How Much Pressure to Obtuate?

[ColColt]

What's the magical floor for a bullet such as those made with ww's(BHN 12) to obturate? Would it need at least about 16,000 psi for instance? It seems I've seen a formula for this but can't recall where. The reason I ask is that I just acquired a 38 Special(Model 15-3) and was going to load some with 6 gr of HS-6 with the 158 gr boolit and according to Hodgdon's website that yields about 16,000 cup which I can't relate to in psi but assume it's around 12,000 or less psi. I may have to go to a softer boolit such as with isotope lead around BHN 10.

Am I way off in my thinking or close to the specifics desired? Another load I want to test is 4 gr of Universal for about the same pressure as the load with HS-6 and velocity about equal. In short, do I need a softer boolit than ww's?

[geargnasher]

The magical floor you're looking for is just over 15K PSI with 12 BHN alloy. HOWEVER. My advice is to worry about something else. The only thing you need to worry about with "OBTURATION" is that the boolit obturates the bore: Meaning it's big enough in the first place that it corks-up the barrel and doesn't leak gas, so it doesn't lead. If it's big enough to obturate the bore, you won't have to worry about whether or not the alloy was soft enough to "bump-up" and seal, or whether the pressure was high enough. If it fits, you can forget all the numbo-mumbo-jumbo.

Make sure the boolits fit the cylinder throats and are bigger than the groove diameter, and choose a powder that burns at it's peak efficiency with your boolit weight and desired velocity.

HS6 never worked for me in .38, too erratic at safe pressures. Universal is hit and miss with that one, too, but I can give you a lights-out combination for .357 Mag with it. I have the best luck with Clays, Bullseye, Titegroup, and Red Dot in the .38 with RFN or RN boolits, Bullseye is still the best with flush-seated wadcutters. As always, YMMV.

Gear

[ColColt]

I like HS-6 for the 357 and 44 Magnum and was thinking perhaps it would do well with the 38 as well...we'll see. 231 is another good powder I'll give a try since I have several pounds of that as well.

The throats barely take a .357" pin gauge and won't accept anything larger so, I'm sizing the boolits to .358". I haven't slugged the barrel as yet but "assume" it's right at .357" until I can do that. I'm a little squeamish of Bullseye. Too easy to double charge or triple charge. I've seen that happen with a friend and he destroyed a super nice revolver.

I have several molds for the 38 clan and all but one are swc's...no full wc's.

[geargnasher]

Have you though about Trail Boss? It's great in the .38, and no danger of double-charging. You'll find, when you start looking at volume vs. charge levels, that most any powder can be double-charged in a .38 with a typical RN or SWC boolit.

Gear

[ColColt]

Never have tried Trail Boss. My current powder stash consist of Unique, Universal, HS-6, Power Pistol, 2400, 231 and AA#5. Maybe during the next gun show I'll pick up a pound of TB. I just thumbed through my Speer and Hornady manual and Speer doesn't even list TB. Hornady list it but it yields the lowest velocity of most powders listed for cast boolits.

[Bigslug]

Not a physics major, but it stands to reason that if you've got enough pressure to cram a bullet to the dimensions of the rifling, and get it out the barrel, the back end of the slug is going to deform from the blast.

[fecmech]

The .38spl cartridge along with Bullseye,231, or Unique and a good lead bullet is like bread and butter or maybe bacon & eggs. IMO slow powders in that cartridge are a waste of time. You can do anything you want and do it well in the .38 with fast powders. You want 850-900 fps with a 158?? BE,231,Unique will do it and do it well. Want to go 700 fps with a wadcutter?? BE and 231 will do it well! Want to go 1000 fps with a 120-125 gr bullet?? You guessed it, BE,231,Unique.

[ladOregon]

gearmasher has a very good point. A powder with enough volume so it is impossible to double charge is a good fail safe. I always look in every case before seating bullets. But some of the very small charge powders can be double charged. scary thought.

[jandbn]

ColColt,

Don't be surprised if you can't find a 1 lb. bottle. Trail Boss is too bulky to fit in the Hodgdon's standard 1 lb. bottle.

[felix]

3.8g 231; 5.0g Unique are USUALLY very accurate loads in the Special case with standard 150-158 Keith types. Yes, Unique/Universal are temperamental and come into their own incrementally on the powder scale. 5.0 is powerful and accurate in about every Special I loaded for and should work OK for any reasonable hardness, like for WW air cooled. ... felix

[44man]

If the boolit fits, even a primer alone that pushes a boolit into the bore will obturate.

[Doc Highwall]

Instead of thinking about enough pressure to obturate the barrel, think about what geargnasher said about having the bullet large enough that it swages down to the groove size. Size your bullets .001"-.002" larger then your groove size.

I size my bullets for my 308 Win at .310" with 30:1 alloy and shoot them at 1800-1900 fps with zero leading. I have shot five shot groups as small as .305" at 100 yards and 1.600" at 300 yards doing this.

The first three rules of cast bullets are.
Rule #1 Bullet Fit.
Rule #2 Bullet Fit.
Rule #3 Bullet Fit.

[Old Caster]

I don't think a definitive answer could be given as to how much pressure is needed to obturate a bullet a given amount because as the diameter changes the action on the rear would vary disproportionaly . This is the same reason why a .45 is so much easier to make accurate than a 9mm when it comes to lead. It is harder to swage the bullet down with 45 brass than it is with a 9 and easier for it to obturate up. Just make sure it is right to start with and don't worry from there and this would include swaging down from the brass. -- Bill --

[ColColt]

ColColt,

Don't be surprised if you can't find a 1 lb. bottle. Trail Boss is too bulky to fit in the Hodgdon's standard 1 lb. bottle.

That's got to be some straw like powder to not fit in a pound container. I may pass on it since the velocity figures for it compared to others are not that great.

I have shot five shot groups as small as .305" at 100 yards and 1.600" at 300 yards doing this.

Well, Doc-I don't think I'd ever attain groups like that. I couldn't get better than 1" at 100 yards with a custom made 270 Winchester back in the early 70's and that was a curly maple stock made by Hal Hartley using a Douglas Premium barrel and an FN Supreme action. I think groups of that order are fantastic.

I think by me using a .358" boolit in .357" throats with a good charge of Universal or Unique there would be no filling or accuracy problems. Still, I like to run tests with different boolit weights and powder combinations. I did that with the 44 Magnum and found a great accuracy combo and will stick to that. Each gun is a bit different. I'm thinking maybe 6 gr of HS-6 with the 358429 boolit would be a good combination.

[geargnasher]

"I'm thinking maybe 6 gr of HS-6 with the 358429 boolit would be a good combination."

Like we said, not likely in a .38, but try it anyway. BTDT. It will be very dirty, but it's not "ideal" for the .45 Colt at SAA pressures, either, and it is one of the most accurate powders I have for them. POI is right on with milder loads, too. Dirty doesn't always mean inaccurate.

Gear

[ColColt]

I sort of based that on some good groups with the 158 gr LSWC in 357 using 8 gr of HS-6(one of the targets here) and thought it may be worth a try in the Special case.
I think this was 12-15 shots. HS-6 has been a good powder for me in 45 ACP and 44 Magnum as well as some loads in 357. The good thing about the 38/357 clan is you can't hardly use a bad powder with them.

[geargnasher]

That's dandy in the .357 Magnum, but you're going to max out the .38 pressures at a little over six grains. Like I said, try it and see. Also try 231 since you have it on hand.

Gear

[popper]

If it's big enough to obturate the bore, you won't have to worry about whether or not the alloy was soft enough to "bump-up" and seal

Agreed. But the same thing happens in the case. Pressure expands the case to the chamber but CB doesn't bump, so you get gas cutting on the base. No problem if the CB is then extruded down to bbl size unless the base is damaged enough to cause a problem when exiting the crown. Then you have accuracy problems. Think sooty necks with plinker loads and hard CBs.

[MT Gianni]

Reread what Felix had to say about Unique.

[ColColt]

I haven't tried Unique as it's always a bit more dirty than Universal which I have some loaded currently at 4.5 gr and the same load with 231. It doesn't meter as accurately as Universal in my DU-O-Measure being a flake powder. I'll try Unique as well as right now I'm not culling any particular powder. Like I said, the 38 clan is so versatile you'd be hard pressed to really pick a bad powder for it.