help guys! i'm more confused than ever!

[mozeppa]

i just got done slugging the bores on my 4 favorite shooters....and i'm puzzled!

i've read time and time again that you slug the bore from muzzle end back to the cylinder or bolt face for auto's
you take that diameter and add 2 thousandths for cast ....and that's the diameter that you should use.

here's some measures i got using mics (not calipers)

S&W model 629 44mag.
using a pin gage set as "go & no-go" the cylinder throats are .429
using same set of pin gages to see what the exit hole in the barrel is ....and got .417 pin will go in.... .418 pin will not.
then i slugged the barrel with a soft lead sinker....pounded it thru ...and it mic'd out at .429

to my way of thinking...the 44 mag sounds about right ...or is it?

.....get ready for weirdness.

colt king cobra 38/357 mag
using a pin gage set as "go & no-go" the cylinder throats are .359
using same set of pin gages to see what the exit hole in the barrel is ....and got .346 pin will go in.... .347 pin will not.
then i slugged the barrel with a soft lead sinker....pounded it thru ...and it mic'd out at .352 really??? is that right?

S&W 686 38/357 mag
using a pin gage set as "go & no-go" the cylinder throats are .358
using same set of pin gages to see what the exit hole in the barrel is ....and got .346 pin will go in.... .347 pin will not.
then i slugged the barrel with a soft lead sinker....pounded it thru ...and it mic'd out at .353 really??? is that right?

lastly ...

star model A 1911 in 38acp (took a few factory loads apart j-words measure .355) ?
using same set of pin gages to see what the exit hole in the barrel is ....and got .349 pin will go in.... .350 pin will not.
slugged the barrel with a soft lead sinker....pounded it thru ...and it mic'd out at .351

so.... why do they measure so differently?
should i just be concerned with cylinder throat diameters and for get the bores ?
or am i doing things wrong?

i have several 357's these are my fav's all of them have bores that are a good .007 smaller than the cylinder throats.
one is .008

please ....somebody break this down for dummy me!

[dubber123]

The 629 I would personally like to have larger throats, but I have at least 1 revolver with cylinder throats and barrel the same that shoots VERY well. 1" at 50 yards. For the others, I would size to snugly fit the throats and not worry a bit about the bores, as it is smaller than that measurement. Just me, but I have had good results doing it this way.

[Outpost75]

On ANY firearm you want to size bullets to fit the unrifled portion of the chamber ahead of the case, before the rifling starts.

Doesn't matter if it is a rifle, revolver or autopistol. You want to measure the THROAT diameter, and IGNORE the bore and groove diameter of the barrel. It DOESN'T MATTER!

Size bullet to enter cylinder throat or ball seat ahead of case mouth without resistance. Exact fit without marking is desired, but lead-antimony alloys will grow a bit as they age harden, so 0.0005" that is 1/2 of a thousandth of an inch under, is perfect.

[jcren]

Did you notice if the slug got tighter right before the forcing cone? Frame constriction is fairly common, I have a 38 snub that measures .357 until the frame area where it squeezes to .354.

[44man]

Don't bother with pin gauges in the bore, only gives bore size when groove is more important.
The S&W with .429" both should shoot OK with .429" boolits but would not hurt to go .4305" throats and use .430 boolits. If it shoots, leave it.
It is why we slug, because so many guns vary. Factories don't much care if a factory load will get through.

[DougGuy]

IGNORE the bore and groove diameter of the barrel. It DOESN'T MATTER!

As long as the throat is GREATER than groove diameter, it doesn't matter. If throats are SMALLER, it matters A LOT ;)


Size bullet to enter cylinder throat or ball seat ahead of case mouth without resistance. Exact fit without marking is desired, but lead-antimony alloys will grow a bit as they age harden, so 0.0005" that is 1/2 of a thousandth of an inch under, is perfect.

You need some wiggle room, .0005" to .001" is advisable. It is not uncommon for a boolit to grow .0003" to .0006" as they age harden. When I first started reaming and honing cylinders, I sized my SBH throats to a perfect fit on a .432" Lee C430-310-RF boolit, 3 months later only half the loaded ammo would go completely into the charge holes..

__________________________________

Pin gages. Very good tools to measure I.D. with. I use them on everything I work on. You *DO* need the incrementals, the half thousandth sizes in between, for more accurate work. A whole thousandth is still guesswork at best. Pin gage sets that run in tenths of thousandths are pretty much unusable with revolver cylinder throats, by the time you get one that goes into a throat, it is several tenths smaller than the actual throat diameter.

I use a Dorsey DBL248 dial bore gage that reads in .0001" with a range of .248" to .500" which really cuts fine hairs when it comes to wanting to know exact diameters of small holes. Pin gages will get close, pin gages in half thousandth increments get really close, but the dial bore gage will accurately read to the tenth of the thousandth every time and if you want to know exactly how EVEN the throats are with one another, this is an indespensable tool to have on hand.

You can get close by observing the amount of drag against a pin gage but this will not tell you if the throat is oval, belled, or convex, it only tells you that this throat is snug compared to another throat. The dial bore gage disspells all mysteries within the throat.

A close fitting pin gage will gage the amount of resistance your fingers can feel from throat to throat and if this is pretty close, this is the *most important* part of the throats. This is the resistance the boolit will be subjected to, and this is what controls pressure in each chamber which affects recoil impulse in the shooter's hands, which ultimately affects point of impact on the target.

[DougGuy]

Mozeppa there are some irregularities in your slug dimensions. Your pin gaged throats seem correct, and your pin gaged bore diameters seem correct, they are reporting the same size as the pilots I use for throating barrels, and these pilots ride on top of the lands not in the grooves. So .346" is correct for the majority of .35 caliber barrels measured across the lands (a.k.a. bore diameter).

The figures you are reporting for groove diameters seems quite a bit smaller than they should be. I suspect your mic may be flattening the slug where you are measuring? Something in your method there is escaping being measured or read accurately. None of the firearms you listed will truly have .007" or .008" difference between groove diameter and throat diameter.

Both revolver and autopistol bore/groove diameters can vary quite a bit. Generally Ruger and S&W have the most consistent barrels. Generally Ruger and S&W have the most consistently smaller throats. There are exceptions.

[44man]

Rifling depth will vary also and I believe a little deeper for cast is better.
The best in a RB gun can reach .010" because of the patch. Many guns will average .003".
Screwed up my mind to see the Marlin Ballard at the same .003" as the Micro Groove. True Ballard is deeper.
Only the BFR revolvers have been true. 45-70 has a .450" bore and a .458" groove. .004" depth. Throats are .4595" so it takes a .459" boolit. The .475 has a .465" bore and a .475" groove for a .005" depth. Throats are .4765 so I shoot .476" boolits. They got it right. More depth stops skid better. More pressure needs more depth.
I think some very high pressure revolvers could live with .006" depth.
I don't know how Doug feels but he has never been wrong. The hardest lead we shoot will "BUMP" as well as jacketed.

[mozeppa]

doug guy....my mic is spot on with the pins....and as far as flattening the slug... the mic has a "clutch" that ratchets when the anvils meet any resistance.

if i sent you all my cylinders for .357 mag....and had you make them all .358 at the cylinder throats would that work? (so i wouldn't have to cast,and size to 3 different throat sizes?....or is there more that would screw things up by doing that?)

pardon my ignorance but that seems like a logical question for a beginner.

[Mitch]

sounds like something is wrong with your groove dementions.how big are the sinker you are usesing.lead my be shaveing off if they are to big or they are not big enough to get a good reading. from the pin of .346 for your bore size you should have a groove size of about .357.Unless you have a very bad thread constriction and there is no thread on an autoloader.so something is going wrong with your barrel slugging.If you have some boolits casr for it try one of them.Make sure you barrel is very clean befor you slug and be sure to use some lube in the bore.i use LPS it works good for lubeing lead in the bore.

[DougGuy]

doug guy....my mic is spot on with the pins....and as far as flattening the slug... the mic has a "clutch" that ratchets when the anvils meet any resistance.

if i sent you all my cylinders for .357 mag....and had you make them all .358 at the cylinder throats would that work? (so i wouldn't have to cast,and size to 3 different throat sizes?....or is there more that would screw things up by doing that?)

pardon my ignorance but that seems like a logical question for a beginner.


No it's a perfectly logical thing to want to do, so you have the same size boolits and common die settings across all the guns of the same caliber. A good many shooters send me their .357 cylinders to ream to .359" and sometimes to .360" so that the boolit diameter that works in their leverguns, also works in the revolvers without the aggravation of having separate sizes, different dies, just to get them all to shoot well. Leverguns are a LOT more picky with boolit diameters than revolvers so once you get the levergun figured out, it is MUCH easier to make the revolver accept the same ammo than it is to get a levergun to shoot well with the smaller boolits that fit in the revolver cylinder.

If you send the cylinders, I will make them right for use with .358" boolits. They will come back to you between .3585" and .3588" to give a little wiggle room for age hardening, when alloy grows b/c of antimony content. Your throat diameter needs to be minimum .0005" to .001" over boolit diameter so that this doesn't bite you in the hind side later. Ask me how I know this from experience..

44man is right, I like a deeper rifling as well, it gives more to grab on. There may be less pressure due to less resistance with 5 wider and shallower lands ala S&W, but for cast boolits, I find the Rugers to be about as good as one could want it to get. Their rifling is tall, well profiled, and grips a cast boolit effectively. My favorite combo for Ruger rifling profile and twist rate is 50/50+2% alloy with Felix lube or SPG lube. This alloy, that I can scratch with a thumbnail, and the smooth sided RF boolit profile with the wide flat meplat takes to the Ruger barrels like a duck to water.

For a Ruger .357 magnum, your groove diameter *should* be right at .357" and the pilot I use most often is .346" so that's .0055" of rifling height per land for a difference of .011" in total. That's why I was thinking something in your measurements is not coming out like it should. The .45 calibers are right at .451" and have been for years, and I use either a .442" or a .4425" pilot. The pilots that come in the Brownell's chamfer cutter kit (forcing cone tools) are .346" .416" and .442" for .38/.357 .44 and .45 caliber revolvers. Not only are these made to fit fairly snug in the bore, but you can use them as a measuring tool as well. So you can figure the difference between the pilots and the known groove diameters and this will give you the rifling height profile. .0055" in the .38/.357 barrel, .007" in the .44 magnum, and .0045" in the .45 ACP/.45 Colt barrels.

Now.. if you are slugging by pushing or tapping all the way through the bore, and you have ANY constrictions or "lawyer warning ridges" inside the bore, they are going to swage your slug down accordingly. I have often advised to take a plastic cleaning jag and patch it tightly into the bore so that it takes a decent amount of effort to push it through. While doing this, if the resistance of pushing the jag changes, what you are feeling is the bore changing dimensions. For example, many of the older "Circle logo" Ruger .44s from back in the plum bluing days, have a noticeable bulge just in front of the frame/barrel juncture. You can feel this with the jag because it will get so loose you can rattle it with your fingers. Then it gets snug again as you push it through the threaded part of the bore and on out the forcing cone. I have seen this with a LOT of the older guns. It's just how Ruger machined the barrel shoulder and how they were tightened against the frame that causes the bulge.

For yours to come out smaller than what I would consider normal and expected groove diameter, it tells me that there may be a constriction somewhere in there. If you use the tightly patched jag method, and the jag gets real hard to push when it gets to the frame, this is thread choke you are feeling and also why the slugs are coming out smaller. You can use the slugs to measure how much choke there is by pushing one all the way through, and then putting a wooden or aluminum or brass rod say 2" shorter than the breech face to muzzle measurement, and you can use it to push out a slug that you only drive in, until it contacts the rod. This will give you a measurement of groove diameter at the muzzle. You may find this slug to be much closer to .357" and then you have found why your other measurements aren't sounding like they should. If there is a difference in the diameter of the two slugs, this is approximately how much choke is in the barrel.

[mozeppa]

Thanks Doug,