Slugging the bore and throat

[Yondering]

In a conversation with another member here, I realized it might be helpful to post a pic of some bore and throat slugs, for those who haven't slugged their barrels.

Here's a pic of some throat and bore slugs I had laying around. These are all made by pushing a soft lead ball or bullet into the barrel; throat slugs are made by pushing the slug in past the chamber, but no farther, while bore slugs are pushed all the way through the barrel. The two on the left are throat slugs; you can see the lip from the front of the chamber on the left, then a smooth section (the throat), and the start of the rifling on the right. That top left one is a 41 Mag bullet, cast from wheel weights, used to slug one of my 10mm barrels.

The two on the right are bore slugs, pushed all the way through one of my 45's.

Of course, the idea here is to measure these to determine your barrel's inside dimensions, so you can choose bullets correctly sized for your gun. I do this to all my guns, but got a good surprise recently with a brand new G19, turns out the bore is .357", not .355". That could be part of where the "don't shoot lead in a Glock" myth came from.

[EdMc]

Thanks for posting. How much leade do you look for? Or, is that more related to the bullet you are using?

[REDLINE]

Very informative.  Thanks for posting.  I suppose to some that is common knowledge.  Not to me though.  Glad you posted the pic.

[Yondering]

Thanks for posting. How much leade do you look for? Or, is that more related to the bullet you are using?

Length of the throat will dictate the OAL (unless restricted further by the magazine). Diameter of the throat is the more important measurement here though. It should be a very slight taper though, so you'll need to measure in several locations. For example, the top left bullet in my pic above measures .403" just in front of the chamber, then tapers to .401" before the rifling starts. A bore slug through that barrel shows a .400" bore, so that's just about perfect for a 10mm/40 S&W. This tells us that a .401" cast bullet is the right size, and the .403" dimension near the chamber means that .401" cast bullet can be seated with some of the front driving band exposed outside the case, without causing a bind.

[EdMc]

Concise explanation. Thanks.

[The_Shadow]

The information listed in the pages of  this link will also show other factors that may determing COAL besides throat and magazines...while this is for the 38Super it does apply to evey bottom feeder.
http://www.38super.net/Pages/Overall%20Length.html

[Yondering]

The information listed in the pages of  this link will also show other factors that may determing COAL besides throat and magazines...while this is for the 38Super it does apply to evey bottom feeder.
http://www.38super.net/Pages/Overall%20Length.html

Well, sort of, although all of that is still determined by the throat and magazine, in relationship to the bullet profile. Good link, the pertinent stuff is at the very bottom. How the bullet fits the throat is the critical point. One thing that article doesn't go into is the diameter of the "freebore" section of the throat; in some barrels this diameter is too tight, and doesn't act as freebore at all, but instead pinches the front of the bullet's driving band as it's chambered, which can raise pressure. This seems to be a common issue with LW barrels in particular.

The magazine stuff is good too, although like you said, the numbers presented are specific to the 38 Super. It's important to keep in mind that the 10mm and 40 S&W were designed for flat nose or hollow point bullets from the start, unlike many other rounds out there. The pictures showing interference of flat nose bullets at the front of the magazine is a good visual, and is why I don't like to use a meplat larger than .300" or so in my Glock 10mm magazines.

[The_Shadow]

Thanks Youndering you are correct, I mainly wanted to show the angle of the tipping action during feeding, in relation to bullet nose shapes, as many are using the WFNGC type bullets and have had feeding issues.  The SWC designs can also pose problems in the tight chamber throats  in relations to the freebore.  Therefore I wanted to add this to your "bore and chamber throat" segment so others can grasp the details of what to look for in the effort to solve any issues.

[sqlbullet]

Good info and pics!

I haven't really bothered much with checking the throats on my auto-loaders.  In general I find if I size the bullet to the bore, I get good results from self-stuffers.  On a revolver I always slug the cylinder, forcing cone and bore separately.

[Yondering]

Thanks Youndering you are correct, I mainly wanted to show the angle of the tipping action during feeding, in relation to bullet nose shapes, as many are using the WFNGC type bullets and have had feeding issues.  The SWC designs can also pose problems in the tight chamber throats  in relations to the freebore.  Therefore I wanted to add this to your "bore and chamber throat" segment so others can grasp the details of what to look for in the effort to solve any issues.

I hear ya. I know you understand all that, just wanted to make sure it was clear to others. The pictures of the angle of the rounds in the magazine, while different in double stack mags, is another good topic of discussion, in relation to the meplat size and binding in the magazine. There have been some good discussions of this on the Castboolits and 10mm reloading Glocktalk forums, as you know.

[ctious]

I slugged mine.  Found the throat to be .4025. The full slug through to be .400. The part I found that I did.not like was I testeda few a little deeper from chamber and found it took a good inch before the slug got down to .400. Is that normal? I would think it would size down a lil faster. Might explain why my barrel tends to be dirtiest just in front of the chamber down the first inch. Not that its bad dirty. Just that it is compaired to the rest of the barrel.

[Yondering]

I tested a few a little deeper from chamber and found it took a good inch before the slug got down to .400. Is that normal?

No, that's not normal. What barrel? I'm assuming the rifling starts somewhere in that inch, not an inch of freebore? Dimensions like that can be a result of lapping the bore too much, and using a lapping compound that breaks down too fast so it doesn't cut all the way down the bore.

If that were my barrel, I'd try some cast bullets sized at .402" (or even .403" depending on bullet design, if there's not much exposed driving band), see if that fixes the leading issues. With jacketed bullets, there's not much you can do, except just live with it.

[ctious]

I tested a few a little deeper from chamber and found it took a good inch before the slug got down to .400. Is that normal?

No, that's not normal. What barrel? I'm assuming the rifling starts somewhere in that inch, not an inch of freebore? Dimensions like that can be a result of lapping the bore too much, and using a lapping compound that breaks down too fast so it doesn't cut all the way down the bore.

If that were my barrel, I'd try some cast bullets sized at .402" (or even .403" depending on bullet design, if there's not much exposed driving band), see if that fixes the leading issues. With jacketed bullets, there's not much you can do, except just live with it.

The rifleing starts where it should.  I only have about 1300 rounds down it. My one mold drops at .403 I will have to try them unsized. It's a lone wolf barrel. It shoots really well just that first lol bit gets dirty. Maybe I should get another barrel.

[sqlbullet]

At .403 you might start to have issues unless the chamber is generous.  Or the bullet might get swagged down during crimping.

[Yondering]

Yep, being a LW barrel, the chamber may be too tight for .403" bullets, unless you've opened up the chamber a bit. Just check them by hand before shooting.