Does slugging the bore really hep?

[snaketail]

This is yet another post about slugging the bore of your firearm.

I have a .454 Casull rifle that I use to shoot .45 Colt in Cowboy Lever Action Pistol Cartridge matches.
My accuracy is “all over the place.” It has been suggested that I’m shooting a boolit that is too small in diameter. I’m loading a store-bought cast boolits in 200gr and 250gr that are marked .452 and measure .452 and .451. Running around 1000fps.
So, now I’m ready to slug the bore and take a measurement of the results. I plan to use a cast boolit that is .002 overbore in hopes of gaining consistent accuracy.

But, if I drive a soft lead sinker the entire length of the barrel won't that give me the smallest internal measurement? What if my barrel is .455 at the breach end and .451 at the muzzle (or the other way around!)? The results of my slugging the bore will not indicate the breach measurement – only the smallest portion of the bore.

I may be over thinking this, and I can the logic and safety reasons to use the smaller measurement, but it seems like the process is flawed.
If the bore is loose until the muzzle won’t the boolit wobble around until it gets to the smaller bore… OK bad choice of words, but you see what I’m getting at.

Will using a boolit that is .002 over my smallest bore measurement help my accuracy?

Michael

[ku4hx]

If your loads are accurate, boolits don't tumble and don't lead your throat or bore, then slugging is a waste of time. On the other hand if you have any of these three problems, or some other, slugging is a useful diagnostic tool.

Will .002" over bore diameter be more accurate? Only your gun can answer that.

[Oreo]

Slugging is one of several measurement techniques and you are correct in your assessment of it. Another technique is to make a chamber cast including the breech end of the rifling.

[Char-Gar]

"Slug the bore!", has become a mantra that is thrown out as the first response to every question regarding cast bullets. All to often it is not needed and can be confusing's to some people.

A cast bullets needs to be larger than the groove diameter of the barrel in order to keep the hot gases in their proper place, which is behind the bullet.

The truly critical dimension for cast bullet accuracy is the diameter of the chamber throat. The reason is a bullets that is a good fit to the throat will enter the rifling straight which is conducive to accuracy. A cast bullet that is undersized may hit the rifle at an angle and that does nothing good for accuracy.

It would take some real barrel making magic to come up with a barrel that has a chamber throat smaller than the groove diameter. Chamber throats will be the same size or larger than the groove diameter. I don't know of one that is the same size, as most are larger by at least .001 and most often more, sometimes allot more.

There are several ways of determining the chamber throat;

1. A throat slug
2. Measuring the inside diameter of a case fired in the chamber
3. Trial and error

It really doesn't matter how you come up with a cast bullets that is a snug fit in the chamber throat, as the results are the same. If you know this number, the size of the bore become irrelevant unless by some alien machining methods it is smaller than the bore.

You don't want a cast bullet larger than the chamber throat as it will shave lead and that is not a good things. A cast bullet that is .0005 to .001 smaller than the chamber throat is about where you need to be. A cast bullet that fits the throat can be several thousand larger than the barrel groove diameter and still give good results.

Now, if you are using a cast bullet with a long nose that rides on top of the lands, you need to know the diameter of the lands to be certain the nose is well supported by the lands and does not slump upon firing. Again, there are different ways to do this.

1. Measure the land diameter with a pin or small hole gauge
2. Trial and error to see the bullet in a dummy round will engrave on the rifling.

If you are using a bullet design that does not require the bullet nose to be supported by the lands, you can skip this step.

Bottom line, is "Slug the bore!" is often a response that does not deal with the needed issues of bullet fit. Bullet fit is important, but all to often folks don't know what is required for good bullet fit.

PS: This is in response to the OP about a rifle. This post is "rifle specific". Handguns play by different rules.

[snuffy]

Michael, you should be able to tell if you have any tight spots in your barrel by how a cleaning patch acts when pushed through the bore. That's assuming you're using a caliber specific jag and a patch that result in considerable resistance when passed down the barrel. It will get tighter or looser as it encounters tight or loose spots in the barrel.

I suspect it's your use of commercial cast boolits, they're usually way too hard for use, they don't *"obturate" when fired. Also you're probably using a light charge of fast burn rate powder to fit requirements for SASS use. The combination of a hard boolit, and not enough pressure to obturate, results in poor accuracy.

Since you asked us about "store bought boolits" I again assume you don't cast your own? Or just this caliber? We really need the whole picture of what you're using and the complete loading recipe to fully analyze this.

[runfiverun]

that extra jump with the 45 colt brass isn't helping either.
think about how low the centerline of your boolit is from the centerline of your barrel.
now add in distance.
how is the boolit supposed to enter the barrel straight ?

go up and re-read the two posts above mine then put them all together.

[Wayne Smith]

Read Char-Gar's post at least three times. Remember, or if you don't know, learn, that the classically accurate cast boolit barrels (think Pope) were smaller at the muzzle. If you find that your barrel is in that condition bow down and thank God! If it is the opposite you may need a new barrel, or at least a shorter one - many military barrels are like that because of careless cleaning. Cutting off or drilling out the worn part often solves the problem.

[mdi]

I like to know as much as I can about my guns. I slug the barrels (of so-so importance) and either slug or use pin gauges for the cylinder throats of my revolvers. The bore dimensions are good info (like the tires on your car, do you know without looking what size they are?) and handy, and the throat size is important for sizing bullets. I put a tag on each slug and put it away for future reference and record dimensions in my load book so I can size bullets for each individual gun. For my semi-autos I would use char-gar's methods for finding chamber dimensions along with a slug to determine groove diameter...

[Mohillbilly]

Remember the muzzle puts the finial launch . If the muzzle is loose/ damaged from improper cleaning or assembled wrong at the builders , ( bigger hole at the muzzle than the chamber ) , it will shoot allover the place . The other fellows are right about the chamber/throat fit ( big end ) and pinched barrel (fire or hand lapped cure ) The idea is to have a very slight taper from breach to muzzle , with an slightly over size slug to seal the bore . There many ways , as soft or hard lead boolits or a slightly larger or smaller size will make improvements , after you know what the Rifle sizes are . And if it shoots good enough for you leave it alone ....

[fredj338]

IMO, the cyl throats are as important if not more. If the bore is 0.451" & your throats are 0.450", accuracy will suffer & you will get leading. My SSRBHH was just that. So I opened the throats to 0.451" & use a .0452" bullet. Accuracy was much better & leading almost gone.
The issue for me a slugging is I have more than one of every caliber, so I go larger dia & run with that. In most guns it works fine, but I do have a few picky ones that need more study.

[Randy C]

If I was going to slug a gun I think I would try it this way if applicable.
http://www.youtube.com/watch?v=KuNoo4m6jso

[wmitty]

snaketail:

Use the largest diameter slug the throat will accept / allow to chamber. Softer the alloy; better the chance it will obturate, but like Char-Gar said, there aren't many rifles with a throat smaller than groove diameter...

[DougGuy]

Lots of good solid info here. If I may add this much. Store bought .45 Colt ammo means one thing. Pressure below 14,000psi. At that rate it would be difficult for all but the softest lead boolit to obturate enough to seal the bore, and then you have the .100" shorter .45 Colt brass which wouldn't allow the boolit to reach the chamber throat unless it was a full wadcutter seated out long. That right there is enough to allow the boolit to deform before it even hits the rifling, and not enough gas pressure to obturate it to the bore once it gets there. I don't see where slugging the bore will tell you anything really useful like it would with a revolver.

[MT Gianni]

Slugging the bore does not help. The knowledge gained from slugging the bore can help immensely if you understand bullet fit and do what is necessary to make it happen.

[303Guy]

You don't want the boolit to bump up to fill the gap between case mouth and leade. To avoid that you'd want a harder alloy but then the boolit must fit the throat well so obturation is not needed. Taking a throat slug would help to this end.