Paintball Weight, Diameter, Volume, and Surface Area Effect on Velocity

[nak81783]

I'm not shy about the fact that I hate modern, crappy paint. I personally think it's likely good coming off the line, but poor shipping and storage degrade its shape by the time I get to use it. I'm also not shy that, because of this crappy, modern paint, I don't think barrel kits are effective or worth the cost. Closed bolts should overbore with RedLeaderSB's barrel detents. I use .693 CP 14" Advantage barrels. If/when those get beat up too much from the local cement bunker course, I'll use the stock barrels that came with my Etha 3, 3M, and EMF100 (.689, 14.5", Shaft 4, I think). Overboring is less efficient and louder, neither of which concern me, but it is the best protection from barrel breaks. My argument is one ball may be too small, one ball may be just right, but then the next ball is too big and breaks in the barrel. Even one barrel break a day is too many, in my opinion.

I fully admit a good paint to bore match (slight underbore) can result in 30+ fps gains compared to overboring.

I also admit that if (and it's a big if) you can be certain that no paint in a bag/case will exceed a certain size, you should underbore a bit based on the the upper limit.

However, I don't think barrel kits in .005" increments (or smaller) are effective when paint can vary .015"+ in a bag. I've seen over .040", but evidence of .036" is here - https://www.mcarterbrown.com/forum/p...s-rate-of-fire

All testing below was done with an Etha 3M with a CP 14" .693 Advantage barrel, which measured .694 at breach and .693 at muzzle.

All that said, I've recently heard that the air blast will deform a paintball some to make a seal in the barrel. This seems feasible, to some extent, so I wanted to see the effect of weight, diameter, and volume on velocity. At the same time, the charts will show diameter variation, which, I think, backs up my view on barrel kits. The first two screenshots below show all the measurements taken to test this on 3 different paints. It also shows a control group with very consistent (weight and diameter) Nylon riot balls. They're meant to be eye charts, as the testing didn't prove much. Not shown (even more eye charts), I sorted by weight, seam min dia, seam max dia, polar min dia, polar max dia, ball sphericity, and ball/ellipsoid volume to see if there was any relationship to velocity. I found none.

I've alway thought velocity is most dependent on how each ball loads - the surface area presented to the air blast and how it interacts with the ID of the barrel. For this test, I had to carefully load each ball to get the seam diameter (which is typically larger than the polar axis diameter) perpendicular to the axis of the barrel to the best of my ability. It was tedious, and I'm sure it still wasn't perfect. I had each ball forward against the detents (not against the bolt) just for the sake of consistency, and to limit, however little, how much the ball would roll as the bolt pushed it into the barrel. I had to carefully line the gun up with the chrono, so the ball didn't shift in the breach as I moved the marker. I should have started with this. It was less work, and begins to show a correlation. See screenshot 3 below. Some of the shots that don't correlate may be excusable due to other factors, weight or the repeatability of the marker (as demonstrated with the nylon balls), for example. I understand average surface area has to go out to 3 decimal places for most shots, and up to 5 between two shots, to sort by average seam surface area, but the data is what it is.

I will try to repeat the test a couple more times to see if the correlation peeking its head continues.

I welcome constructive criticism and thoughts.

21-Mar-2026 Edit: I was reviewing this for something else, and I realized I had a bad formula for the average weight of the nylon riot balls. It should be 3.667 grams, not the 3.604 grams shown.

[flyweightnate]

Wow...

First, you must be a patient man.

Second, you have conclusively proven our paint options are trash and trash. The spread on mass alone is astounding, let alone the .030" size differences.

I should make a shaker with sized screens, roll up to a tournament, and charge $20/bag to "size" someone's paint into 4 different lots. Cuz you're right, with a spread that spans 4 or 5 inserts, a barrel kit isn't doing a ton these days.

[nak81783]

I'm patient...to a point...with some things. Other things allow me to immediately and spontaneously concoct curse tirades, like Darren McGavin in A Christmas Story, but not PG rated.

Your sized screen shaker would be snake oil. It all depends how the paintballs are oriented when they meet the screen. We think we're shooting spheres, but we're shooting spherical polyhedrons, something like soccer balls, but with non-uniform facets. It'll fit through one circle in one orientation but not in another orientation.

[Jordan]

Why do we think paintballs deform to fit the barrel?

I thought Tom Kaye conclusively proved they did not, years ago, via a series of tests.

[nak81783]

It sounded feasible, to an extent (maybe ten-thousandths to a few thousandths), but you're not going to make a .670 paintball grow .019 to fit a .689 barrel, without breaking it, in my opinion. If they did deform .010 or more, I think we'd see more consistent velocities and a lot more bounces. Since the velocities are still quite erratic, I believe that's at least evidence towards them not deforming much at all.

[nak81783]

I forgot to include this picture in the first post - a sharpie, calipers, a scale, and lots of measuring. The numbers helped me orient the ball for all the different diameter measurements. Obviously, I had to do another set of 10 for the second data set.

[BretG]

Tom Kaye's test used a clear acrylic barrel and talcum powder and showed that the paintall always rides on 2 contact points. There is always gas leakage around the ball and no significant derormation of the ball. Every shot had dust escaping before the ball and disproved the pressure seal idea.
I think of it as the ball rides a wave like a surfer rather than under pressure like a bullet.

[nak81783]

Screenshot 1: Initial seam oriented load data. The ball was meticulously loaded with the seam perpendicular to the barrel axis. This showed pretty good correlation of average surface area to velocity.

Screenshot 2: Repeat of seam orientation load data - even better correlation of average surface area to velocity.

Screenshot 3: Oriented load data based on polar diameter instead of seam. The ball was loaded with the polar axis perpendicular to the axis of the barrel. This resulted in great correlation.

Screenshot 4: Back to seam orientation but randomly loaded to see if correlation went away. I observed the randomly loaded ball just to see if they loaded in different orientations, but I didn't manipulate the loaded ball in any way. The correlation disappears.

I'm not a stats guy, so if I'm doing something stupid, let me know.

I also edited the first two screenshots (4 sets of data) in the original post. I messed up the volume calculation, but it doesn't change the lack of correlation.

Edit:
1. Screenshot 5: Chart of all 30 balls loaded in a controlled manner.
2. Should I be using a 2nd order polynomial for the trendline and R² value, since the x-axis is Pi x radius² to correlate against the y-axis velocity value?

[cockerpunk]

this is a CRAZY amount of work. kudos for doing it.



and it long confirms what we learned with barrels, accuracy etc ... the paintball is the problem. the barrel, the regulator, the action, etc, all of that is actually far far far better controlled than the paintball itself is. and therefore the issue with paintball guns, is mostly in the paint itself.


one of the things id love to see is a flow visualization of the vortex shedding based on different imperfections in the sphere of the ball. it is my opinion that the source of most of paintball's inaccuracy is due to a non-spherical ball having preferential (ie not random) vortex shedding. i first started to notice this when using the golf ball dimpled powered balls.

[uv_halo]

I'd say not even simply preferential but, a dynamically preferential vortex shedding due to the spin of the paintball.

[uv_halo]

Nice Work!!

It's got me thinking about some things:

The Weight: Unless the paintball specification for sizes and weights has changed since 2014, these balls are at the upper limit. In 2014 at least, the max weight was 3.5g For perspective, here's some old 2008 data (average weights over 20rds each):

• Marballizer: 3.1126
• RPS Stinger: 2.8076
• DXS RecSport: 3.1561
• MonsterBall: 2.6006
• Diablo Heat: 3.0405
• XBall Gold: 3.1504

The Dimensions: I think getting accurate measurements of ellipsoids can be difficult, especially so with a pair of calipers. Now that I think about it, I think these days, it may be worth getting some 3D scans of paintballs (given that scanning services have gotten so much cheaper), while measuring their weights. Once digitized, it should be relatively easy to see how misshapen these things are. Assuming that they have gotten a little bit smaller than paint of 2008 (as many folks believe) that would mean that these balls have a better sectional density and thus a better ballistic coefficient than the older paint. A little min-maxing on the industry's part?

[nak81783]

I’ve never seen an ASTM certification on a box of paint that I can remember, so while their weight is at the upper limit, which is good for sectional density, as you point out, I don’t think the paint companies care much about the ASTM spec. Or they don’t guarantee it once it leaves their facility.

Unfortunately, sectional density and ballistic coefficient don’t mean much if it doesn’t fly straight.

I’m confident in my measurements. The first handful I checked multiple times to make sure it would repeat, and every time I went back to check the outliers, the measurements repeated. If you use the thicker part of the calipers, further up the throat, it gives some grace to ensure you’re getting the actual widest part of the ball on that orientation.

I won’t claim to be an expert on scanning, but for the stuff I’ve used and seen demo’ed, I wouldn’t trust it down to .0005” or even .001”. Give me calipers and micrometers any day, especially for stuff like this. CMMs and scanning are useful for profiles, true positions, and other GD&T, but hard tooling still wins the day on small diameters like this. The scanning companies talk a big game, but when it comes time for the Gage R&R, their claims usually fall apart pretty quickly. Notice I singled out scanning in that last sentence. Conventional CMMs (touch probes) can be extremely accurate and precise; they’re just time consuming to use for stuff like this when calipers/micrometers are available.

[uv_halo]

nak81783 I think you don't understand how the ASTM works. Good news, I have a thread just about it. That being said, there is no marking requirement for the ASTM, that's a marketing/business decision of the manufacturer. A paintball manufacturer wants to be sure their product adheres to the specification because if someone gets hurt, and independent testing shows it's the paintball's fault, the manufacturer will be found liable.

Totally fair point about measurements. I've used calipers quite a bit myself in measuring First Strike rounds. I've had plenty of engineers point out that getting the diameters would be best done with ring gauges. And based on my experience with dealing with the seams on FS rounds, I agree.

I'm not an expert on 3D scanning either, so that was a bit of an assumption given how much I see it used by folks replicating small objects. I wouldn't be lookign to obtaining absolute measurements but rather relative measurements (in other words, replicating the shape and proportions rather than the absolute size).

From my understanding of how paintballs are made, there is nothing that says a paintball has one pole perpendicular to the seam or, even a single pole at all. If I use a globe's latitude and longitude as an imaginary reference: The "south" pole (as in the southern most point away from the seam) for a given ball could be at LAT 5S and LONG 5E, while the "north" pole could be at 10N and, 25E. The dies are as round as they can afford but, the rest of the process dramatically influences how the "ball" ends up. Then there is the issue of when the seams don't line up.

[nak81783]

My statement about paint companies not caring about the ASTM was wrong. It was more a jab at poor quality paint, but as I’ve said before, it’s probably good coming off the line and degrades during shipment and storage.

I’m quite familiar with ASTMs and other specs/certs/testing. I have to deal with them more than I’d like.

[cockerpunk]

not sure surface scanning is truly accurate enough. you could just model some common distortions at common magnitudes, and see how they effect the vortex shedding.

and i think your right about spin. we found that most of that of the time spin was very very low, but it was enough that in full flight, the ball would spin a few rotations. that would expose different shapes to the air, in different directions, causing the net force of a non-random vortex shedding to move. and thus why paintballs kinda wiggle around sometimes.

[uv_halo]

I wasn't sure either but, I suspect it would be useful for determining the shape, and proportions.

I remember the improvement rntlee over on TechPB found when he used paper towel sabots to keep them from spinning up in the barrel.