Slugged my mosin barrel...confused by the readings.

[SirNomad]

Howdy,

I slugged my Mosin/Nagant 91/30 barrel this morning. I've never done this before. The caliper gave me readings between .301 and .309. I checked the caliper on some surplus 7.62x54R ammo, just the jacketed bullet, and got .310... so is my caliper off, or am I just misunderstanding what I should be seeing here?

[Ballistics in Scotland]

Assuming those are the measurements from land to land and from groove to groove, they are pretty much OK. More significant is that the slug should push all the way through the barrel without encountering tight or loose spots - except that some throat erosion in a well-used rifle is normal. A slight reduction towards the muzzle is fine, but loosening towards the muzzle is usually bad.

[SirNomad]

It was definitely feeling a little bit looser near the chamber, but that could have been from lead rubbing off, no? Definitely not looser near the muzzle end. I have 3 more mosins to slug and I'm wondering if using a cast lead .357/.38 bullet might make more sense to use for slugging than a mystery lead fishing sinker.

[HangFireW8]

I always slug 3 times, throat, muzzle and all the way through.

Soft lead is best.

[GhostHawk]

Get lucky and find a tight Finnish barrel on your Mosin?

I feel lucky that both of mine slugged at .312 I have heard of as big as .316 which would be tough to find a mold for.

[SirNomad]

It's a 1935 Tula.

[SirNomad]

And I just did the 1934 Izshevsk, and got .301 on the smallest diameter, .313 on the largest. So, considerable variation? What size cast bullet (gas checked?) would I even use to reload for both? Hmm...I still haven't done my son's 1925 ex-dragoon.

...edited to add, I just did my son's ex-dragoon, and it's the same as my 1934 Izshevsk.

Now, since I'm new here, I'm going to have to read about different loads! I have a feeling that my .309 Tula will not like the same rounds as the .313 Izzie and ex-dragoon.

[runfiverun]

I wouldn't worry about the loads I'd be looking for some goofy mold that throws numbers like that.
301X310 is just about every 30 cal mold out there.

301 x 313 hmm maybe a 314 mold [304 nose] and a nose sizer to make it 302.

[northmn]

Had a carbine that slugged at 317, which is why I HAD it.

DEP

[HangFireW8]

Howdy,

I slugged my Mosin/Nagant 91/30 barrel this morning. I've never done this before. The caliper gave me readings between .301 and .309. I checked the caliper on some surplus 7.62x54R ammo, just the jacketed bullet, and got .310... so is my caliper off, or am I just misunderstanding what I should be seeing here?

In a pinch you can use a Sierra .30 jword to check your caliper calibration. I have yet to see one that is not .3080".

[Wayne Smith]

And slug with pure lead only. Two part and three part alloys have a springback and will give you large readings.

[Outpost75]

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 PURE LEAD 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 PURE LEAD 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.

In a typical Finnish M39 7.62x54 chamber the chamber neck is 0.340". Typical case mouth wall thickness of Norma or Sako commercial 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 Finn chamber in an M24, M27, M28, M28/30 or M39.

It is not unusual for WW2-era Russian and later Chicom rifles to have throats as large as .316" and groove diameters of .314". 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.

As a general rule the largest diameter of cast bullet which chambers and extracts freely, without resistance, will shoot best. For most Finnish rifles this is .311-.312" and for Russian and Chicom rifles .313-.314".

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.

[victorfox]

all this talk made me remind a history i read somewhere of how the Sowjets spy poisoned a Finn ammo/gun developer in order to take the "secret" of the Finns' great accurary. Turns out the Finns used larger boolits to the bore while the sowjet ammo was something like .002 smaller. They got what they wanted but the man was dead and his home maid who was the perpetrator and accomplice of the spy thrown in jail. I can't remember where I got this but it's said to be true.

[HangFireW8]

The Finns used a tighter bore and accuracy qualified/re-qualified each and every rifle. The Soviets did neither.