Does a dirty barrel cause a drop in velocity?

[PinesAndProjectiles]

We all know that you don't clean a barrel until the accuracy starts to fall off.

When the carbon and copper start to build up in the barrel, does it also create more friction between the bullet and the barrel, causing velocity to decrease?

I was up way too early this morning and these are these are the kinds of things that go through my head.

 

[Ledzep]

Not appreciably. A stripped spotless clean dry barrel can be slow for 1-3 rounds but most everything after that is pretty much the same until rifling starts burning away.

 

[Supersubes]

I’d guess no. Not in any meaningful way anyway. If there was more friction between barrel and bullet, more pressure would be developed by the burning charge, which would offset the friction somewhat. Hard to say how much. You can look at moly coated bullets as an example. At the same charge, moly bullets go slower than non coated bullets due to lower pressure generated by the more slippery (coated) bullet.

New/never fired barrels, and barrels that are squeaky clean tend to shoot the very first round slower than the average. Then round number two will be on or very near the average. For a custom barrel anyway.

 

[PinesAndProjectiles]

New/never fired barrels, and barrels that are squeaky clean tend to shoot the very first round slower than the average. Then round number two will be on or very near the average. For a custom barrel anyway.

Not to stray from the topic, but I saw this last weekend. I installed new barrel and the very first round chrony'd at 2741. The next nine were in the 2770-2800 range.

Your thoughts on more pressure equal more velocity are spot on and I hadn't thought about that.