9mm cylinder throats

[Piedmont]

I keep searching on the web for 9mm revolver info and there just isn't much out there. Since putting my 9mm Security Six together, I was slowly getting my butt in gear and ready to order a bunch of moon clips for it. Something made me slug the cylinder throats. I remember now, .357 bullets weren't even close to going in the front of the cylinder. I had assumed I would be dealing with .357 or .358 throats. Why would Ruger cut a special diameter? Well, they certainly wouldn't have gone smaller than .355, let's see what they are.

So I slugged two throats and got .351". That is not a misprint and I have slugged a whole lot of firearms, there was no procedural error. I even rechecked my micrometer and it was fine. I guess Ruger wanted to be certain the gun would never be accurate. The cylinder is off getting reamed to .357". My barrel slugged .3565 as near as I can determine, but it is a .357 Mag barrel. I did slug the 9mm barrel from the original gun in 9mm (it was a police cut-up gun, destroyed to keep us all safe) and it measures .3560".

The purpose of this rambling post is to ask you guys with 9mm revolvers to slug your cylinder throats and tell us what they measure. Is mine an abberation?

[John Traveler]

I suspect the tight 9mm throat diameter is to reduce the chance that bullets would jump the case mouth and tie up the revolver. As you know, the 9mm cartridge is not roll crimped like the rimmed cartridges are.

S&W historian Roy Jinks even wrote once that in airweight revolvers, there was a problem with bullets jumping crimp. It was decided to reduce the diameter of chamber throats to prevent bullets from jumping their crimps and jamming the gun.

[Sprue]

Piedmont,

I just measured all 6 throats and they are ALL .351 as is the forcing cone. (for 9MM Speed Six)

As for accuracy, I've never noticed anything out of the ordinary as for POI.

I also have a 4" Stainless Security Six .357 and the throats measure .359

I've never had a leading problem with either.

Also have Security Six 2.75 inch blued in .357 and the throats are all .359 as well.

Hope this helps

Sprue





Hi Sprue, That is a pretty Ruger Speed Six 9mm you have. I just recently took a cylinder from a police "destroyed" gun like that and dropped it into a 4" Security Six. I am writing because I slugged two cylinder throats and got a measurement of .351 in both. So I sent the cylinder off to a smith to get them opened to .357. I figured it was going to be hopeless at .351. I put a thread on this in the "Wheelguns" section asking others what their 9mm revolvers measured in the throats.

I got no response. Since I know darn well you have one of these (grin), I was hoping you would give me at least one more data point.

Thanks in advance,
Piedmont

PS How accurate is your 9mm?

[The Virginian]

I think getting the chambers properly reamed will help a lot especially with cast bullets. Let us know how it turns out.

[Piedmont]

I am going to keep this thread updated for posterity.

Adrians posted the other day that he had measured his SP 101 9mm cylinder throats and they, too, were .351". So Ruger continued the practice into the SP line.

My cylinder is back from the pistolsmith who reamed all my cylinder throats to .357". Before sending it off I had only one range session. The single cast load I had fired featured an LBT 125 grain .358" truncated cone with Chinese surplus powder (long gone) for a velocity of a little over 1,000 fps. had produced a ten shot 6" vertically strung group. That same load was retested today with my now larger cylinder throats and put ten into a round 3.35" group. The groups were fired from between my knees with a back rest at 25 yards. I think we can call that an improvement!

The other load, a type of jacketed ball, that I had initially tested had pierced some primers so I didn't want to subject my breech face to more of that.