Pressure and GC boolits

[Marlin Junky]

Here's a simple question that I don't have an answer to 'cause I don't own pressure equipment (or I'm just not paying close enough attention):

Will a long .30 gaschecked boolit (let's say RCBS30-180FN wearing Hornady GC's) that registers BHN 15 shoot accurately with 40K loads?

Most of my success with this boolit in the 30-30 has been with slower powders with the exception of WC-820 at up to 16 grains.

MJ

[Bass Ackward]

Here's a simple question that I don't have an answer to 'cause I don't own pressure equipment (or I'm just not paying close enough attention):

Will a long .30 gaschecked boolit (let's say RCBS30-180FN wearing Hornady GC's) that registers BHN 15 shoot accurately with 40K loads?

Most of my success with this boolit in the 30-30 has been with slower powders with the exception of WC-820 at up to 16 grains.

MJ


MJ,

Depends. Like how it is supported in the throat. Or what shape and size throat you have. How smooth your bore is. How tall your rifling is. How large is your case capacity. How fast a powder speed are you burning. Etc.

Just bear in mind that all weight is accumulative on the base. Therefore, leaving accuracy aside, the bullet that has the highest pressure / velocity potential will be the lightest bullets per caliber.

And it also depends on your accuracy standards. If 1 1/2 MOA is acceptable, then 40,000 is doable. If you want to stay sub MOA, then about 34,000 is my ceiling.

[Marlin Junky]

Bass,

1.5 MOA is all I expect from my leverguns so your answer is encouraging. Basically, I am trying to avoid the heat treating step while attempting to get RCBS-30-180-FN to shoot well with the 16 pounds of DP68 I have on hand. My testing at 23 grains of DP68 produced a ES=99 (15 rounds) and I'm trying to figure out what I need to do (if anything) before going to 24 and possibly even 25 grains of DP68. 22 grains of DP68 was real bad. Three 15 round strings with 30-180FN and 22 grains produced the first ES=96, the second ES=188 and the final ES=148 (in that order). The primers were always Fed210M and the lube has always been the same very slick homemade concoction of three oils: mineral, castor and extra light olive.

MJ

[Bass Ackward]

The primers were always Fed210M and the lube has always been the same very slick homemade concoction of three oils: mineral, castor and extra light olive.

MJ

MJ,

Why not be a little bass ackward. In the spirit of starting that bullet off easy, use a pistol primer and see what happens. Pistol primers are safe up to about 45k.

[Larry Gibson]

Will a long .30 gaschecked boolit (let's say RCBS30-180FN wearing Hornady GC's) that registers BHN 15 shoot accurately with 40K loads?

Most of my success with this boolit in the 30-30 has been with slower powders with the exception of WC-820 at up to 16 grains.

MJ

Many ask the same type question such as; why can I shoot a 120 gr cast bullet accurately out of a .30 carbine at 1900 fps and 40,000 psi but can't get any accuracy out of my '06 with the same bullet at 40,000 psi?

The answer is RPM not pressure. The .30 carbine has a 1-16" twist and the bullet is doing around 85,500 RPM. With it's 1-10" twist the '06 bullet is cranking 180,000 RPM (at 2500 fps - about where you'd get 40,000 psi). That is almost 100,000 RPM difference. RPM is critical because the faster the bullet spins the greater any defects or imbalances in the bullet cause the bullet to yaw, pitch and wobble. That causes inaccuracy.

Your success with slower powders is because they cause a slower initial accelleration (has to do with the time pressure curve of the internal ballistics) to the bullet. Hence the bullet is not distorted as much. Less distortion means you can attain a higher RPM, i.e. velocity and maintain accuracy. At least to a certain level of RPM.

Pressure certainly has an influence, especially as it relates to the time pressure curve of the bullets accelleration. However, with regular cast bullets such as yours the key limiting factor to accuracy is going to be RPM.

Larry Gibson

[Marlin Junky]

Larry,

My question wasn't intended to be specifically related to accuracy or external ballistics. I am noticing large ES's on the chronograph with 22/23 grains of DP68 and 196 grain 30-30 boolits. I just wanted to know if my BHN 15 boolits will handle 40K so I can take the DP68 charge up a grain or two in an effort to get a better burn. Basically, I just need to find a use for my 16 pounds of Accurate DP68, which is probably the same as WC-680.

Bass,

Believe it or not I don't have any large pistol primers and all my 30-30 primer pockets have been reamed to fit WLR's & Fed210's. I've decided to make a heat treating rack for my skinny boolits so I can play a BHN vs. pressure game. If I'm going to confuse myself, I need to do it my way. :-D Thanks just the same for the interesting suggestion (no sarcasm intended).

MJ

[joeb33050]

Many ask the same type question such as; why can I shoot a 120 gr cast bullet accurately out of a .30 carbine at 1900 fps and 40,000 psi but can't get any accuracy out of my '06 with the same bullet at 40,000 psi?

The answer is RPM not pressure. The .30 carbine has a 1-16" twist and the bullet is doing around 85,500 RPM. With it's 1-10" twist the '06 bullet is cranking 180,000 RPM (at 2500 fps - about where you'd get 40,000 psi). That is almost 100,000 RPM difference. RPM is critical because the faster the bullet spins the greater any defects or imbalances in the bullet cause the bullet to yaw, pitch and wobble. That causes inaccuracy.
Larry Gibson

I've thought about this for a while, believe I read the same some years past. The SS plain based breech seaters are in the game with the CBA bolt gun guys, and I'm thinking that it's the difference between starting with the bullet in the rifling and having the bullet leap into the rifling. If there's truth in that, then I'd suspect that slower twist would shoot better than faster twist, and slower twist means shorter = lighter bullets. The modern 30BR guys are shooting 125 gr. bullets in 17" twist guns with great results. If leaping-into-the-rifling hurts accuracy, maybe short bullets and fast twist will shoot better than those 10" and 12" twist 30 calibers. Maybe the bore-rider long bullet high BC attempt to minimize wind effect, for 200 yard shooting, is the wrong way to go. Sort of like 14", 16" 22 cfs for so many years.
joe brennan

[Ricochet]

How are you lubing with that mixture of three oils? Seems like a nonhardening liquid lube wouldn't leave enough on the boolit to do the job. I do know that grease that exudes oil can kill powder.

[Marlin Junky]

How are you lubing with that mixture of three oils? Seems like a nonhardening liquid lube wouldn't leave enough on the boolit to do the job. I do know that grease that exudes oil can kill powder.

Ricochet,

It's 3 oils, grated ivory and a couple pounds of beeswax. Slickest s**t I've ever made... prob'ly too slick, but it do leave a clean bbl.

MJ

P.S. And a couple Tbs. of sheep squeez'ings.

[Ricochet]

OK, thanks!

[Larry Gibson]

Larry,

My question wasn't intended to be specifically related to accuracy or external ballistics. I am noticing large ES's on the chronograph with 22/23 grains of DP68 and 196 grain 30-30 boolits. I just wanted to know if my BHN 15 boolits will handle 40K so I can take the DP68 charge up a grain or two in an effort to get a better burn. Basically, I just need to find a use for my 16 pounds of Accurate DP68, which is probably the same as WC-680.

MJ

Guess I missunderstood this question; "Will a long .30 gaschecked boolit (let's say RCBS30-180FN wearing Hornady GC's) that registers BHN 15 shoot accurately with 40K loads?"

With regards to pressure effecting accuracy; the slower burning powders will, at the same pressure as fast burning powders, have a tendancy to be more accurate. This is because there is not as much obturation during accelleration as with the faster powders given the same peak pressure. Now having said that it is the deformation (obturation) of the bullet during accelleration that creates imbalances. These imbalances are amplified by RPM, Thus the faster you push an unbalanced bullet the greater the inaccuracy. If the faster powder creates the same imbalance at lower velocity as the slower powder at the higher velocity the accuracy will be pretty much the same. However, with the normal design of cast bullets as the one you mention the limitation is going to be RPM. How much RPM the bullet will take given the amount of unbalancing obturation depends on the rate of acceleration which depends on the speed of the powders burning rate.

Sounds confusing doesn't it? But it's not. With standard cast bullets and normal loading techniques the best accuracy level at the highest velocity of a regular cast bullet is determined by RPM. I have found that RPM level to be somewhere between 130,000 to 140,000 RPM. Faster accelleration from a faster powder lowers that RPM threshold. Also as you suspect, with all loads you want minimal ES of the velocity as that is a measure of consistancy.

Larry Gibson

[Ricochet]

I don't see how you can have less obturation during acceleration with a slower powder than a faster powder at the same peak pressure. Deformation of the metal is a function of applied pressure. If anything, applying the same peak pressure with a slower powder should somewhat increase obturation by maintaining the peak pressure longer. Perhaps the difference is in when the peak pressure is applied, the faster burning powder perhaps peaking when the boolit is still in the throat where it may not be fully supported, and the slower powder peaking after it's fully swaged into the rifling?

[grouch]

Ken Molahan (I think) wrote an article a few years ago about exploring the theory that the limiting factor might be muzzle pressure. Seems all his experiments - which were extensive - supported this theory. This was inspired by the pictures in the Lyman manual of the deformed bullets from short barrels to show obturation.
If I remember correctly, he abandonned this theory when paper patched soft lead bullets at high velocity came to his attention.
Maybe there's something to this theory that bears some more consideration. Maybe someone with a very long - barrelled rifle could do some comparison tests to shed some light on this.

[Larry Gibson]

I don't see how you can have less obturation during acceleration with a slower powder than a faster powder at the same peak pressure. Deformation of the metal is a function of applied pressure. If anything, applying the same peak pressure with a slower powder should somewhat increase obturation by maintaining the peak pressure longer. Perhaps the difference is in when the peak pressure is applied, the faster burning powder perhaps peaking when the boolit is still in the throat where it may not be fully supported, and the slower powder peaking after it's fully swaged into the rifling?

Take your car and park it in a level parking lot with no brakes on and out of gear. Get behind it in another car bumper to bumper and slowly accelerate pushing the front car to 30 miles per hour. The front car will move with no damage to either car. Now do the same thing except stomp on the accellerater with the rear car until until you reach the same 30 miles per hour. In the second test you will reach 30 miles per hour in less time and in a shorter distance. The bumbers of both cars will be damaged. Same peak pressure, i.e. same amount of force needed to move the car to 30 miles per hour. However, since the force was applied over a longer time in the first test the same speed was reached without damage. It is the same with acceleration of the bullet in the barrel with slow vs fast burning powders. Longer time pressure curve equals less obturation all other things being equal. That's a fact.

Larry Gibson

[VTDW]

Marlin Junky,

As you know our Marlins are individuals and what works in one does not necessarily work in the other. Hard cast boolits generally like a very healthy dose in order to obturate and grip the grooves. Load em up and shoot em.:drinks: I usually find an accurate load on the lower end and usually find another accurate load toward the upper end of the spectrum.

Dave

[Ricochet]

Larry, in your car example the peak pressures applied are VERY different.

You're confusing impulse with force.

[Jon K]

MJ,

The guy that can give you more specifics is Trailblazer, he's using 180 or heavier to shoot the HP Silhouette Match with a 30-30 lever.
If he doesn't chime in on this one PM him.

Jon

[Bass Ackward]

Larry, in your car example the peak pressures applied are VERY different.

You're confusing impulse with force.


Ric,

That's the problem that I have with the lead hardness chart where they claim a top end limit multiplied times a factor of 4. It becomes obvious that they didn't understand this concept either as the top figure they list for each mix generally is a figure obtained with what would be considered cast speed powders and heavy bullets.

The real culprit is inertia and twist rate. The heavier the slug per caliber, the more pressure will affect the base because the base will attempt to move before the nose. But the faster the bullet is moving when a pressure peaks, the more pressure it can take or the less effect with obturation it will have. Therefore, slower ramping pressures will exhibit less obturating effects.

This is where Larry and I differ. Larry believes that it is RPM in every case that is the limiter. I believe it is the pressure and acceleration curve (if you don't have some ridiculous twist rate that you have to deal with). So the lightest bullets per caliber will always have the highest accurate velocity potential at the same hardness. So the faster the twist rate, the steeper the angle of the rifling in the bore which aids in retarding forward bullet motion and thus increasing pressure on the base.

[pdawg_shooter]

Your BN15 bullets will handle 40k fine if you size them to bore dia. and paper patch them !