Slugging a 303 barrel?

[Ds J]

All 303 barrels do not have the same calibre.

The measurement between the grooves is supposed to be .312" / 7.92mm. (wikipedia)

The measurement between the lands is supposed to be .303" / 7.69mm.

When reading one learns that the measurement between the grooves varies from .311" / 7.89mm to .318" / 8.07mm. (Dr Google)

To get proper accuracy from a barrel, one must have a bullet which fits the barrel. Hence one needs to slug the barrel. I have done this a few times, and the results are not completely satisfying yet.

For my last measurement, I melted down a few air rifle pellets in a .32 pistol case. This gave me an elongated slug of 7.6mm diameter. I then proceeded to whack the slug slightly with a hammer to fatten it up a little. Only then did I hit the slug through the well oiled 303 barrel.

First question:
How does one measure a five-grooved slug accurately enough to determine the true calibre?

Second question:
Why do the measurements differ?

I did the whole melt-and-measure procedure twice and got different results: the first one measured an even 7.9mm diameter across the widest points of the slug, and the second turn it measured 8.05mm across the widest points of the slug.

[Tripodmvr]

All 303 barrels do not have the same calibre.

The measurement between the grooves is supposed to be .312" / 7.92mm. (wikipedia)

The measurement between the lands is supposed to be .303" / 7.69mm.

When reading one learns that the measurement between the grooves varies from .311" / 7.89mm to .318" / 8.07mm. (Dr Google)

To get proper accuracy from a barrel, one must have a bullet which fits the barrel. Hence one needs to slug the barrel. I have done this a few times, and the results are not completely satisfying yet.

For my last measurement, I melted down a few air rifle pellets in a .32 pistol case. This gave me an elongated slug of 7.6mm diameter. I then proceeded to whack the slug slightly with a hammer to fatten it up a little. Only then did I hit the slug through the well oiled 303 barrel.

First question:
How does one measure a five-grooved slug accurately enough to determine the true calibre? I have seen a gunsmith using a tool that has a collet that expands when a central knob is turned. I presume it has 5 fingers that open to fit into the grooves. This can now be checked against a circular pipe that has a specific diameter.

Second question:
Why do the measurements differ?

I did the whole melt-and-measure procedure twice and got different results: the first one measured an even 7.9mm diameter across the widest points of the slug, and the second turn it measured 8.05mm across the widest points of the slug. I am taking a guess that the lead is harder than it should be and spring-back occurs

X

[big5ifty]

First, get someone else to measure it as well.

Two people can get a different result measuring the same thing.

Second, try using a micrometer instead of a vernier. It's easier to deform what you're measuring with a vernier.

[Ds J]

Thanks Folks, will do.

[Treeman]

Put your slug in and push it down abit. Then put a rod in from one side another from other side and tap the plug a few times to expand it properly before pushing it through. It just fills out better, yes pellet gun pellets work well.
The slug must be long enough to avoid rocking in the bore, have a reasonably long bearing surface, this also helps when using a vernier.
A vernier is not the best tool as mentioned, the way it is held applied and used will affect your reading.

[Ds J]

How long should a measuring slug be? I had 13mm of usable bearing surface.

[Treeman]

How long should a measuring slug be? I had 13mm of usable bearing surface.

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Although I do not think there is a rule, I would say 30 - 50 % longer than the bore is wide. I helped a bloke once who could not read his slug, when I got there he had a piece of 0.5 cm oddly shaped lead that he had moered through the bore a few times in frustration - it looked like a badly made rugby ball, all rounded, no parallel edges. 

[Ds J]

Put your slug in and push it down abit. Then put a rod in from one side another from other side and tap the plug a few times to expand it properly before pushing it through. It just fills out better, yes pellet gun pellets work well.

I think this will be the way to go. Proper pressure to form the lead.

[oafpatroll]

Another thing that helps is having the purest lead you can get so that it conforms relatively easily. Lead plumbing pipe and flashing seem to be really soft. The few times I've done it I have also lubed the barrel. Not sure if that was a bad thing to do but it seemed to work reasonably well.

[Treeman]

Another thing that helps is having the purest lead you can get so that it conforms relatively easily. Lead plumbing pipe and flashing seem to be really soft. The few times I've done it I have also lubed the barrel. Not sure if that was a bad thing to do but it seemed to work reasonably well.

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This is about the way its done, soft lead and a good lube.

[big5ifty]

I did this myself yesterday on a .375 barrel.

I started with a 45 caliber muzzleloader ball which is pure lead, worked it a bit, then ended up with a slug by passing it through a .377 sizer. I then tapped it into the muzzle about two inches , then tapped it back out.

I want to try a sulphur cast of the last two inches of muzzle to verify the slug.

[big5ifty]

Done. This is the last few inches from the muzzle.

Mix of moly powder and sulphur powder, around 1 to 5 ratio.

Plug the barrel with a felt wad.

Melt in a cast iron ladle, pour.

[Treeman]

The reasoning in regard to the moly powder ?

[big5ifty]

Some say that the sulphur on it's own might stick, so they add a bit of graphite powder. I don't have graphite powder but I do have a bucket of moly, so I just used that instead.

It's one of those things that I tried without expecting it to actually work, and it turned out very well.

[Ds J]

Which other materials can be used for such a cast?