S&W 686 357MAG with .354" bore?

[LeadIsBest]

Hello all,
I have been lurker for a while and I have gained a wealth of information from this site.

I am looking for some advice. I slugged the bore of my S&W 686 357mag last night and I came up with some strange results. The slug measures .354”. I slugged the gun two more times and came up with the same measurement. I measured a GP100 that I have and came up with about .3755” so, I don’t think the problem is my process or measurements for slugging the bore. Can this be right? Can the bore really be that tight? Am I doing something wrong?

[454PB]

Welcome to the forum!

S&W's are hard to measure because of the odd number of lands and grooves. Are you sure you measured the widest point?

Generally it's best measured by turning the slug and "feeling" for the widest measurement.

I've owned about 8 S&W .38's and 357's, and never found one that tight.

[LeadIsBest]

Thanks for the input. That's exactly how I was measuring the slug. I started at .358” locked the set screw on my calipers and slowly spun the slug between the caliper jaws. Then went down in .001” increments until I felt resistance. That measurement was .354”

[jameslovesjammie]

My 686-5 has a barrel of .356 and cylinders of .357. I have never experienced leading and it is one of the most accurate revolvers I have ever shot.

If I remember correctly, the old Pythons had .354 barrels, which is part of why they had stellar accuracy.

I measure mine the same way you guys did.

The only other possibility is that you have some constiction where your barrel meets the frame. Do you have leading in the farthermost section of the barrel?

[LeadIsBest]

I have been getting poor accuracy and some leading. I have heard some people describe it as a wash. After firing 12 or so rounds I can get all the lead out with solvent covered patch. It doesn’t seem to be in one spot in the barrel.

The load I am using is:

13.0g 2400
CCI550
357446 or Lee 158 with GC sized to .358” with Thompson’s Bear Lube Cold
About 1250fps

So, should I just buy a smaller size sizing die? Maybe .356”?


Thanks for the help

[richbug]

Is it a 5 groove barrel?

[LeadIsBest]

Yes, it is a 5 groove barrel.

[LeadIsBest]

I guess what I am thinking is:

-do I just try some smaller diameter sizing dies. .356", .355"?

-or is it bad enough that I should try and get S&W to fix it?

what about some of the fire lapping kits? Would they remove .001+ of metal from the bore?

[Bass Ackward]

I have a rescent manufacture 627 that started life out as a .354 bore. After 2600 rounds it is now close to .356.

I was sizing .356 using them in .357 throats. But now the throats cleaned up to about .3575, so I started sizing .357. The bore just gets better and better as the rough and low spots clean up. And the accuracy is picking up as well.

I suspect that it will stop at some point. When it does, I will quit chasing it.

My advice is to quit worring and just shoot it.

[richbug]

If .358 bullets will chamber, shoot them as such, the gun will size them down for you.

[Catshooter]

I have had a 686 with a .3555 bore, with mostly .356 to .357 throats. Since I had other .357s, I sized all my boolits to .359. It shot very well with them and didn't exhibit excessive case expansion with heavy to max loads.


Cat

[machinisttx]

My 686-5 has a barrel of .356 and cylinders of .357. I have never experienced leading and it is one of the most accurate revolvers I have ever shot.

If I remember correctly, the old Pythons had .354 barrels, which is part of why they had stellar accuracy.

I measure mine the same way you guys did.

The only other possibility is that you have some constiction where your barrel meets the frame. Do you have leading in the farthermost section of the barrel?

Pythons featured a choked bored barrel and a 1-14" twist. IOW, the bullet got squeezed down as it went through the bore, and the 1-14" twist is optimal for wadcutters.

[MtGun44]

I'm wondering if you don't have a tight spot where the bbl screws into the frame?
These are more common by far than .354" bbls and the slugging can tell the same
story. Fire lapping is the usual fix or replacing the bbl.

Bill

[S.R.Custom]

I once had a model 19 that was that tight. It was accurate, did not lead, but it was a very dirty gun, in that it blew an inordinate amount of smut out of the cylinder/barrel gap. A mere 12 rounds would render the exterior of the revolver filthy.

[LeadIsBest]

How do I tell if there is a tight spot in the barrel by slugging the bore?

[S.R.Custom]

Clean the bore and look down the muzzle. If it's there, you can see it. Looks like a gentle, wave-like ring in the bore at that point where it enters the frame.

Also, you'll feel it as a slight constriction as you push the slug (or a tight fitting jag & patch) through the bore.

[LeadIsBest]

Thanks again for all the great info. I can't see anything if there is a tight spot where the barrel meets the frame. In fact the barrel has a mirror finish with very few tool marks when I compare it to my Ruger. I would just leave all alone if I was getting better accuracy but currently my cast bullet groups are twice the size of the jacketed ones. Right now I am leaning towards trying a .356" size die.

[shooting on a shoestring]

LeadIsBest - please don't be offened, but I don't believe anyone's (including mine) measurements on the outside diameter of a 5 land boolit. And really, it probably doesn't matter as you can't reasonably do much about it. What are you chamber throats measuring? Also are you water quenching the boolits?

A bunch of this hobby is experimentation, so go ahead and try a .356 sizer. Theory says that will either have no effect or make it worse. If you see something different, heads will turn.

2400 is a good powder choice, but using magnum primers with it is not.

I suggest you close your eyes to the barrel diameter, focus on the throats, get your boolits bigger than your throats, water drop them, use standard primers, stick with 2400 and shoot it a bunch.

[LeadIsBest]

My throats measure .357" measured with pin gages and I am currently sizing to .358"
I had a .360" size die which gave me the same results.
I am water dropping them now. I haven't tried standard primers but I will next time I go out. Thanks for the help.

[9.3X62AL]

L-I-B......

I lean toward SoaS' answer. Assuming your groove diameter IS .354" and the throats run .357", things could be worse.

[LeadIsBest]

Shoestring,
I thought you wanted your cylinder throats bigger than your boolit so they don't size it down?

For example; if you bore was .357" and you sized your boolits .358" then it would be best if the throats were at least .3585"

Do I have this backwards?

[jameslovesjammie]

Boolit should be bigger than the throat.

Thoat should be bigger than barrel.

When you fire the cartridge, the throat should align the boolit with the forcing cone by swaging it down, and the the forcing cone should finish the job and swage it down to barrel diameter.

[Ed K]

When it is said throat should be smaller than boolit, how will loaded rounds fit? I can understand it there were a leade or taper of some sort but if the throats are a straight cylinder, how can you chamber those rounds without force?

I thought ideal boolits were supposed to be about 0.0005" under throat diameter.

[shooting on a shoestring]

Jameslovesjammie has it right. Boolit should be bigger than throat, throat should be bigger than bore.

Ed K the answer is slight force. Grossly oversize and of course it can't load if your leading band has to swage way down into the throat. However, a thousandth over has not been a problem for me. Most of my loads don't have the entire leading band in the throat, just the leading edge of it.

[anachronism]

I too, think you have thread choke at the frame juncture. I've had a couple of 686s come from the factory this way. One wasn't too bad & it came out with fire lapping. S&W rebarreled the other one at no charge. The new barrel also had choke in it, but was lappable. My range rod wouldn't pass through either bore.

[LeadIsBest]

I was trying to get an idea if there was just a tight spot in the barrel if the whole thing was tight. I don't know if this is a good way to measure this but I started a slug down the muzzle about .250" but left enough out that I could pull it back out with pliers. The portion that was just inside the muzzle was .354" also. What do you think?

[Ed K]

Most of my loads don't have the entire leading band in the throat, just the leading edge of it.

In a revolver everything forward of the crimp groove is in the throat.