TargetTalk

I attempted to slug the barrel of my Pardini SPBE .32 ACP today and came up with 0.314". Is this possible? All along I thought it was supposed to be 0.313".

Maybe you just horribly screwed up. It just can't be!!!

Rover wrote:Maybe you just horribly screwed up. It just can't be!!!

That's what I thought so I did it again. I still horribly screwed up.

Mine is 0.314"

Depending on the quality of your micrometer. .314 may be right. Was the actual measurement between .313 and .314?

Beezer;
How did you slug the barrel?
From the muzzle or the chamber?
Just an inch or fully thru the barrel?
What did you use as the slug? And dimension before slugging?

The first time I slugged the barrel with just a .314 diameter bullet thinking the barrel was .313. The second time I melted a slug in a case with an ID of about .316 and knocked it out with a bullet puller. Both times I pounded it through the muzzle end about 1/2" and punched it out with a wood dowel. I used a good set of Mitutoyo digital calipers and ended up with a maximum OD of 0.3140 both times. If my procedures needs improvement please let me know.

0.313 - 0.314 is within the error range if you measure it with a caliper. Use a mike to be sure.

Also, I'd push the slug all the way, in through from the chamber and out the muzzle. Be careful when it exits so it doesn't get damaged.

Greetings,

The Pardini 32 ACP barrels I have encountered measure 0.313-0.314".

It is called manufacturing tolerance.

No need to worry, just size your bullets accordingly.

Cheers,

Dave

don't forget to set your outside mic to a standard to get the correct size.

Beeser,
I slugged my barrel, using 0.314 LSWC inserted into the chamber and pounded through with a dowel; it measured 0.313 across the lands. That bothered me since XTPs and Gold Dots are both 0.311; how can they be accurate? But they are.

I've dug up bullets fired at my home range and all (Gold Dots, XTPs, and Oldcaster's bullet) show prominent rifling marks. I guess the explosion plus the friction makes the bullet expand and grip the rifling in the barrel (at least that's my 'logical' conclusion).

It's fun to learn about these precision shooting tools. I guess Pardini knows what they are doing. Dipnet

Dipnet wrote:Beeser,
I slugged my barrel, using 0.314 LSWC inserted into the chamber and pounded through with a dowel; it measured 0.313 across the lands. That bothered me since XTPs and Gold Dots are both 0.311; how can they be accurate? But they are.

I've dug up bullets fired at my home range and all (Gold Dots, XTPs, and Oldcaster's bullet) show prominent rifling marks. I guess the explosion plus the friction makes the bullet expand and grip the rifling in the barrel (at least that's my 'logical' conclusion).

It's fun to learn about these precision shooting tools. I guess Pardini knows what they are doing. Dipnet

Strange how much variance is involved. Following a suggestion from another forum member I used a Mitutoyo micrometer to measure my slugs and came up with .3142" - .3143". The Hornady XTPs that I have measured anywhere from .3115" to .3120-". The Gold Dot's were the most consistent at .312", which is what Speer shows in their specifications. I agree, it's been fun learning about this stuff and frustrating at the same time. My latest direction is to try another batch of T&B bullets but will get them as cast from Travis and size/lube them myself using a Lube-A-Matic, .315" sizer and whatever lube comes my way. I still plan to use N310 but haven't decided on a load yet. Any suggestions?

As for the XTPs and Gold Dots having prominent rifling marks and achieving accuracy my take is that there is enough contact on the grooves to accomplish this. In other words contact on the lands may not be necessary. And since these are jacketed bullets leading is not an issue. Anyway, that's my novice thinking.

Beeser,
Variation in the diameter of the Pardini’s barrels is normal and typically increases as more barrels are cut with a particular button or mandrel, due to wear of the cutting device. However, we are only talking about ± 0.002 inches in a pistol barrel designed to shoot < 2 inches at 50 yards. In comparison, Shilen match rifle barrels must measure within 0.0005" of their reference diameter and can’t vary more than 0.0003" in diameter for the length of the barrel. Dipnet

My barrel slugged .3138 and it was one of the earliest guns to be sold. It would just about be necessary to measure the bullets with the same mic at the same time to be sure of measurements that close and I don't think it is necessary to get the bullets sized that closely unless you are using very hard lead. My gun shot .313 or .314 just about equally well but I finally settled on .314 thinking it was the wiser thing to do and felt like it was just a bit better.

Some things to keep in mind:

-The barrels are almost certainly not round to 0.0001" tolerance.
-Measuring accurately with a mic to 0.0001" is very challenging, especially when measuring something round and soft like a lead slug. It requires good, well practiced technique and a very good quality mic. If you spent less than $200 on your mic, I'm not sure I'd trust it to 0.0001".

If I were to slug my barrel (which I haven't), I'd probably measure the slug diameter using a toolmaker's microscope rather than a mic, because I would not trust myself to get an accurate measurement. I'd almost certainly end up slightly crushing the slug, resulting in too small a measurement. I run a lab at Arizona State where we do ultra-high precision CNC machining (~0.00004" tolerance). The hardest part of the work is the measurement.

cgroppi wrote:Some things to keep in mind:

-The barrels are almost certainly not round to 0.0001" tolerance.
-Measuring accurately with a mic to 0.0001" is very challenging, especially when measuring something round and soft like a lead slug. It requires good, well practiced technique and a very good quality mic. If you spent less than $200 on your mic, I'm not sure I'd trust it to 0.0001".

If I were to slug my barrel (which I haven't), I'd probably measure the slug diameter using a toolmaker's microscope rather than a mic, because I would not trust myself to get an accurate measurement. I'd almost certainly end up slightly crushing the slug, resulting in too small a measurement. I run a lab at Arizona State where we do ultra-high precision CNC machining (~0.00004" tolerance). The hardest part of the work is the measurement.

No toolmaker's microscope and the Mitutoyo micrometer cost less than $200, so what can I rely upon for a measurement with my untrained and novice abilities?

Here is my take, if you have a load that is < 2" at 50 yds. then spend your time dry firing and practicing at the range. Be secure in the fact you have a gun/load combination capable of taking you to High Master - if you do your part. Don't fill your mind with doubt because something doesn't measure as you think it should.

Fixate on the X.

- Dave

beeser wrote:

cgroppi wrote:Some things to keep in mind:

-The barrels are almost certainly not round to 0.0001" tolerance.
-Measuring accurately with a mic to 0.0001" is very challenging, especially when measuring something round and soft like a lead slug. It requires good, well practiced technique and a very good quality mic. If you spent less than $200 on your mic, I'm not sure I'd trust it to 0.0001".

If I were to slug my barrel (which I haven't), I'd probably measure the slug diameter using a toolmaker's microscope rather than a mic, because I would not trust myself to get an accurate measurement. I'd almost certainly end up slightly crushing the slug, resulting in too small a measurement. I run a lab at Arizona State where we do ultra-high precision CNC machining (~0.00004" tolerance). The hardest part of the work is the measurement.

No toolmaker's microscope and the Mitutoyo micrometer cost less than $200, so what can I rely upon for a measurement with my untrained and novice abilities?

You can do it with a Mitutoyo micrometer, just don't trust the answer you get to 0.0001". Be sure to use the little knob on the end (the ratchet stop) to get a consistent tightness. Measure the slug at several angles across the diamteter. The variation in those numbers is telling you your uncertainty: barrel non-roundness and variations in measurements. Also, remember that all your measurements are likely 0.0001"-0.0002" small because the slug is soft, round lead. No matter what you do, you'll always crush the slug a little bit. The micrometer itself is most likely to have a fixed offset, meaning it will measure relatively consistently when you measure the same thing repeatedly, but it will not measure the right absolute number. This could be calibrated out of you have access to a set of gauge blocks, but these cost a lot more than a good mic. Without calibration, I'd add in another +/- 0.0001"-0.0002" uncertainty for the mic error.

I'd guess the actual error will end up being around +/- 0.0005".

I'm surprised Chris didn't mention it, but just holding the slug in your hot little hand will change the dimensions. Just tack that onto your "WAG".

Rover wrote:I'm surprised Chris didn't mention it, but just holding the slug in your hot little hand will change the dimensions. Just tack that onto your "WAG".

That would explain the margin of error I get when I mic my M&Ms.