“How to slug a barrel” - Looking for the best resources

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I have slugged many of my airgun barrels. In the end, while it’s nice to know your groove diameter, it doesn’t necessarily lead you down a certain path. There’s not a magic formula so to speak, that I have found.

Sometimes full groove contact has proven best and sometimes just bigger than bore riding is the ticket. And many combinations in between. Especially across pellets AND slugs.

Pellets are more forgiving, in my experience. Head to bore relation seems to be the key. In this case, a set of pin gauges is a way to determine bore diameter. I can’t confidently measure bore by slugging, some may.

I have come to the conclusion that the target seems to be the BEST way to measure for projectile diameter.

Just my opinion, I’m sure I’ll continue slugging barrels like I have always done. Just not convinced it really tells me much.

Dave
 
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There are many methods and ideas about slugging barrels but airguns offer us a new challenge...

The Choke

In reality, the slug does not need to be driven all the way down the barrel, it only needs to be pushed far enough down from the breech end to imprint the lands and grooves.

Pushing the slug all the way through results in two unwanted effects: 1.) Extra lead sloughes off the slug making you think that the bore is tighter than it actually is. 2.) If present, the choke shrinks the diameter of the slug also giving you the impression of a smaller than actual bore diameter.

I grease the bore from the breech up about 2" and PUSH the slug in a couple inches then PUSH the slug back out.

"What slug should I use?"

I strongly suggest a dead soft round ball.
 
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I agree with sb327, I've slug ed all kinds of barrels and can't say I learned all that much from any of it. There just is no set formula, that's true. The only thing that I've found from slugging is finding tight and loose spots in barrels to know where to concentrate lapping. But honestly, barrels today, especially on premium gun, and even many cheaper guns, are so good they only slightly benefit from lapping. It really only helps noticeably if you get a lemon, so I have found.
 
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sb327 said:
Pellets are more forgiving, in my experience. Head to bore relation seems to be the key. In this case, a set of pin gauges is a way to determine bore diameter. I can’t confidently measure bore by slugging, some may.
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This is how I do it.. I pin gauge the bore & If I really need to know groove Ill slam a big slug in it & back out. I do write the data down for reference but why I really do it is to find barrel problems... all my LW barrels have problems that had to be delt with for my sanity. The dang things do wierd sh.. when there machined & tight spots are present where they shouldn't. Also I do not trust anybody's crown jobs at this point & take care of those with long slugs & pin gauges.
I'm a pellet shooter for the most part & consistent bore is my game with 0 problems at crown. So I would just slug for groove & pin gauge for bore. But this will not be the entire story at all for bad spots in barrel. try running a proper pin gauge thru & you will know.well,...bore at at least. I prep out the tight spots ot then run a good pin gauge slowly thru with a dowel.. tells me alot Good pin gauges won't have a sharp shoulder but definitely harder then the barrel steel I play with so I go slow, very slow.
You could also send it off to a willing barrel maker for proper measurement I suppose.
 
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A rifle barrel and chamber is measured with cerrosafe. Linotype will work too. It's a hot lead casting that does not shrink when it cools. I only know one fellow who has actually done it. He didn't learn much from the exercise.

A soft, short lead slug of the appropriate diameter pushed in a couple inches from the breech and then pushed back out gives you a good idea about those first couple inches. The same with the muzzle end. Past that "slugging" a barrel won't tell you much.
 
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"Linotype or eutectic alloy is a broad name applied to five categories of lead alloys used in manufacture of type, especially for the Linotype machine, each with three to five sub-classifications. One alloy is composed of lead with 4% tin and 12% antimony."

Casting your breech with linotype will result in poor results.

Cerrosafe is NOT a slug, as said above it's for chamber and headspacing.
 
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Firewalker said:
"Linotype or eutectic alloy is a broad name applied to five categories of lead alloys used in manufacture of type, especially for the Linotype machine, each with three to five sub-classifications. One alloy is composed of lead with 4% tin and 12% antimony."

Casting your breech with linotype will result in poor results.

Cerrosafe is NOT a slug, as said above it's for chamber and headspacing.
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To be clear I did not say cerrosafe was a slug. I contrasted cerrosafe with using a soft lead slug.

Cerrosafe and linotype both are used for castings of the barrel, throat and chamber to attain accurate inside measurements. Either could certainly be used to determine the dimensions of an airgun barrel. Of course the low temp cerrosafe would be best.

Linotype has very little shrinkage. Thats why they use it for typesetting. It has been used for casting and measuring inside dimensions on machine work for many years. There may be alloys that won't work well. Those weren't the alloys I was speaking of.

We both know exactly what I'm saying and know it to be correct. Why the rub over minutia?
 
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Bedrock Bob said:
To be clear I did not say cerrosafe was a slug. I contrasted cerrosafe with using a soft lead slug.

Cerrosafe and linotype both are used for castings of the barrel, throat and chamber to attain accurate inside measurements. Either could certainly be used to determine the dimensions of an airgun barrel. Of course the low temp cerrosafe would be best.

Linotype has very little shrinkage. Thats why they use it for typesetting. It has been used for casting and measuring inside dimensions on machine work for many years. There may be alloys that won't work well. Those weren't the alloys I was speaking of.

We both know exactly what I'm saying and know it to be correct. Why the rub over minutia?
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Because it's inaccurate and not prescribed in gun smithing.

No, it's not correct, nor has it been for over 100 years of scholastic gunsmithing.

I carried a gunsmthing license for 37 years, I cast thousands of chambers an slugged as many barrels on direction of a master gunsmith.

Nothing here you mentioned is minutia.

In engineering, linotype was used in much larger spaces where shrinkage was recorded in the Engineering Handbook (Bible) but is not suitable for barrels.

Shrinkage is the key, had you researched this, you would have known.
 
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DesertSilver said:
For slugs they should be between.0005 and.001" larger than the groove diameter. That's from lani52 or roach Creek. As far as pellets????
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I carried this same thought process over from my pb casting days. In larger caliber airguns, it worked out sorta ok. I ended up ~ .0005” below groove. That’s only in a couple cases and I do not suggest that as a rule. It’s just what worked in those guns.

Smaller caliber, like a .22, I have found likes to be smaller in relation to groove than the big bores (in most cases, not all).

This is why I can’t, with confidence, suggest a formula for the correct size projectile based on bore/groove dimensions.

All the suggestions I have seen on the internet have been right…..and wrong.

Airguns are just way more sensitive to the relationship than pb and I believe friction plays as much into this as projectile control.

Dave
 
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