Completely newb question about boolit fit

[jamesp81]

I measured my GP100's chamber throats and they came out to .358. The groove diameter in the barrel is coming out to slightly over .357, roughly .3572. What is the proper sized boolit here?

I've been shooting light loads with 18 BHN 148gr wadcutters that measure out to just s shade under .359 (which surprised me since they're Missouri Bullets and are advertised as .358). I still get some leading in the forcing cone and right at the beginning of the rifling. It's annoying, but it's not too bad to remove from there so I just kind of suffer it and clean it occasionally.

Should my cast boolits be .001 over groove diameter or .001 over chamber throat diameter? Is it safe to run .359 boolits down a .3572 bore? And are my leading issues likely related to too hard a boolit for a light load? And if I decide to start loading full house magnums, is 18 BHN a good hardness for that application?

[Grandpas50AE]

I think what you are getting is some leading from the boolits shaving in the throats, since your boolits are a little larger than the throats. If I recall correctly, the boolits should be same as throat diameter or .001 under, but still at least .001 larger than the groove size of the bore. Are these boolilts purchased, or cast yourself?

[Iron Mike Golf]

I go for equal to throat and .001 over groove diameters. Bigger than throat will either swage down or shave down. That depends on the back edge of the throat, boolit shape, and boolit hardness. In your gun with that hardness, a RN might swage down and a WC shave down.

If it shaves down, then you got lead scraps can give you your leading. Does it keep building up as you shoot more rounds (meaning dozens then hundreds) or does it build up just so far and then stays that way?

[williamwaco]

I measured my GP100's chamber throats and they came out to .358. The groove diameter in the barrel is coming out to slightly over .357, roughly .3572. What is the proper sized boolit here?

I've been shooting light loads with 18 BHN 148gr wadcutters that measure out to just s shade under .359 (which surprised me since they're Missouri Bullets and are advertised as .358). I still get some leading in the forcing cone and right at the beginning of the rifling. It's annoying, but it's not too bad to remove from there so I just kind of suffer it and clean it occasionally.

Should my cast boolits be .001 (.001 or .002) over groove diameter or .001 over chamber throat diameter? Is it safe to run .359 boolits down a .3572 bore? ( Yes, even .360 would be fine) And are my leading issues likely related to too hard a boolit for a light load? ( Probably but you never know until you try something different This combination never works for me) And if I decide to start loading full house magnums, is 18 BHN a good hardness for that application?(NO That is way too hard for any handgun unless you are hunting elephant. I mix alloys for 9 to 12 BNH for .357 and .44 Mag. )

Remember, the .44 mag was developed by Elmer with 1/20 tin/lead. That is about BNH 10.

All the referrences in the old casting literature to "Hard Cast" are referring to Lyman No 2 at BNH 15.

[Calamity Jake]

Those Missouri bullets are to hard and have the wrong lube for your low volisity loads.
Hardness should be 11-15 with a softer lube, something like 50/50 or slighty harder.

[Maven]

"Should my cast boolits be .001 over groove diameter or .001 over chamber throat diameter? Is it safe to run .359 boolits down a .3572 bore? And are my leading issues likely related to too hard a boolit for a light load? And if I decide to start loading full house magnums, is 18 BHN a good hardness for that application?"

Jamesp81, I have a ca. 1992 Ruger Blackhawk and find I get better accuracy when I size my CB's to .359", which gives me an interference fit in the cylinder throats. I.e., it takes a firm push with a wooden dowel to push them through the cylinder and out the throats. As for the 18 BHN, I'd try something softer, say 10 - 12 BHN for low velocity loads, but 18 should be OK for full house work. If you're up for and experiment, try to reduce the BHN of those CB's by placing several dozen of them on an old cookie sheet (Don't use it again for food products though.); preheat your oven to 400 deg. F and leave them for ~45 mins. Turn off the oven and let them cool to room temperature. Remove and tumble lube with Lar's Xlox (See our sponsors.) or Lee Liquid Alox. Load with 3.0 - 4.0gr. Alliant Bullseye or an equally fast powder. That should reduce leading and give excellent accuracy.

Let us know how you fare.

[mdi]

I like to size my bullets to the same diameter as the ID of my cylinder throats. My Ruger and S&W get .431" bullets and my Dan Wesson gets .430" because that's the size of the cylinder throats (and I know that the throats are larger than the groove diameter). My .38s and 357 get .358" bullets. My alloy (wheel weights + range scrap+ ?) runs about 10 - 12 BHN and I get very little or no leading. With my limited experience I would say to try bumping up your load and/or running .358", softer bullets which may help the leading. I've found that bullets larger than cylinder throats seem to spray lead on the cylinder face and in the forcing cone, but That may be just me...

[jamesp81]

For full house magnum loads, would 18 BHN still be too hard of a boolit? I'm thinking of trying the advice I got elsewhere and starting out with 10.0gr of 2400 with 180gr boolits and working up from there.

These boolits will be commercial, btw. I don't have a good place to do casting and even if I did, I don't have the time, not if I actually want to ever shoot what I'm loading.

[gray wolf]

size to the cylinder throats,
18 BHN is not 10--11--or 12

With a smile--don't let me repeat myself

[Char-Gar]

I measured my GP100's chamber throats and they came out to .358. The groove diameter in the barrel is coming out to slightly over .357, roughly .3572. What is the proper sized boolit here?

I've been shooting light loads with 18 BHN 148gr wadcutters that measure out to just s shade under .359 (which surprised me since they're Missouri Bullets and are advertised as .358). I still get some leading in the forcing cone and right at the beginning of the rifling. It's annoying, but it's not too bad to remove from there so I just kind of suffer it and clean it occasionally.

Should my cast boolits be .001 over groove diameter or .001 over chamber throat diameter? Is it safe to run .359 boolits down a .3572 bore? And are my leading issues likely related to too hard a boolit for a light load? And if I decide to start loading full house magnums, is 18 BHN a good hardness for that application?

1. .358
2. .359 won't hurt, and it is unlikely you will see any degrading in accuracy from .358. It may be there, but you won't notice it.
3. .18 BHN is harder than necessary but will do just fine.
4. Some commercial bullets are good, but most are nothing but a source of frustration for the user. Most likely these are the source of your leading.
5. Without more information, that is the best I can do.
6. 14 to 14.5 grains of 2400 under a 150 - 160 grain bullet is a very good load in the 357 Magnum round. Use standard and not magnum primers.

Good luck with your loading and shooting

[geargnasher]

First, what size boolit that your gun will prefer depends on a lot of things and might require some experimentation. Some of us have better luck with boolits sized larger than the throats and seated to nestle in the chamber forcing cones, some have better luck going slightly under the cylinder throat size.

Second, based on your information, you don't have much room for error, so I'd say .358" is where you need to be to start out, that way you can still chamber your rounds reliably with a variety of boolit nose designs and yet still have boolits that are a bit larger than the groove dimension.

Third, I think you have a totally separate problem from your "static" boolit fit causing the leading. Static fit is what I call the way the boolit fits the gun when loaded in the cartridge and the cartridge is chambered. The boolit is prepared so that it's larger than the groove dimension and seated (and selected for design) so that it pilots in the throat and gets a straight start, tight gas seal, and suffers minimal damage going through the gun. Dynamic fit is the bugaboo. The boolit must seal the powder gas adequately or you will have leading due to gas abrasion and the resulting particles being ironed on the bore by the passage of the boolit like gold leaf. If you have leading in the bore, you either have a rough spot or a gas leak. Gas leaks mean the dynamic fit of the boolit has been compromised. This can be due to a number of factors including powder burn rate, alloy composition and toughness, lube, etc. But I find that revolvers tend to have a real problem losing this dynamic seal right after passing through the forcing cone due to a factor called thread choke.

Thread choke is common on many revolvers, and is extreme in many. When the barrel is torqued to the frame, the threads tend to crush and the resulting displaced metal makes a tight spot in the barrel. After passing through this tight spot, the boolit can be squeezed down smaller, spoiling the proper static fit you started with, and after passing through, no longer fits tightly enough to seal the remaining gas pressure. So gas leaks happen, and leading occurs past the restriction.

Another big thing is rough forcing cones, for which Ruger is famous. If the lead is torn from the boolit by the tapered lead-in to the lands rather than smoothly pressed into the sides of the boolit as it engraves, the lead shrapnel gets smeared onto the bore and subsequent shots keep builiding it up and dragging it through the barrel. Harder, more brittle boolits have a worse tendency to shave off lead as they engrave than soft boolits do, but both will benefit greatly from a smooth, polished forcing cone.

If you drive a soft lead slug into the muzzle a little way and drive it back out with short pieces of brass rod inserted through the frame opening from the breech end, then drive another pure lead slug all the way through and compare the to measurements, you can determine the amount of thread choke your revolver has and if it needs to be addressed by lapping or reaming. Inspect the forcing cone, particularly the land "ramps", for perpendicular machine marks that could be tearing lead away from your boolits. If it doesn't look as polished as the inside of the barrel, it probably needs some work.

Gear

[geargnasher]

Here are a couple of pics I have handy of a revolver with a very rough forcing cone, but notice it doesn't lead beyond that point because there's no thread choke. The forcing cone looks like it was cut with a rusty railroad spike. The thing had no visible grooves when I first got it because the cylinder throats were in the neighborhood of .356" for the .357" groove and the undersized boolits had leaded the grooves completely. After a good ream to .3582" the thing shoots quite well. I could polish the forcing cone to remove this last bit of leading tendency, but it's accurate and the leading doesn't get any worse after many boxes of rounds, so I just leave it alone. If it were a target pistol I'd polish the cone. Yours probably has a combination of roughness and thread choke causing the leading past the forcing cone.

http://castboolits.gunloads.com/imagehosting/thum_89094ea76ffff2284.jpg (http://castboolits.gunloads.com/vbimghost.php?do=displayimg&imgid=2503)http://castboolits.gunloads.com/imagehosting/thum_89094ea771dca089c.jpg (http://castboolits.gunloads.com/vbimghost.php?do=displayimg&imgid=2504)

Gear

[Char-Gar]

A rough barrel forcing cone can indeed be a problem. The old Pre-War Smith and Wessons had little more than a bevel at the breech end of the barrel. I have even seen much touted Colt Pythons that look like the cone was cut with a cast iron burr.

I have the tooling and have recut a several of score forcing cones. PROPERLY done it can do no harm and often gives a boost in accuracy and a reduction or elimination of leading.

Standing in the waters of calm reflection as I get older, and have seen many firearms ruined by folks trying to improve on them, I am reluctant to counsel folks to remove metal from their firearms until all attempt at solving the issue by loading the proper ammo have failed.

Recutting a revolver forcing cone, is not rocket science, but a fellow must have the right tooling, know what he is doing, and the patience to do it right and not remove more metal than necessary.

I have also seen a very few pistols with "micro-machine hickies" in the barrel. Nothing, I mean nothing would cure them of leading until these were removed. My favorite Smith and Wesson K-38 was one of these. These can be removed without resorting to fire lapping. I am certain that will do the job also, but also might remove metal in places where you don't want it removed. You can't put metal back once it is gone. A fellow needs to make dead certain that he won't want it back, before he removes it.

[MtGun44]

Start at throat diameter, and unless is so spectacularly accurate as to totally preclude
further experimentation (rare, but it could happen) try + and -.001 after that.

As to being .002 over the groove diameter - this is totally unimportant from a pressure
standpoint. A lead boolit being soft will not take much to size it down a few thousandths.

Bill

[44man]

I measured my GP100's chamber throats and they came out to .358. The groove diameter in the barrel is coming out to slightly over .357, roughly .3572. What is the proper sized boolit here?

I've been shooting light loads with 18 BHN 148gr wadcutters that measure out to just s shade under .359 (which surprised me since they're Missouri Bullets and are advertised as .358). I still get some leading in the forcing cone and right at the beginning of the rifling. It's annoying, but it's not too bad to remove from there so I just kind of suffer it and clean it occasionally.

Should my cast boolits be .001 over groove diameter or .001 over chamber throat diameter? Is it safe to run .359 boolits down a .3572 bore? And are my leading issues likely related to too hard a boolit for a light load? And if I decide to start loading full house magnums, is 18 BHN a good hardness for that application?
.358" or even .3575" There is just no sense in going over throat size. Over throat does NOTHING, NOTHING.
TOO HARD, don't fall into that trap. Too soft and skidding the rifling or a constriction at the threads. A rough forcing cone can be polished.
If you want the truth, a .38 wad cutter with a shoulder to align the cylinder would just start to do better at 25 to 30 BHN.

[MtGun44]

If the throat is perfectly round and the boolit is perfectly round, so that you measured
the ACTUAL throat, not the minor diameter of an oval, then you shouldn't need over
throat diam. In the real world, I have found a couple of revolvers that prefer +.001
diam. I have not bothered to spend a lot of time trying to prove exactly why, but
my experimental data says it is true sometimes and my theory is that some of
the throats are not round.

Most of the time throat diam or even -.001 works very well.

Bill