Slugging Barrels for Lead Boolits Really Necessary?

[Edward429451]

We all know that boolit fit to barrel is king if one wants to reduce or eliminate leading in their gun and enjoy the benefits of cast. No question on that. So go slug that barrel and see what you have, then oversize .001 or .002 and you're there.

Slugging a barrel sounds like a pain in the butt. I admit to never having done it though. I have cheated and skipped the slugging and simply went to a larger size from nominal jacketed size. This has worked for me, but am I wrong for doing so? If so why?

I think that newbs getting into cast lead loading may be discouraged somewhat by the advice to slug the barrel. Why not just suggest going .002 over nominal, and begin with a start load, working up the load in your gun?

This seems to me to be a perfectly safe alternative to slugging as long as they really begin with a start load and do not reduce a max load by whatever to start. Increasing diameter will increase pressure so beginning with a start load would be the critical element, yes?

I'm ready to learn if I am wrong. What say ye?

[zxcvbob]

Slugging the barrel should be one of the steps in diagnosing a problems (also the cylinder throats of a revolver.) If there is no problem, it seems kind of pointless to me.

[dunerjeff]

Yes and no. To go .002" over can increase pressures IF your barrel happens to be a correctly sized(many match types). Many barrels do actuelly measure +.001 so the 2 over works 80% of the time.If what you are doing works then its good. When I first started I didn't slug either and bought the over sizeed bullets, which after trying a dozen different powders and up/down charges ect...(leading,leading, and more leading).Bought some different bullets,same size though,and still more leading.
I slugged my barrels and found it was .003 oversize(Beretta 92's)I then bought some .38bullets(.358 sized) and all my problems went away.Sure would have saved some time and $ to slug first and buy accordingly. I now have another 9mm gun thats does need the smaller bullets so I can use up those others.
I got into casting and bought molds so I can size to whatever a barrel "prefers".
In the case of .40S&W,you can only get .401 lead bullets,both of my '96's are.4025 and they will lead like crazy with those bullets,but I saved myself some $ by knowing I needed bigger bullets than what was available.

[FrankenMauser]

You can always use the "trial and error" method to find out what a firearm wants. The reason cast bullet shooters (and milsurp shooters) suggest slugging the bore, is because it can eliminate a lot of time and money wasted on reloads that don't work, and/or time wasted on hours spent scrubbing lead out of the barrel.


As dunerjeff discussed, most barrels are not at the 'nominal' dimensions, and most are over by 0.001-0.002". Some, like .444 Marlin, are frequently over (from the factory) by as much as 0.005".

It is not uncommon to find 9mm barrels with a groove diameter of .357-.358". If you went with a .357" projectile (for 0.002" over nominal), you'd likely encounter a lot of leading.

I have a .380 with grooves that measure .358" x .359" at the muzzle, but are significantly larger than that mid-way through the bore (estimated to be in excess of .363"). I bought 500 lead bullets for it, quite a while ago, and couldn't figure out why I couldn't get it to stop leading. Once I slugged it, I knew why. Those .356" bullets were just rattling down the oblong, over-sized barrel. Had I slugged it, to begin with, I never would have wasted my time or money on the wrong bullets.
(Since it's rather difficult to get .359-.360" bullets appropriate for .380, it has the loose section, and I don't want to size down Makarov bullets... it now gets hollow base .356-.357" bullets that can expand to the proper size and 'ride the grooves'.)

It is not uncommon to find 30 caliber barrels with narrow bores and deep grooves. Rather than the nominal .304" bore / .308" grooves, you may encounter a .302" bore and .309" or even .310" grooves.

.45 Auto and .45 Colt share the same problem. One .45 Auto might have a groove diameter of .451", while the next is clear up at .454". And, its sibling .45 Colts might have a groove diameter of .452", while the next is at .456". Going with a slight over-size of the nominal bore dimensions won't work with those deep grooves.

And, then.... you can get into arguments about revolvers and rifles.
Many people believe it is more important to size the bullet to a rifle's throat, than to the groove diameter. Rifle chambers are quite variable, and the throat changes along with them. Since the throat is also the most erosion-prone part of the barrel, it's also the part that gets enlarged faster than anything else. Without slugging (or taking a chamber cast)... you'll never know what size the throat actually is.

And, revolvers have even more areas to worry about:
Do you size it for the cylinder throats? -Are they smaller than the bore? Smaller than the forcing cone? Are the mis-matched?
Do you size it for the forcing cone? -What if the bore is smaller?
Do you size it for the groove diameter? -What if there's a constriction where the barrel threads into the frame (very common)?

Revolvers can be quite a headache, if you have mis-matched dimensions. But, slugging before aimlessly tossing lead bullets through them can provide some insight into what might be the best starting point.


If you don't want to slug your barrels or take chamber casts, don't. Just throw a bunch of lead through it, until you find something that works.
But, when we suggest slugging it... we're just trying to help, by saving you some potential frustration (and bore scrubbing).

[armoredman]

Ah, this answers some questions - my Beretta PX4 Storm 40 slugs out at .402, .002 over nominal. It leads badly with slugs sized at .401, and has so-so accuracy with factory jacketed .400 bullets. I am hoping to try some Berrys' plated .401 bullets.
Having said that, my Lee mold actually drops at .404, so I have tester rounds loaded with the wider dimension bullets waiting for range time. I am looking forward to hopefully some improved results.

[Quizcat]

Anybody slugged the bore of a Taurus PT140 yet? Just wondering what you may have come up with.

[Edward429451]

I gotta admit, Frankenmauser laid down some pretty good ideas of why to slug a barrel. I'm not experiencing any problems now but would I open a can of worms on myself to slug my 44s? Suppose my SBH turned out to need .430 boolits and the RH wanted .431s? Would shooting .431s in both of them then be detrimental in any way?

[Quizcat]

My Taurus PT140 is slugging at between .3985" to .3990" using a ID dial caliper, and also measuring an OD slug.

Based on what I've read in this thread and others, I am assuming that it's not uncommon for a bore to be at .3990, and that, theoretically, the .3990" dimension would be appropriate for a .401" lead bullet with respect to the bullet being able to seal the bore.

[Crosshair]

I just load some up and try them. if things work fine then there's no need to slug.

[Quizcat]

I have about (18) rounds loaded up, 6.2 grains of Power Pistol, crimped using the standard bullet seating/taper crimp die, OAL =1.125". I am going to be looking very specifically for signs of leading in the bore after testing is complete.

[armoredman]

I tried the .404 unsized home cast lead bullets in my Beretta PX4 Storm with the .402 sized barrel, and had these results - the targets all looked pretty much the same.

[FrankenMauser]

I gotta admit, Frankenmauser laid down some pretty good ideas of why to slug a barrel. I'm not experiencing any problems now but would I open a can of worms on myself to slug my 44s? Suppose my SBH turned out to need .430 boolits and the RH wanted .431s? Would shooting .431s in both of them then be detrimental in any way?

I would actually shoot .432s, if I had that combination. But, .431s might prove to be acceptable, as well.

I must admit... I had never gone to the trouble of slugging my SBH, until last night. I don't shoot it as much as I used to, and leading has never been a real issue.
Being an older model, I expected the cylinder throats to be different.

I don't have a set of pin gages to properly measure the throats. So, I went to the next best thing: a variety of cast and jacketed bullets, ranging from .427" to .435", that I verified were actually round with my micrometer. I slip-fit the bullets until I found one that was easily moved, but sat there if I let go. That indicates it's within 0.0002" of the actual size.
I have 4 chambers with throats between 0.4315-0.4323", and two between 0.4330-0.4335". That may explain why a six-round group usually has 2 holes standing out from the rest.

As expected, the barrel is constricted where it threads into the frame. I didn't get a good measurement and will have to slug it again. But, I know that it's tighter than 0.4275".
The groove diameter of the barrel is actually tighter than expected, at a few ten thousandths over 0.428".

With those measurements, I plan to stick with the largest bullet diameter I can chamber without resistance. It'll probably come in somewhere around .432-.433".

But, now that I know this groove diameter is pretty decent, I'm giving serious thought to lapping the constriction out. If I get that constriction opened to match the rest of the groove diameter, or even 0.0005" larger, it should turn into one helluva shooter and eliminate all size-related leading. Since Ruger will ream these throats to the same diameter for free, I plan to take them up on that offer after the Elk hunt. I may even try arguing that the a .433"+ throat is a ridiculous mate for a .428 barrel, in hopes that they fit an entirely new cylinder with identical chambers/throats.

[floydster]

If you cast bullets and don't know the proper bullet size for your gun, you may just as well throw the bullets at the target

[Edward429451]

That's interesting. My SBH does the same thing. Two flyers, which I assumed was me not holding steady, but perhaps I can yet bring them into one ragged hole. I have extra sizing dies I can have opened up. I'm tempted to go straight to .432 and just see what effect it will have on my groups. As long as my moulds cooperate, lol, or some beagling may be in order. I get plenty of sizing at .430 so I think there wont be a problem getting .432 or certainly .431 at least. I'll have to cast some more to check them, everything's sized and lubed.

Back on topic, I'm understanding that no it is not absolutely necessary to slug the guns. One can trial & error and get it right...eventually. My trial & error method has been satisfactory and certainly gives me hunting level accuracy, and I have no leading problems to speak of beyond slight flakes. Beyond that, I'm left wondering if target type accuracy with some bragging rights isn't a small tweak away...

I'm all for that. Since it will take some time to send out a die to be opened up, I think I will go ahead and slug my SBH. It will be easiest since I can remove the cylinder. I've been neglecting my 44s since my acquisition of the 45 Colts anyway. I shoot them but stopped experimenting with them in favor of the 45s. Good thread.

[dunerjeff]

Armoredman did the Px4 have any leading with those rounds? The target looks pretty good,being an auto it what we hope for.

Quizcat, I doubt it that your barrel is .399, being measured with calipers is just a guess. I would use a mic only. Very, very few barrels are even as small as .400(standard), so to be undersize?? not likely.

[armoredman]

No, leading was very minimal. Did this with it today, same loads. This is two five shot groups, the two taped over flyers were learning that this gun HATES having the trigger jerked at all, glacial speed only...the tape on the sides was holding it on the backing.

[Nathan]

With an even number of lands and grooves, calipers provide an educated guess. A slug would fine tune this to real knowledge in like 5 min.

or you could order 100 pcs of:
+0.000
+0.002
+0.004

but then you would wonder if the odds were better and order .001 and .003!

So, the $15 kit for slugging the bore gets lots cheaper.

[F. Guffey]

It has been years since I have talked about slugging a barrel, seems the strong got weak and the weak past out, anyhow, before the Internet there were at least three different methods.

F. Guffey

[F. Guffey]

Years ago and forgot, it is assumed the barrel diameter is the same from start to finish, different methods and or techniques allowed for barrel lapping, the plan was not to waste time lapping the larger diameters.

F. Guffey