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Discussion Starter #1
This has been bothering me for some time since I have barrels of different bore sizes. I know that the more matched my barrel diameter to the paintball is, my velocity will go up.

When t comes to paint, my field uses both GI SPortz and Valken so I purchased a caliber at my local Home Depot and measured a few of my paintballs.

The box says .68 and yet on the average, my paint is measuring roughly .6875 inches and I did the test several times with different balls I had laying around my closet (mind you my closet is in my office that has the A/C on all the time, so I would say it is climate controlled.

Now the real kicker is the paintball barrels as I know that paintball can swell and deform.

My Rap4 rifled barrel which is supposed to be .68 is actually .71875 inches.

My Lapco Big Shot Assault barrel that is stamped .687 actually measures the same as my Rap4 barrel (.71875 inches).

I did the same test with all my barrels and none was exact.

What gives?

Am I going something wrong?
 

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probably, how are you measuring the barrel's bore? and the paintballs?

in reality, it doesnt matter what the numbers are, do a blow test with the paint in the barrel that will give you the best way to physically see how they are sealing, it doesnt make a difference as to what the numbers say if its not sealing its not sealing

this is partly why I have always found ball sizers unnecessary when you are running a barrel kit, who cares what the number is as long as the ball seals properly in the barrel
 

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Actually I have done the "ball through" the barrel test. Some of my older, larger bore barrels, the current paint just falls right through. I was just curious when I realized that mair speed did not change between my Rap4 .68 barrel and my Lapco .687. I would have thought the velocity would go up with the .68 but it was pretty much the same as my Lapco.
 

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Paint size varies wildly by make, model, quality control, storage, humidity, etc. Even from batch to batch. "68 caliber" paint can fall anywhere from .670" to .695", though it's been trending on the minuscule side in my experience lately.

Here's a random post I made a few months ago on another forum:

me said:
[rambling musings]

with regard to the over/underboring discussion above, it's been my experience that....

  • Consistent overboring (paint can easily fall through the barrel with no friction) provides a consistent velocity, is less efficient than underboring, is more forgiving with bad paint, but is a PITA on pumps/closed-bolt markers due to potential for roll-outs.
  • Consistent underboring (a tight fit that will generally blow out) provides a consistent velocity, is more efficient than overboring, is less forgiving with bad paint and prevents roll-outs on pumps/closed-bolt markers.
I prefer to modestly underbore when possible on my pumps both for the anti-roll-out measure and the efficiency gains, and generally overbore open-bolt semis because it's a hassle-free way to avoid suffering bad paint and I don't mind losing a bit of efficiency when I'm already using a constant air tank. I try to avoid the middle ground, where some paintballs roll out and others get stuck, whether due to seam orientation or poor batch quality. This type of paint/barrel fitment is the least consistent.

[/rambling musings]
 

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Discussion Starter #5 (Edited)
Paint size varies wildly by make, model, quality control, storage, humidity, etc. Even from batch to batch. "68 caliber" paint can fall anywhere from .670" to .695", though it's been trending on the minuscule side in my experience lately.
I would like to thank those people that have responded but the question was not so much the paint as I measured it at .6875 and I know that paint does swell and shrink and the manufacture can sometimes be imperfect.

I also know and thank everyone as to the differences between underboring, bore matching and overboring.

My real question was the barrels. Why did each barrel that I measure, show a bigger inner diameter than what the manufacturer claimed. My original career was in engineering and I understand about tolerances, and that even metal can expand or shrink, but why were my caliper reading so much off what the manufacturer claimed (and it wasn't that the caliber was not zero'd out properly).

Other than just dropping a few test balls through each barrel to see whether it goes through easily, has to be blown through or just jams, is not to me the best test, and makes purchasing future barrels difficult if I cannot trust what the manufacturer says.
 

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Discussion Starter #7
The Lapco and all the other barrels are stamped with the inner bore size. The one I purchased from RAP4 does not have it stamped but this is from their webpage:

"NEW .68 Caliber Tactical Rifled Barrel. The high quality of these rifled barrels will give you supreme performance on the field! The new Tactical Rifled Barrel is designed for scenario paintball markers such as those from Tippmann, BT Paintball, and our RAP4 T68! These barrels are designed to accept a handguard, front sight, rail system and even grenade launchers! These new Tactical Rifled Barrels are designed on the outside for looks and engineered on the inside to greatly improve accuracy and reliability. Available in 12", 14", 16" and 18"

The Tactical Rifled Barrel is key to converting a paintball marker for scenario play. It can be used as an attachment point for all manner of accessories, as the Tactical Rifled Barrel is scaled from military specs and takes most attachments without anything modifications. The barrel can accommodate rails, mounts, laser sights and grenade launchers without making any changes to the marker or the barrel.

The Tactical Rifled Barrel will not just make your marker look and feel better, but will also enhance your accuracy as well!

This barrel will fit ACI Illustrator, BT, BT-4, Procarbine, Tippmann A5, and Tippmann X7 markers."
 

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the calipers are your problem, you need to be using a bore gauge since the calipers will be measuring to the corner of the flat surface that contacts the barrel which is not the actual bore

measuring cylinder bore - ClassicOldsmobile.com

someone asked basically the same question here, scroll down to the photos

back when I got my freak kit I had heard that there were some bad batches made and wanted to make sure mine was not one of them so I used the bore gauges and a micrometer and all the measurements came out to within .001 of what was printed (rounding error and slight differences between measuring it with the bore gauge and then measuring the bore gauge with the micrometer) well within acceptable tolerances
 

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but its still a difference so while yes I agree the numbers should be coming out smaller, it doesnt really matter because with calipers you are not going to be able to measure the actual bore anyway

although if the narrowest part isn't at the neck (which isn't uncommon) then that would hold true.
I wouldnt be surprised to find this is true, the bore they stamped is further in the barrel than the calipers can reach

While I am not sure of lapcos QC process, I can say of Rap4 that they have some pretty bad QC. My father in law way back when he got his T68, it came with the wrong main spring in it, basically he could not chrono under 320 at all. While at D day, just while passing through the Rap 4 tent we saw at least 2 people with the same issue with markers they bought from the rap 4 tent (and in reality I spent maybe a half hour in that tent total through the week and saw 2 people have this issue) the guys running the tent knew what was going on, apparently it is not uncommon for Rap 4 to send out these markers with the wrong springs installed, how hard is it to realize the color of the spring you are putting in a marker? anyway, I wouldnt be surprised to find out bad QC happens in other parts of the Rap4 factory as well
 
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