Ruger GP100

[smokeater]

Bought a used but in really nice condition GP100 4.2" 357Mag. Looking at bore, riflings appear very shallow. Is this normal? S/N dates mfg in 2009. Other than that, bore is spotlless and shiny as a mirror but doesn"t want to group cast bullets plain base or gas checked.

[Divil]

Seems weird. My former .357 GP 100 shot cast accurately. My current .38 Special only 3 inch GP also shoots cast accurately. I would recommend a 140 ish flat based boolit backed up by a middle charge of Win 572 or Win 244. My picture shows my 3 inch .38 shooting DA at 7 yards a cast boolit loading of (accurate custom molds) 36-127-Q, tumble lubed with LLA, then a light coat Rooster Jacket, with 4.6 grains of Win 244.

[megasupermagnum]

They typically have .004" to .005" deep rifling.

[dannyd]

They work

[Wag]

Mine shoots just fine with j-words. I'd check your bore diameter and see if you're cast boolits are too small.

--Wag--

[smokeater]

Not really all that knowledgeable in the revolver realm. Checking with Hornady jacketed 125gr XTP-FP I have one chamber that the bullet falls straight thorough without any resistance and rest of chambers half the bullet protrudes on face of cylinder. Reason asking is that the rifling in the bore is fairly faint. May try sizing .359 and see if accuracy is there. I think I may also have a .360 sizing die as well.

[Silvercreek Farmer]

A trip to Doug for throat uniforming is well worth the money.

What does one of your sized boolits do when you try the same thing?

Try shooting a group using just the large throated cylinder and then one of the tighter cylinders.

[Bigslug]

Have you measured anything?

Off of memory, bore should be something around .346-.347" and groove .357-.358". Cylinder throats about .357-.358.

Have you checked for frame crush (barrel constriction at where it screws into the frame)? This is VERY common on modern revolvers and is usually to the tune of about .001" smaller than the rest of the bore. Common result of this is leading immediately after the constriction caused by gas blow-by around the now too-small diameter bullet. Look up how to fire-lap your barrel. Not hard, and may fix your accuracy issues.

[megasupermagnum]

A .001" thread constriction likely will not cause any serious accuracy problems. I'm sure it isn't helping, but most people get them to shoot fine that way. If it's closer to .003", then that's when things get bad. The only way you will know what you have is to measure it with a slug. Slug only near the muzzle, then slug all the way through the bore. This will tell you what you have for a thread constriction. It will also tell you your bore and groove diameters, which will also let you know your rifling depth. Lastly is the cylinder throats. since you say a .357" bullet doesn't fall through most of them, I'm going to say they are rather tight. Dougguy is the one to contact to fix that, his prices are very reasonable. Before you do that, you want to make sure the problem isn't your barrel. Make sure you don't have a bad thread choke, or any other issue.

[smokeater]

Went back to Step 1 today. Loaded jacketed and cast bullets to around 1100fps or lower and shot a 5" target at 12yds and kept all but 2 rds out of 18 on target with group concentrated in and around center. Shots were offhand. Think I was running more horsepower than I was qualified for and also out of practice due to reloading shortages. Feel much better tonight. Will stay slow and short till I get my act together then expand in 5yd increments so as to not trying to shoot beyond my ability.
megasupermagnum; a jacketed XTP will protrude onto cylinder face but not go thru without pushing however, I have one chamber that the bullet will fall straight thru. What course of action is necessary, open the others up to same as the looser one? My cast bullets are sized .359 and none of them fall thru. Next experiment will be with cast and see if the loose chamber throws the bullet askew or if they are trying to group. I've shot handguns but the 357 is the only gun that I have that has that much recoil. Mainly 9mm autos and 38Spec and one foray with a Ruger 44Special single action.

[Thumbcocker]

Dry fire. Dry fire. Dry fire.

[jrayborn]

12 Yards is a good start. Keep practicing. Sounds like there is promise. Try a few groups at 25.

[MT Gianni]

Try single action at 12-15 yards with loads in the 38 special range. When they tighten up, increase the velocity. My nephews shot great with 158 gr bullets.