run into a couple of oddities, though. I have a 13 ft lb rifle that will shoot right with any 20 ft lb one that I've run across in wind drift and uses 8.4s. Also, the current barrel on my USFT blows significantly less that a pile of others I've tried. Is it bore dimensions, leade, or crown.... not sure but I've prepped a fair pile of barrels with great care and these stand out. Also, when we were testing barrels for the ART project, one of the things I documented was the measured drag (to compute bc) and did so for each barrel at a couple of velocities for each pellet. These barrels were prepped to a spec by LW and it was pretty interesting to see correlations between the choke and bore profile and the bc.

Well... not sure what all that was worth... just trying to relate a little bit of experience.

Bob

Thank you, Bob. I'd say your input is worth a lot..........At least .02 cents 

 

[zx10wall]

Interesting that you say that, Derrick. We can't change the spin RATE easily but can change the velocity and that changes actual RPM. I've puzzled over the 16.1s in 177 in that respect. Several barrels shoot them extraordinarily well at 50 but they decay badly by 100 and can only be extended a little bit by higher velocities. 10.3s and 13.4s do not seem to suffer like that. Barrels are 1 in 17 now. Would you go slower twist, or faster?

Bob

I'd, if resources were unlimited, go with a slower twist. Pushing faster delays the bad results very little and I'd think adds detrimental aspects to the equation. 

 

[Arzrover]

I know the Prophet 22 barrel is slower twist and the ART barrels are slower twist but in the PB world, heavier and longer projectiles are typically spun faster. We are subsonic but doesn't that still apply?

Bob

 

[thomasair]

For me....when the numbers don’t add up to the observed behavior, I look very closely at how the numbers were obtained. That’s usually where the trouble is. It’s extremely easy to come up with crap data. It’s really hard to come up with valuable data. I really don’t put any value into data obtained by someone else, because I have no idea how they came by it. If it’s important to me, I will take the time to do the testing on my own while introducing the least amount of variables that I possibly can. I may come to the same conclusion. To believe that all data has equal value is asking for trouble.

Mike 

 

[zx10wall]

I know the Prophet 22 barrel is slower twist and the ART barrels are slower twist but in the PB world, heavier and longer projectiles are typically spun faster. We are subsonic but doesn't that still apply?

Bob

The simple answer is no. Pellets are drag stabilized. Bullets/slugs are spin stabilized. 

 

[zx10wall]

For me....when the numbers don’t add up to the observed behavior, I look very closely at how the numbers were obtained. That’s usually where the trouble is. It’s extremely easy to come up with crap data. It’s really hard to come up with valuable data. I really don’t put any value into data obtained by someone else, because I have no idea how they came by it. If it’s important to me, I will take the time to do the testing on my own while introducing the least amount of variables that I possibly can. I may come to the same conclusion. To believe that all data has equal value is asking for trouble.

Mike 

Very true. Being lazy, as I'm guilty of sometimes, is just easier. The easy road is broad. The narrow road is much less traveled but leads to truth. Trusting others data blindly is a recipe for being manipulated. 

 

[Franklink]

For me....when the numbers don’t add up to the observed behavior, I look very closely at how the numbers were obtained. That’s usually where the trouble is. It’s extremely easy to come up with crap data. It’s really hard to come up with valuable data. I really don’t put any value into data obtained by someone else, because I have no idea how they came by it. If it’s important to me, I will take the time to do the testing on my own while introducing the least amount of variables that I possibly can. I may come to the same conclusion. To believe that all data has equal value is asking for trouble.

Mike 

Trust, but verify.

Here's some data from a little experiment I did a little over a year ago. There are some holes in the methods but its better than just saying, "well don't you know, everybody says such and such so its just gotta be true."

Measured BCs from 3 different .177 barrels (BSA, 12land and groove LW and a polygonal LW). All in the 18-20fpe range. Used the two distance collection method (0.3 yards/1 foot and 53 yards). Ten shot averages for speeds plugged into Chairgun. JSB 10.34, from the same tin. 5600feet above sea level.

BCs:

BSA-0.0308

12 L&G LW-0.0351

Poly LW- 0.0354

From those numbers alone, the BSA SHOULD be the pellet getting pushed the most by any crosswinds. 

Next step was to shoot in some nasty winds to see. NWS reported 18-21mph winds that day, from right to left, but somewhat protected in this location. 

Shot ten shots at each target. No hold off, no timing for gusts-just held on corresponding blue dot.. Target was at 53 yards. 

Repeated that 7 times, new piece of paper each time. So, 7, 10 shot groups from each gun. 

Used caliper to measure how far EVERY single shot (210 shots) got pushed from the aim point. 

The average horizontal deflection (measured by distance between center of pellet hole to the line bisecting the aim point) was:

0.963 inches for the LW traditional

0.627 inches for the LW polygonal

0.924 inches for the BSA cold hammer forged.

More than one disparity in that data. The traditional LW had the second best BC but the worst deflection. The BSA had the worst BC but the second best deflection. The poly did not have a 30% better BC than the 12 land and groove, but DID have roughly 30% less deflection.

So maybe I'm guilty of a bit of hyperbole when I say that poly barrel only needs half as much hold-off, seems like it's maybe 30%. But why?

 

[zx10wall]

Let's say we have a tin or sample of perfectly produced 10.34gr JSB pellets; same weight, shape & size. Wouldn't we all agree that if we could propel these pellets with precision, without disturbing their perfectly smooth finish, these unaltered pellets would be more aerodynamic than pellets, from this same perfect sample, propelled through a bore that imparts intrusive gouges on their heads and skirts?

If the answer is yes, wouldn't it be logical that a bore that imparts the least intrusive changes to these pellets would produce better aerodynamic projectiles?

 

[Franklink]

So BC doesn't account for imparted rifling marks?