Barrel Slugging Measurement

Hey guys,

I want to figure out the groove diameter of my bore so I can cast the correct size bullet for my 9mm.

I took some advice from a local gunsmith and tried it myself... and I got the lead bullet he gave me to use stuck in the barrel. I carefully got it unstuck and now I’m no longer interested in slugging my own barrel. The gunsmith refuses to do it for whatever reason...

I did take a inside diameter measurement with my calipers of the end of the barrel and consistently got .3575/.358. Can I use this measurement? Is there something I’m missing by measuring the barrel this way?

Thanks

Try using cerrosafe instead of lead. Same procedure as a chamber cast.

https://www.brownells.com/gunsmith-t...y-prod384.aspx

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Dial calipers or digital calipers are considered to have a tolerance of +/-.001". So they really aren't accurate enough for this type of measurement. This is why the standard process is to use a soft lead slug to obtain a measurement using a micrometer. If you use a gas check turned around backwards and a brass rod very near the bore diameter, the lead won't be so inclined to become impacted around your driving rod and become stuck. Some light oil applied to the bore helps as well, but just a thin coat. Too much can affect your results.

Since joining this forum, I think I have seen more threads asking for help with 9mm leading and accuracy problems than probably all other calibers combined. So if you're not rather seasoned as a bullet caster and reloader, you've chosen a task that can be challenging and frustrating at times. Not trying to discourage you, just be prepared. This project may be a cake walk, but more likely a trip thru a bed of hot coals.

Thanks everybody. I’ll try it.


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Best approach: do a Cerrosafe chamber cast to include a short section of the bore. That'll tell you all you need to know regarding bullet selection- throat diameter, leade, and groove diameter. (And measure it with micrometers, not a dial caliper.)

There used to be a "sticky" but I couldn't find it, so save this for future reference.

"Slugging the barrel" ala the old Lyman handbooks applies only for straight-cased, black powder style rifles chambered for "Cowboy" cartridges using revolver ammo, or rifle cartridges like the .45-70 in which the rifling simple starts in front of the case mouth with little or no transition.

For everything else, to determine the correct bullet diameter for a rifle, the groove diameter of the barrel is NOT the determinant.

INSTEAD you want to measure the "throat," or "ball seat", which is the unrifled portion of your rifle barrel ahead of the case mouth, before the rifling starts.

The best way for accurate measurement and easiest to do is to upset a throat slug, to make a "pound cast." For most .30 cal. rifles an easy way to do this is to take a dead soft, pure lead slug, such as a factory .38 Special 158-grain LRN projectile you have pulled, and drop it into the chamber. Then use a 3/8" diameter brass rod to drive the slug into the neck of the chamber until the 3/8" diameter of your brass rod comes to a hard stop against the chamber shoulder. Then just knock the slug out from the muzzle, and you will effectively capture the chamber NECK diameter (which determines safe release clearance, limiting maximum bullet diameter), the transition from the case neck to the "throat",
(the UNRIFLED portion ahead of the chamber mouth before the rifling starts) and the conical "forcing cone" section at the origin of rifling.

In REVOLVERS you want to measure the diameter of the cylindrical "throat" or "ball end" of each chamber in the cylinder ahead of the case mouth. This is best done by using a set of round steel gage pins. Lacking these, remove the cylinder and tap dead-soft pure lead round balls or slugs through each chamber and measure them with a micrometer.

A method widely used for rifles is to start with a sized case with DEAD primer in its pocket. Heat your lead pot, then fill the sized case with DEAD primer plugging the flash hole, and generously overflow the case. After the lead cools, clean all spilled lead off the case exterior, then file the exposed lead FLUSH to the case mouth.

Now take a piece of PURE lead buckshot or a short chunk of pure lead wire and drop it into the EMPTY chamber, letting it fall into the throat of its own weight. (With very long throats use a longer piece of wire or a SOFT bullet with long bore-riding nose and not a long grooved section).

Insert your lead-filled dummy case and GENTLY tap it into the chamber using a piece of brass rod until you can close the breech. You are using the lead-filled dummy case to force the lead slug into the ORIGIN of rifling. In short throated barrels it helps to drive the slug first into the origin of rifling, far enough to chamber the lead dummy behind it, then close the bolt and upset the slug against the lead dummy using a Brownell Squibb Rod threaded onto the end of your cleaning rod. You don't need to use a hammer, just raise up and let the weight of the rod when dropped make many light taps of the squibb rod against the slug until you get a clear "ringing" sound. It need go no farther!

What you want to measure is the diameter of the UNRIFLED portion of the chamber forward of the case neck BEFORE the rifling starts! Extract the dummy and GENTLY tap the lead slug out and measure it. THAT is the diameter you want to size your bullets to! Using Cerrosafe, etc. is more trouble and you then need to compensate for shrinkage, etc.

The upset pure, dead-lead slug is exact and straight forward! If you forget EVERYTHING you ever read about slugging barrels and simply cast chambers from now on, and get bullets to FIT THE THROAT you will be far happier in the long run.

The limiting factor in safe bullet diameter is neck release clearance. You MUST measure the neck diameter of the chamber on the cast. A correct pound cast will get you a portion of the case mouth and its transition angle to the throat or ball seat, so that you can measure neck diameter at the mouth and throat diameter of the ball seat. The loaded cartridge neck diameter must not be larger than 0.0015" SMALLER than the chamber cast at that point, to ensure safe expansion for bullet release.

In most rifles the largest bullet diameter which chambers and enables extraction of a loaded cartridge without debulleting or the bullet telescoping deeper into the case is best. Light marking of the forepart of a rifle bullet which does not cause debulleting upon extracting a loaded round, without telescoping the bullet is OK.

Attachment 225872Attachment 225873Attachment 225874

In a typical .30-'06 a bullet diameter of .310" is correct for a new SAAMI chamber, as this represents the ball seat diameter of a "minimum"chamber. If your barrel has been fired more than 1500 rounds with full-power jacketed loads .311" will be a better fit. In a milsurp rifle which has been fired much a .312" will fit just fine and still have adequate neck release clearance when using GI brass in a typical military 03A3 Springfield or US M1917 chamber. It may provide useful information to assemble a DUMMY round with larger .313" or .314" bullet and mark the case neck with Dychem or black felt tip marker and attempt to chamber and extract it. Marking or rubbing on the die marker will shot you where the fit is too tight. If your cases are a bit long, or your crimp excessive you will get a bulge of the case mouth or shoulder. If so, this test will reveal it, so that you can make the necessary adjustments, trimming cases and/or backing off the seating die to reduce the amount of crimp.

Measuring a pound cast is really necessary to get best performance most milsurps. This is because large groove diameters and huge throats are common , especially in wartime .303 British rifles and in Russian M91/30s and Chinese copies whose chambers are notoriously sloppy! In these rifles a bullet with full-diameter, tapered forepart, rather than the typical Lyman "double-cylinder" groove diameter body and bore diameter forepart is best. Accurate 31-215B is a good example of the full-diameter, tapered forepart bullet designed to fit WW2-era .303s and 7.62x54Rs:

Attachment 225875Attachment 225876

Let's review an example of chamber neck diameter and bullet fit:

A typical Finnish M39 7.62x54 chamber neck is 0.340". Typical case mouth wall thickness of Norma commercial brass is 0.013," so .340" minus twice neck thickness (0.026") = .314", minus 0.0015 for safe radial expansion of the neck for release clearance = .3125" max. bullet for a typical Finn chamber in an M24, M27, M28, M28/30 or M39.

WW2-era Russian and Chicom Mosin-Nagant rifles very often have throats as large as .316" and with groove diameters of .314", like a wartime British .303! If you expect anything resembling normal accuracy you MUST cast your chamber, measure it, and then buy a mold which fits your THROAT, not the groove diameter of the barrel. Use .30 cal. gaschecks, pressing them on by hand and then pushing the base of the bullet against a table edge until the gascheck is bottomed against the bullet shank. Only then size the bullet. Otherwise the GC will not be seated squarely on the base of the bullet and any hope for accuracy goes out the window.

And there you have it and with correct bullets which "fit" your milsurp rifle can shoot like this:

Attachment 225877Attachment 225878

I take it you are talking about a 9mm autopistol. If you are talking about a rifle, then read again the post by Outpost 75.

I have measured the bores of a number of American and Euro 9mm auto pistols and they have ranged from .354 to .357 in the barrel groove. The American barrels tend to run smaller than the Euro barrels.

Just size your bullets .358 and be done with it. It won't hunt a whit (whatever a whit is) to shoot the larger lead alloy bullets down a smaller bore.

The use of a set of calipers to measure the barrel groove will not give you accurate measurements.

You can apply exactly same principles above to an auto pistol barrel. Easy to do with removable barrel.

My experience has been that most Euro barrels chambered to CIP or NATO STANAG 4090 need .357-.358 bullets.

Most commercial US barrels with SAAMI chambers require .356 bullets, but many have throats so short and tight they won't shoot lead accurately unless reamed. Some also have tight, minimum chambers so that rounds loaded with a bullet larger than .355" won't even go into the gun!

DougGuy can correct the above for you if you send him the barrel only.

Seating bullets deeper so that rounds past a "plunk" test may cause a DANGEROUS pressure spike. DO NOT seat bullets in 9mm any shorter than the published source of pressure tested data recommends!

Yes, it’s a 9mm Beretta Px4. Mainly wanting to cast to save money but also learn a new skill. This isn’t the first time that I’ve read to just get a .358 mood/sizer and just use that. There are a bunch of opinions out there. Thanks so much for the info. I’ve got the info saved, Outpost. Thanks everybody. I’ve got a lot to digest once I get home from work.


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