Slugging GP-100 .357

[bcr]

In order to get ready to start casting for it, I just finished slugging the barrel and cylinder throats of my SS GP-100 (357).

I learned that it has five grooves, so I guess I can't measure the groove diameter that way. The six cylinder throats each measured a very consistent .357".

What do you guys recommend for size diameter, and how am I going to get the barrel groove diameter?

[Silent]

I'm running a .358-125-RF 50/50 as dropped in my SP101 (snubbie .357). With a Molybee lube, I have no leading issues. Tho I may switch to a different lube since Molybee smokes a lot and kinda stinks :P

[bhn22]

Size to your throat dimensions, and see how things work from there. It's still worth slugging the barrel to get a feel for any restrictions in the bore and to identify any potential thread choke. It is possible to measure 5 groove bullets with a special V-block setup, but I don't know anyone who has one anymore.

[bob208]

take the slog to a machine shop. there is a mic. that will measure 3 sides or they couls put it on an opticcompearer.

you mite have better luck at a place that sharpens end mills

[fecmech]

If you have a mic you can just rotate the slug to catch the corners of the opposite grooves.

[Charlie Two Tracks]

I have the same revolver. I did the throats like you did and for barrel, I tapped a pure lead 158 RNFP through it. As the boolit was even with the muzzle I marked the boolit and the edge of the muzzle with a dry marker so I would know how it was aligned. After pushing the boolit all the way through, I did it again by aligning the marks so it went through the exact same way. I did this four times and the fourth time I could push it through by hand. I could feel that there was more resistance at the muzzle and it decreased as I pushed it through. I could tell there was no restriction where the barrel screws onto the frame. No, I did not find out what the diameter was but I found out the barrel was good. The revolver shoots cast boolits great.
edit: this was done on a 6" barrel.

[geargnasher]

What Femech said.

I have a GP100, 4" SS that had .3556" to .3575" throats, a .3558" forcing cone restriction, and a square .3570" groove diameter past that. Oh, and some out of roundness in the front sight area and the side with the encyclopedia roll-engraved on it. I reamed the throats with a .3580" chucking reamer (and homade chamber pilot) and polished them after firelapping the restrictions out of the barrel. It is important to fix the barrel, if necessary, first, then fix the cylinder because the cylinder throats will get worn from the firelapping. Reaming and polishing the cylinder helps a lot. Gun drives tacks now and doesn't lead a single bit with .358" boolits. I also had a custom .357" expander spud machined for my Lee PTE die so my .358" boolits remained .358" after seating.

Gear

[MtGun44]

.38/.357s are much less variable IME than the .44s and .45 revolvers. I wouldn't sweat
it too much. Size to .358 to start and see what you get, you'll probably do just fine.

Bill

[NHlever]

You can try wrapping aluminum foil around the slug tightly, miking it, and then subtracting double the thickness of the foil. From a practical standpoint you can also just try the slug in your cylinder throats to see how it fits. If it won't pass through, you have a problem, and need to then find the diameter of the slug. If it does pass through you only have to know the diameter of your throats, and size your boolits accordingly.

[Harter66]

Maybe slug like Charley but turn the boolit 180 giving you 10 lands and a place to measure on the slug. Just a flash passing thought.