So I slugged the bore and determined ?????

[zymguy]

I don't know how to post links, but in the Cast Boolits sub-forum, there is a 'sticky' for 'how I do a pound cast'.

http://castboolits.gunloads.com/show...o-a-pound-cast

[Hannibal]

http://castboolits.gunloads.com/show...o-a-pound-cast

Yes, that's the one I was referring to. As for a chamber casting, Cerrosafe is the product I've seen recommend most, and I believe it comes with instructions.

Hope this helps.

[Outpost75]

Here is a reposting of one of mine:

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 the unrifled portion of the barrel forcing cone or "ball seat" ahead of the case mouth, before the rifling starts. The best way to do this is from a chamber cast or upset throat slug.

Most accurate for measurement purposes and easiest is to upset a throat slug, or as some people call it a "pound cast."
Start with a sized case with DEAD primer in its pocket. The way I do this is to heat the lead pot, then fill the sized case with DEAD primer plugging the flash hole, and generously overflowing the case.

After the lead cools, clean all spilled lead off the case exterior, then file the exposed lead FLUSH to the case mouth.

Take a piece of PURE lead buckshot or 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 you can 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 let the weight of the rod 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 clearance. You MUST measure the neck diameter of the chamber on the cast. Most chambers have enough clearance ahead of a fired case mouth that a properly upset throat slug 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. This is absolutely essential for custom target barrels which often have tight-necked chambers which require neck-turned cases. As a general rule the largest diameter of cast bullet which chambers and extracts freely, without resistance, will shoot best.

For instance in a .308 Winchester target rifle with .339" tight-necked chamber and using case necks turned to 0.012," maximum bullet diameter is determined by"

[neck (.339")-2(neck wall thickness 0.012)] - 0.0015 = 0.3135" for a "fitted neck" in which fired cases do not require sizing, but bullets will be held by case springback only. For necked sized fixed ammo, subtract another 0.0015" or .312" IF the chamber ball seat is that large. In a new barrel chambered for jacketed bullets, probably not. Min. SAAMI throat as on the pressure test barrel is 0.3105".

Unless you know your throat is smaller, try .310". If the barrel has been fired more than 1500 rounds with full power jacketed loads .311" will be better. If you shot a couple seasons season of highpower with it, .312" will fit just fine.

John Ardito set all of his CBA benchrest records shooting .312" bullets in his .308 Win. and wildcat .30 cal. rifles.
This is seldom a problem in the Russian M91/30s and Chinese copies chambers are notoriously sloppy!

In a typical military 03A3 .30-'06 chamber the minimum neck diameter per the drawing is 0.340".

Typical case mouth wall thickness of LC, FA, TW, WCC or other USGI brass is 0.013," so .340" minus twice neck thickness (0.026") = .314", minus 0.0015 for safe expansion = .3125" max. bullet for a typical worn GI .30-'06 chamber in an 03A3 or US M1917. A new barrel will have a .3095-.310" throat and you should size to that diameter for a "new" replacement USGI or Criterion barrel.

As a general rule the largest diameter of cast bullet which chambers and extracts freely, without resistance, will shoot best.
For milsurp .30-'06 rifles showing ANY MEASUREABLE throat wear this is .311-.312."

[swheeler]

I want to try this on the SMLE I'm fooling with. It slugs .312" tight the whole way through. I'm going to try some Sierra 180 .311" sp that are already loaded first. I've cleaned it up to sell for a friend but I'll probably buy it.......and a mould........
Best, Thomas.

Sent from my SAMSUNG-SM-G930A using Tapatalk

Give it a try Thomas and report back.

[leadman]

A slugging of the bore on an old mil-surp rifle can give you some valuable info. It can tell you if there is a loose spot or a tight spot in the bore. Without a borescope a pit or built up rust may not be seen. It will also tell you if the gun has been cleaned from the muzzle and suffers wear there. I do a slug of the throat area seperately and will normally size the boolit to match depending on the neck size of the chamber.
With my 71 Mauser the neck is .008" smaller than the throat so I use 3 grains of poly-fil in the case to help seal the gases in the bore.

[Multigunner]

SMLE Mk1 bores were "reverse taper lapped" for the last 14 inches of the bore to the muzzle.
This was an attempt to get the same velocity and range when using the MkVI cartridge as the longer LE barrel by reducing friction.
The Cordite Mk1 propellant used with the MkVI cartridge in use at the time reached maximum pressure at ten inches of bore travel.

Reverse lapping was dropped early on but replacement of those barrels was not authorized till around 1917.
The SC HV marked barrels intended for best accuracy when using the MkVII cartridge are most often encountered but reverse lapped barrels still show up occasionally.