What is the Purpose of Knurling on Slugs?

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Ezana4CE

Ezana4CE

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I’ve been trying to figure out if knurling reduces or helps a slug's velocity. So far I’ve read that knurling can reduced leading and it can hold lubricants better. I haven’t found much more on the topic. I’ve seen some .45 slugs that @nielsenammo makes that are knurled and I’m wondering why just that caliber and what’s the purpose? What are the other benefits of this feature of slugs for airguns?
 
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It will obviously depend on the application. Knurling bumps up the size of a slug. If your slug already fills the grooves, it may slow it down. If your slug barely rides the bore, it can increase sealing without drastically increasing friction which can help velocity.

I knurl slugs for my huben so the slugs fit the mag snug, align to barrel better and doesn’t significantly increase friction/fit with a large slug. I bump .218 slugs up to .223 and my accuracy/consistency improved significantly.

It obviously helps hold lube.

The way I knurl is very repeatable and is all homemade stuff.

Dave
 
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sb327 said:
It will obviously depend on the application. Knurling bumps up the size of a slug…
I knurl slugs for my huben so the slugs fit the mag snug, align to barrel better and doesn’t significantly increase friction/fit with a large slug. I bump .218 slugs up to .223 and my accuracy/consistency improved significantly.

It obviously helps hold lube.

The way I knurl is very repeatable and is all homemade stuff.

Dave
Click to expand...
@sb327 How does knurling increase size? You can bump the size up from .218 to .223 via knurling? How are you knurling your slugs?
 
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Ezana4CE said:
@sb327 How does knurling increase size? You can bump the size up from .218 to .223 via knurling? How are you knurling your slugs?
Click to expand...
If you have a cylinder of lead and make divots in the surface, the lead that has been displaced has to go somewhere. I tried to make an exaggerated illustration:

IMG_7867.jpeg


I assume that given proper knurling you get a better seal because you've slightly expanded the diameter of the slug and should get a slightly more snug fit.
 
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Thank you MrP. Years ago before there were a bunch of diameter choices per caliber, we would get two files and knurl our slugs. Knifemaker put me onto how to do it because I had the daylight around the slug blues. Knurling helped but it was a pain in the buttocks. It’s part of the reason my second slug gun eventually got put back on pellets and it’s original barrel.

The knurled slugs shot so good I didn’t want to use anything else. But I also didn’t want a life sentence of knurling. I wound up using that brand and caliber barrel later on other builds when all the slugmen started producing the diameter I needed or at least bigger so I could size them.
 
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I cut into the slug positioned between two stacks of utility knife blades. The blades stacks are set parallel on a couple blocks of aluminum with a set spacing between.

The slug is set on one and the blocks pushed together. Then slide the blocks to roll the slug between the blades until the two blocks touch.

I added a picture of two slugs pushed through a barrel, one with and one without the cuts. This was my first experimental setup. I don’t use as many blades now and I have the stacks spaced up some.

Dave
IMG_5606.jpeg
IMG_5610.jpeg
 
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sb327 said:
I cut into the slug positioned between two stacks of utility knife blades. The blades stacks are set parallel on a couple blocks of aluminum with a set spacing between.

The slug is set on one and the blocks pushed together. Then slide the blocks to roll the slug between the blades until the two blocks touch.

I added a picture of two slugs pushed through a barrel, one with and one without the cuts. This was my first experimental setup. I don’t use as many blades now and I have the stacks spaced up some.

DaveView attachment 476059View attachment 476058
Click to expand...
Excellent diy setup!
 
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Corbin has one $260 one at a time or looks like maybe ya could lay 3 in there at a time, looks painfully slow to me,,
I want to just drop them into a tube feeder with a hand or elec crank feeding them through like an old time pencil sharpener ,sort of
www.corbins.com

Lead Bullet Knurling Tool

- Applies a diamond pattern of knurling on lead bullets - Create enhanced lubrication grooves - Versatile tool for various bullet calibers - Improve bullet quality and performance
www.corbins.com www.corbins.com
 
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@jarmstrong I’m aware of that tool and other methods used to creat knurling patterns. What I’m trying to understand are some of the benefits. The one that I have been unable wrap my head around is the creation of an uneven surface resulting in a better or even a good seal in the barrel when shot from an air rifle. I an unable to imagine how that would happen.
 
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When you knurl, it increases the diameter. How? That was explained. You are not removing material by cutting/grooving, you are displacing it by let’s call it smearing. Now the benefit is your increased diameter is a friendly soft material. It’s not dense like just simply using a larger diameter slug. With knurling, it conforms easily. So in a nutshell, if you are not getting the results you want, knurling can accomplish two things. You are using a slug that matches your choke and now you can make it match your leade diameter also. Then is easily crushes down to the choke diameter before Elvis leaves the building.
 
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Vetmx said:
When you knurl, it increases the diameter. How? That was explained. You are not removing material by cutting/grooving, you are displacing it by let’s call it smearing. Now the benefit is your increased diameter is a friendly soft material. It’s not dense like just simply using a larger diameter slug. With knurling, it conforms easily. So in a nutshell, if you are not getting the results you want, knurling can accomplish two things. You are using a slug that matches your choke and now you can make it match your leade diameter also. Then is easily crushes down to the choke diameter before Elvis leaves the building.
Click to expand...
@Vetmx. Those points are not lost on me. A couple of members explained that pretty thoroughly. The illustrations are helpful.
MrP said:
…I assume that given proper knurling you get a better seal because you've slightly expanded the diameter of the slug and should get a slightly more snug fit.
Click to expand...
The above assumption is what I have been unable to make sense of. The expanded areas are parts of an uneven surface. I’m trying to understand how air won’t escape around the projectile.

It’s good that others seem to understand this. I’m just asking for them to help me understand this man’s assumption because I do not get it.
 
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I will say a conventional divot knurl will promote sealing because it will allow the grooves to be filled more (if not entirely). I understand the difficulty understanding this but it doesn’t need the entire surface to be able to seal. As long as the groove gets filled, that groove is sealed. It doesn’t have to be sealed the entire length of the bullet’s bearing surface.

The way I bump them up essentially creates a number of driving bands similar to what a cast bullet has. Altaros turned slugs do as well.

Dave

Here are before and after but not pushed through a barrel.
IMG_6050.jpeg
IMG_6049.jpeg
 
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