bullet size for revolver, GP100 357 mag?

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Shadow9mm

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Ok. I got a GP 100 38spl/357 mag. I was planning on using a 158g SWC for my primary load. I am wanting to make sure I am understanding this correctly.

For cast lead bullets you want the bullet 0.001 larger than the cylinder throat correct? This is to prevent gas cutting as it leaves the cylinder as it exits correct?

As best I can measure with my calipers
cylinder throat is 0.359
The forcing cone measures in at .370
muzzle measures .357

The cast lead bullets I bought are supposed to measure in a 0.359, which is under sized as near as I can tell. For my gun I would theoretically need a 0.340 or 0.341. Also even with the 0.359 it may lose a little size due to being loaded into the brass case correct?

Some JHP .357 pass through no problem
some cast coated .357 sized lead are a very light push fit.
 
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I run my cast bullets at .359 which has worked out just fine in several .38 Spl. and .357 mags. You should be just fine. I run my cast .357s at full power with no leading.
Paul B.
 
This is actually a big subject and opinions abound.

Generally, you want lead bullets to be .001" over bore size. At least, that's the general rule. YMMV.

This is to prevent gas cutting as it leaves the cylinder as it exits correct?
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Mmm, yes. Sort of. More broadly, the bullet needs to seal as it travels down the barrel. Being at least .001" over bore size facilitates that seal. A good seal minimizes leading.

As best I can measure with my calipers cylinder throat is 0.359.
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That's likely not an accurate method. I bet your cylinder throats are closer to .357. I bought those little dowel thingies for measuring 38/357 cylinder throats. The dowels ran from .355 to .357 in .0005" increments. The revolvers I checked (two S&W's and one Python), the .357 dowel just barely fit - just barely. I didn't have a .3575 (wish I did), but I bet it wouldn't fit. This leads me to believe that most modern, well made firearms (like S&W, Taurus, Colt, & Ruger) are probably pretty close to the .357 spec. (This is where I'm probably going to get some disagreement.)

I've been loading and shooting lead for decades. In 38/357, I've never loaded a single lead round that didn't lead my barrels/cylinder throats. I read all the time about folks who run full-house lead rounds with no leading. I have never had such an experience - not even close - ever. I gave up on lead slugs and load/shoot plated bullets instead these days. I like cleaning guns; but I'm too old and tired of scrubbing lead (doesn't come out anyway). Plated bullets makes the loading, shooting, & cleaning experience well - WELL - worth the extra cost. I'm done with lead. (Save 45 ACP - my 1911's don't lead.)

That said, I got minimal leading with soft swaged .358 lead slugs - like Speer's products. Missouri Bullet Co makes soft cast products that are good too. With hard cast, I got minimal leading with .359" slugs. .358 hard cast slugs are the worst. No matter how hot or cool the load, barrel leading after just a few rounds (like 6) leaves lead in the forcing cone, down the first inch or so; and flakes of lead in the cylinder throats (this is with six different guns). Hard cast .358's were the worst for me. Maybe my experience is anecdotal I but tried every combination you could possibly think of over the last 3-1/2 decades to no avail. In fact, the most effective way to minimize leading I have found is to lube the barrel with an oiled patch between every cylinder's worth.
 
OK, just a few points to help you out.
1- cast bullets should be .001" over barrel groove diameter. This is to help obturate (seal) the bore so hot gases don't get past the bullet in the bore, cutting through the lead bullet and causing leading. Slug your bore and go .001" over, or just shoot whatever you have and see which ones work best.
2- Your cylinder throats may very well be .359", Rugers tend to run a little looser than Colt and S&W.
3- Forcing cone diameter has little to do with bullet choice. It is basically a funnel to get the bullets into the barrel. Yes, there are throat specs, but don't worry about it too much.
 
Slug the bore, measure with a micrometer, then size the bullet .001-.002” bigger in diameter so it obturates properly. But, if this size doesn’t push through the front of the cylinder with moderate finger pressure the throats need to be opened up so they don’t swage the bullet diameter down to where it no longer obturates. Another thing to take note of when slugging the barrel is if the pressure while pushing the slug through is uniform through the whole barrel, if you meet additional resistance at any point, especially in the frame/forcing cone it means you have a restriction that needs to be addressed. Or you can make life simple and shoot either plated or coated bullets and avoid these things being a problem, that’s the route I’ve eventually taken.
 
Generally, bullets sized to match the cylinder throat diameter, as long as that is bigger than the bore's groove diameter, will shoot about as well as the gun can shoot.
 
Start with .358" , in a plain lubricated cast bullet , since they are a common and easy size to get , and see how they shoot.

I have tested both .357" and .358" sizes , plain lubricated and saw no difference in my revolvers .

The cast coated .357" bullets that are a light push fit through the throat should be just about perfect !
The fit in the throat doesn't have to be a tight or a hard push fit ... a light push is fine ,
Go with this size if using the coated bullet ,
Gary
 
It can be challenging to get real good ID measurements.

You also did not specify tumble lube, traditional lube only, traditional lube with gas check or PC. Both gas check and PC tend to help avoid leading. I cast my own and use tumble lube for 38 Sp level loads and PC for hotter. I have run full power 357 magnum loads with good accuracy and no leading.

If you can easily push your bullets through any cylinder throat (try each one), you are likely to get leading unless you load them down to 38 SP levels. If it they do not slip through without a good amount of force, they are probably big enough.

For most 357s, a cast at 0.359" has a good chance of working out.

Some guns are more forgiving than expected and some are more demanding. The GP-100 that I load for was pretty easy to deal with.
 
Yes I know all guns are different but my GP-100 loves these at 357 start loads - crazy accurate . Not a lot a kick but more then light 38spl loads .

jpaSpo.jpg
 
Try .358" diameter cast/lead bullets, most likely they'll work just fine. I buy .358 for all my .38/.357 revolvers(10 different ones, none are Ruger). I have used .357 .358 and .359 with success. I buy .358" but they don't all measure .358". I shoot them anyway.
Your mileage may vary.
 
Again, the standard advice is to match the throats. If your throats are .359" as demonstrated by a mic'd pure lead slug (don't trust a caliper with this measurement), then use 0.359" bullets. The CAS shooters have tested this a lot because there are still a good number of 45 Colt revolvers out there with 0.451" groove diameter barrels, but 0.455 or even 0.456" throats to accommodate the old pre-war 0.454" bullets. Repeatedly, you read bullets sized 0.455-0456 shoot most accurately in them despite the much narrower groove diameter of the bore. Anything loose in the throat allows gas cutting as the bullet heads to the forcing cone, and that causes leading and can also unbalance the bullet.

The only exception to the above is when your throats are narrower than your bore's groove diameter. In that case, the gun can seldom be made to shoot jacketed bullets well, much less shooting lead well until you get the throats reamed out to exceed the groove diameter. I have a Ruger Redhawk that prints 240-grain JHPs into under an inch at 50 yards off bags and with a 1.5X handgun scope on top. A friend of mine was so impressed, he got a copy. His would hardly stay on a paper plate at half that range. So he sent it to Ruger complaining about the accuracy and comparing it to my gun. Ruger returned it with a shop note saying they had reamed the cylinders. It then shot just fine.
 
So I don't have a mic.

However
.357 caliper hornady will fall through
.357 coated lead required very light finger pressure
.359 lead bullet will not pass through even with strong finger pressure

Seems like a .358-.359 should get the job done for straight cast or coated bullets.
 
Oddly the only 2 revolvers I've owned that would not shoot were a .45 with oversize throats and a .44 Special with oversize throats. 45 had a .451 bore, .44 was .429. I should have tried .455 bullets in the .45. .44 was just a lost cause. It was a new 24 from S&W, they would not replace the cylinder or set the bbl back(rear of bbl was cut at an angle). If I could have come up with some .433 or .434 bullets??????????
 
Been loading cast bullet's a long time and haven't learned anything new. My thought is to buy a mold designed for the cartridge your loading and go for it. Simply make sure the bullet's are lubed before using them. From one gun to another, min bore to max bore just how much difference is there. Now of course if this is a competitive gun, measuring might make a difference. Get into competitive stuff and the rules change! Buy dies for 357/38 bullet's and go for it! Don't forget lube and gas seals if needed.

One more though. If your really worrying about being .0001" undersize, powder coat the bullet and don't size after applying it.
 
As for gas checks, I found them to be unnecessary in revolvers. I've shot full magnum loads in .357 and .44 mag with regular bullets(no gas check). Alloy, lube and size are more important.
Again, your mileage may vary.
 
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