Ruger gp100 oversized throats.

[J-H-Pointman]

Howdy, I’m new here to the forum and joined to soak up as much info on reloading/casting as one possibly could. While not being new to the reloading scene in general I’m no veteran to the art. I bought a Gp100 .357mag a week ago and planning to reload for it, slugged the barrel and cylinder throats. The bore came out to .357 and the throats came out to .361-.362 I realize this piece isn’t cut for cast bullets due to the blatant gross over sizing of cylinder throats so what in the heck do I do? One fellow said to send it off to Ruger and another said to “fire lap” the bore? The bullets I have are a diameter of .358 and a hardness of 12bhn. I can’t wrap my head around the gun being inaccurate if the cylinder dimensions are off but the bore dia. Are perfect? Any help?

[DougGuy]

Sounds like BUBBA got hold of the cylinder! Did you buy it used?

Call Ruger customer service they rock!

NO to firelapping, don't "fix" the part that ain't broke. And no you cannot size to those throats, the rounds wouldn't even go in the cylinder all the way.

[J-H-Pointman]

Nope bought it brand new.

[ShooterAZ]

First, Welcome to the forum. Have you shot it yet? I'd recommend loading up some of your .358 boolits and give them a try before going to any drastic measures. That's what I'd do anyway...Please keep us posted.

[Outpost75]

Just use cast bullets which "fit" the throat, 0.001" under the pinned diameter is ideal. Use alloy of moderate hardness, 10-12 BHN, book loads, and move on.

Gun will shoot fine.

My 1939 Colt New Service .357 has .359" cylinder throats and .354" barrel groove diameter and is the most accurate .357 I own. I use 50-50 COWW and plumber's lead with 1% tin added, about 10 BHN. No leading even with full-charge loads. Hard alloy is neither necessary nor recommended.

[J-H-Pointman]

Ok sure will.

[rkrcpa]

What did you use to measure the cylinder? .361 seems awfully big from the factory.

[J-H-Pointman]

My midwayusa calipers.

[J-H-Pointman]

That gives me some hope. I’ll report back from the range as soon as possible.

[edp2k]

Calipers can be off by a few thousandths.
Try a pin gage.

[tazman]

Try pushing a jacketed bullet through the throats to see how loose they are. If a .357 slug drops through easily, you are probably close on your measurement. Check some unsized cast boolits for diameter and try sliding them through the throats and see what you get.
If this is your only 357/38 special, you may get by using boolits as cast without sizing them. Just tumble lube and go.
As someone else said, try what you already have. It may shoot just fine without anything special.

[DougGuy]

Just use cast bullets which "fit" the throat, 0.001" under the pinned diameter is ideal. Use alloy of moderate hardness, 10-12 BHN, book loads, and move on.

If throats really are .361" and the SAAMI spec for the chamber itself is .381" and the OP loads a .360" boolit into brass that's .011" wall thickness, .360" + .011" + .011" = .382" at the case mouth, it won't fit in the chambers.

However, an alloy of BHN12 would easily obturate to fill the throats provided the loads aren't light target loads. More than one way to fit to the throats..

[megasupermagnum]

I don't see what's so wrong. I universally load .359" bullets in 38 caliber. The only gun I loaded for that needs a .358" is a S&W J frame. I was recently loading for a Dan Wesson 357 maximum that had .361" throats. I loaded bullet sized .3605", it shot great, and had no problem chambering.

Try .360" bullets in that GP100, and you should be just fine.

[Outpost75]

If throats really are .361" and the SAAMI spec for the chamber itself is .381" and the OP loads a .360" boolit into brass that's .011" wall thickness, .360" + .011" + .011" = .382" at the case mouth, it won't fit in the chambers.

However, an alloy of BHN12 would easily obturate to fill the throats provided the loads aren't light target loads. More than one way to fit to the throats..

The .381 is min. chamber spec. Most run a bit larger with up to +0.005" being fairly common.

[Petrol & Powder]

He's using calipers to measure the throats. I suspect that .361" is not a true measurement.

[nhithaca]

Really need to use pin gauges to accurately measure the throats. If they truly are 0.361" then send it back to Ruger. A new gun should be right.

[35remington]

Yes...measuring cylinder throats with calipers guarantees an incorrect measurement.

[dogdoc]

You get a pin gage set .251 to .500 for around 100 bucks on amazon. I use mine all the time for measuring throats and other things. Well worth it if you are a serious hobbyist.



Sent from my iPad using Tapatalk

[Kenstone]

My midwayusa calipers.
I have read all the recommendations to buy pin gages $$$
A $20 set of ball gages and a micrometer is really all you need.
Half ball gages can measure a diameter close to a shoulder/step and are what I prefer/use.
Ball gages can also check for taper, something a pin gage cannot do.
Here's some:
https://www.amazon.com/s?k=ball+gages&ref=nb_sb_noss_2
And there's telescoping hole gages for bigger holes.;-)
:coffeecom

[Forrest r]

Let your targets do the talking/thinking/measuring for you. Load some of your .358"/12bhn bullets up with old standard loads like 3.5gr of bullseye in some 38spl cases & hit the loud button a couple hundred times.

If you get patterns instead of groups you'll have your answer.

[Silvercreek Farmer]

Do you boolits drop through the throats freely?

[DougGuy]

I have read all the recommendations to buy pin gages $$$
A $20 set of ball gages and a micrometer is really all you need.
Half ball gages can measure a diameter close to a shoulder/step and are what I prefer/use.
Ball gages can also check for taper, something a pin gage cannot do.
Here's some:
https://www.amazon.com/s?k=ball+gages&ref=nb_sb_noss_2
And there's telescoping hole gages for bigger holes.;-)
:coffeecom

Still not as accurate as pin gages, and yes you can find taper with them very easily. Ball gage only measures diameter at a given spot. Pin gage measures 360 degrees and the entire length of the throat which a ball gage cannot do. Since boolit fit is dependent on 360 degrees, the pin gage is the accepted measurement for cylinder and barrel work.

It takes a long time and patience to develop a touch with ball gages and the % for inaccurate measurements is quite high. Not so with pin gages.

[35remington]

Since pin gauges are cylindrical, they most certainly can and do check for taper. Doing so is a simple matter of entering the gauge from both sides of the cylinder.

[Kenstone]

Still not as accurate as pin gages, and yes you can find taper with them very easily. Ball gage only measures diameter at a given spot. Pin gage measures 360 degrees and the entire length of the throat which a ball gage cannot do. Since boolit fit is dependent on 360 degrees, the pin gage is the accepted measurement for cylinder and barrel work.

It takes a long time and patience to develop a touch with ball gages and the % for inaccurate measurements is quite high. Not so with pin gages.

I'm going to have to respectfully disagree here, maybe because of the long time/patience/touch I have in using hole gages.
The long time experience has been recently greatly compressed with the advent of the net and youtube though :coffeecom

I'll just respond to a few of your statements and don't really want to start a debate here.
1.-""Still not as accurate as pin gages""
A ball gage can measure a hole to within a 0.0001".( yes I know standard gage pins are usually -0.0002")
A pin gage can only measure a hole to the increment of the pins, in most cases 0.001".

2.- ""Ball gage only measures diameter at a given spot.""
Once the ball gage is set, it can be moved in/out of the hole to detect taper.

3.- ""Pin gage measures 360 degrees""
A ball gage can be rotated and rocked thru for feel/check for roundness, a pin gage only measures the narrowest part of an out of round hole, without detecting any of the "ovality", and only to the nearest 0.001"(the increment of most pin gages)
Sure you can rock a pin gage to check for ovality but that's not measuring it, it's just "feeling it."


Since pin gauges are cylindrical, they most certainly can and do check for taper. Doing so is a simple matter of entering the gauge from both sides of the cylinder.

4.- ""a simple matter of entering the gauge from both sides of the cylinder.""
Yes, that will work for thru holes like cylinder throats, what about the chamber that ends at the throat step?
How do you determine that diameter(?), it could be bigger than the diameter at the back end of the chambers but cannot be determined with a pin gage.
A pin gage cannot determine/identify a restriction/burr at the mouth of a blind hole, a ball gage can, along with any taper small to big and any wide/narrow grooves in such a hole.

Yes pin gages are a valuable tool to measure small holes but are not the only way and having an alternate is always good.

I understand your views on this, I just have a different perspective.
And as stated, I won't be debating any of this but feel free to comment/respond.
:mrgreen:

[str8wal]

How does it shoot???

[DougGuy]

I'm going to have to respectfully disagree here, maybe because of the long time/patience/touch I have in using hole gages.
The long time experience has been recently greatly compressed with the advent of the net and youtube though :coffeecom

I'll just respond to a few of your statements and don't really want to start a debate here.
1.-""Still not as accurate as pin gages""
A ball gage can measure a hole to within a 0.0001".( yes I know standard gage pins are usually -0.0002")
A pin gage can only measure a hole to the increment of the pins, in most cases 0.001".

2.- ""Ball gage only measures diameter at a given spot.""
Once the ball gage is set, it can be moved in/out of the hole to detect taper.

3.- ""Pin gage measures 360 degrees""
A ball gage can be rotated and rocked thru for feel/check for roundness, a pin gage only measures the narrowest part of an out of round hole, without detecting any of the "ovality", and only to the nearest 0.001"(the increment of most pin gages)
Sure you can rock a pin gage to check for ovality but that's not measuring it, it's just "feeling it."



4.- ""a simple matter of entering the gauge from both sides of the cylinder.""
Yes, that will work for thru holes like cylinder throats, what about the chamber that ends at the throat step?
How do you determine that diameter(?), it could be bigger than the diameter at the back end of the chambers but cannot be determined with a pin gage.
A pin gage cannot determine/identify a restriction/burr at the mouth of a blind hole, a ball gage can, along with any taper small to big and any wide/narrow grooves in such a hole.

Yes pin gages are a valuable tool to measure small holes but are not the only way and having an alternate is always good.

I understand your views on this, I just have a different perspective.
And as stated, I won't be debating any of this but feel free to comment/respond.
:mrgreen:

I actually own a Dorsey split ball dial indicator and a Starrett split ball dial indicator, both read in .0001" there are advantages to split ball as you stated, it's almost hilarious watching the dial sweep as you turn the ball in an egged cylinder throat. I rarely pull these out anymore as you can assume plenty of Rugers have egged throats. I just hone them to size and usually the Sunnen hone has made the throat pretty round by the time it finishes.

I find the gage pin closely mimics what the boolit will be presented to, since gage pins and boolits are both cylindrical in shape, so if there is an oval throat, both boolit and pin will find their own center in the same manner, by finding the least resistance. No other measuring tool assimilates the boolit better than a pin so this is what I use almost exclusively.

[Jtarm]

I could never see spending $100 for a set of pin gauges so I do it the old fashioned way: drive a soft lead slug through the mouth and mike it.

A box of 100 Speer 50-caliber soft lead balls costs $16 and will last for years.

I save the ones used on .44/.45 caliber and reuse on .357s.

[DougGuy]

You can get single pins in a range to do one gun for $25 in half thou increments. Meyer gauge sells singles for 2-3 bucks in the 35 caliber range they have a minimum order of $25 so you could cover .356" to .361" in ten pins and make minimum order.

[str8wal]

I could never see spending $100 for a set of pin gauges so I do it the old fashioned way: drive a soft lead slug through the mouth and mike it.

A box of 100 Speer 50-caliber soft lead balls costs $16 and will last for years.

I save the ones used on .44/.45 caliber and reuse on .357s.

My procedure as well, but I use lead fishing sinkers which can be had in any SG store.

[35remington]

Since the OP is concerned about the diameter of his cylinder throats, and you admit that pin gauges can detect taper in cylinder throats, I will let the obvious statement of agreement that pin gauges can indeed detect taper in this area of concern to the OP stand on its own merits. The statement that pin gauges can detect taper is quite valid.

Let us not discuss other concerns in the area the case fits in rather than the bullet. Pin gauges are a widely accepted standard tool for gauging revolver cylinder throats. That will remain so despite any commentary to the contrary because they accomplish the needed measurement sufficiently well that the proper fix is identified and applied due to their usage.

They do the needed job well. They have a wide range of application to other uses as well. No more needs be said.

[Jtarm]

My procedure as well, but I use lead fishing sinkers which can be had in any SG store.

I don’t find that the case much these days. Most stores I go to are selling zinc.

[Kenstone]

Most cylinder holes are no longer cut in the traditional drill/ream process of the past, but instead are cut with circular interpolation programming.
That method introduces many more variables to hole cutting that can affect all aspects of the resulting hole.
The hole can end up egg shaped/tapered/oval/ribbed/etc. depending on the ramp-up in the program, size of the cutter, cutter sharpness/depth of cut, with any and all these conditions subject to what is acceptable results by the maker.

Something to ponder for those who only envision the old school drill/ream machining process.

These holes considered "good enough" by the maker, can be "improved" by "us" using different processes like reaming/lapping/honing but only by making them bigger...[smilie=b:

https://www.bing.com/search?q=circular+interpolation&qs=AS&pq=circular+int&sc=8-12&cvid=8494DA430CD34CE48431FFE874E09C3E&FORM=QBLH&sp=1
just some info, interesting for some, useless for most,
:-)

[J-H-Pointman]

Ok sorry for the large gap of time from my last post to now work has been hectic lately. I took it to the range and bought some Fiochi .357 mag fmj to test and the shots were all over the target as well as keyholing.

[onelight]

With calipers I would slug the cylinder with pure lead round balls and measure the balls you will be close enough.
I have not had much success measuring a hole with calipers.

[J-H-Pointman]

problem is when I try to use a .358 dia. Slug it falls right through the throat no force needed.

[J-H-Pointman]

I just sent it off to ruger they said they’d fix it.

[gnostic]

With calipers I would slug the cylinder with pure lead round balls and measure the balls you will be close enough.
I have not had much success measuring a hole with calipers.

Onelight nailed it. Because of the radius of the chamber a dial calipers won't work, you can see it when you insert them into the chamber. My bullets drop from the mold at .360-.361, push one through the cylinder and see where you're at. I'd size some bullets to .358 and shoot them, I don't think you have a problem....

[cupajoe]

I find it hard to believe the factory throats are oversized but it is sure possible. I hope Ruger does you right.

[bigted]

I just sent it off to ruger they said they’d fix it.

Cool! Best overall fix.

As for your "calipers" for measuring ... my calipers do me a fine job ... easy now all ye naysayers. I use the "sharp" end of the caliper to loosely measure a hole. If you use strength and begin flexing a good set of calipers, they become useless. They are capable of giving you the closest .001 of an inch ... providing the calipers have been used properly and cared for properly.

Never force a set of calipers.
Never store calipers completely closed.
Never store calipers dirty.
Never use calipers as a scraping tool.
Always use care when using calipers and treat them as a fine measuring tool.

Now having said this, NOTHING can take the place of ball gauges, pin gauges and other bore measuring tools such as snap gauges.

I know this furthers the debate ... HOWEVER ... most of us use these as a GENERAL gauge to get an idea of the nearest .001 inch ( thousanths ) . Rarely do general shooters have need to get any closer that this ... especially cast boolit shooters. Softer lead boolits will expand under powder explosion/burn pressure.

We are not making nuclear high grade weapons here. Just mostly shooting our guns and have expectation of REASONABLE accuracy. If most of us can hit a 2 or three inch dot at 30 yards, we are happy.

[dkf]

You can get single pins in a range to do one gun for $25 in half thou increments. Meyer gauge sells singles for 2-3 bucks in the 35 caliber range they have a minimum order of $25 so you could cover .356" to .361" in ten pins and make minimum order.

That is what I do. I usually just buy the single pin gauges I need for a caliber from McMaster. Class Z are a little over $4 each, can get cheaper elsewhere but MC is convenient and I get them next day.. I don't want to have to run to my shop and get them out of my sets. If you are really anal you can get class ZZ pins in .0001" increments. I have customers that ask if you have precise accurate pin gauges before they will give you the work, there is a reason for that. I have plenty of calipers and a plethera of telescoping and ball gauges for indirect measurement and they are used for +-.001". That calipers not even for that.

[winelover]

Give me oversize throats, all day long, for cast bullets. Under size is the real problem.............throats will act as bullet sizers. No need for pin gauges. Just feed your revolvers cast bullets that will be a loose slip fit in the throats. Instead of buying pin gauges, invest in larger sizing dies or moulds that cast larger bullets. It ain't rocket science.

Winelover

[dkf]

I would rather have undersized throats and open them up to what I need and be able to control the variation in size. When buying a gun (if you can) test the throat size with pin gauges before you plunk down the cash you can avoid doing further work or having a future headache.

[J-H-Pointman]

I will get some pin gauges and keep for future reference. Thanks to all for the insight, I should have sent it off to ruger first inkling.

[TCLouis]

I still have not seen a report back about dropping a bullet of "known" diameter through the cylinder.

[str8wal]

I still have not seen a report back about dropping a bullet of "known" diameter through the cylinder.

Right?!? And if you have to force it through you can mic it after and get an accurate measurement.

[onelight]

problem is when I try to use a .358 dia. Slug it falls right through the throat no force needed.
I use .375 lead balls if you want to make them easier to start run them through a sizer you know is bigger than the throat , remove the cylinder so you don't spring the crane if DA and drive them through with the largest wooden rod that will fit through the chamber. This method is not as precise as a machinist would measure but gets you close enough to determine what will work in your gun , or if you need to send it to the factory or a machinist for proper repair or replacement of the cylinder.