I am very dissapointed with Berger bullets regarding the 338 hybrid bullet.

[Airgunner7]

Well, I can say one thing about giving the bullet a start, it is storing it's own kinetic energy b4 it hits the lands, if it is on the lands the only thing pushing it is the powder. None of the energy has been transfered prior. Nice little shock absorber

I'd imagine this:, you would see a very smooth pressure gain and then a spike as the bullet hits it's resistance. I could see how having more energy and speed it would cushon that pressure spike.

Now changing how much gas gets past the bullet b4 it engages has me wondering about what happens with pressure in front of the bullet....does it suck it along or does it bottle it up? I think this might b the clue to any accuracy issues.

 

[elkaholic]

I agree with you Oliveralan,
This is exactly what I have observed. Especially with bullets seated right on touching the lands.

+2 on that....Rich

 

[edge]

SNIP.

Then run these .458 Lott loads with every aspect of the load remaining constant other than the seating depths, over the chronograph. Record the MVs. MV will be highest with the bullet jammed into the lands, and MV will decrease as the interior casing volume is decreased by progressively seating the bullet further into the casing.

This is the scenario my comments and discussion apply to.

I agree with jamming the bullet into the lands, but that is another scenario!

IMO, even using YOUR scenario, I believe that the pressure will be relatively higher with the bullet pushed down to compress the powder, then as the COAL is increased( DECREASED ) the pressure will drop, Then as the COAL is further increased(DECREASED) so that the jump to the lands gets smaller( LARGER) the pressure will again rise....but that is the effect of the lands, and not the case/bullet relationship...IMO

edge.

PS I mainly shoot smokeless muzzleloaders. In effect I have a nearly unlimited case capacity. I can vary the seating depth, within reason, to increase/reduce pressures.

 

[BryanLitz]

I've been away for a while and am just getting caught up with this thread again. I haven't read every word, but get the gist of what's being discussed and have some comments.

First, Eric was discussing the max acceleration in terms of G forces realized by the bullet to be around 800 G's. He left a zero off, it's closer to 80,000 to 100,000 G's of acceleration at the pressure spike. This is just academic, but quite astounding.

Next, the situations where the standard .338 LM was consistently producing nose slump involved the use of ball powders. Ball powders are typically faster burning and will produce a higher pressure spike for a given MV. Loading the .338 LM with slow burning stick powders like H1000 have not produced nose slump to my knowledge.

Regarding the freebore and seating depth effects on pressure/max pressure and velocity, the following is only my understanding and may not be entirely accurate. It's just how I 'think' it works.

First consider a 'smoothbore' situation, that is, remove the riflings from the barrel and just look at the effect that seating depth has on case capacity. In this case, I believe that loading the bullet out farther (increasing COAL) will reduce the peak pressure and MV. Reason is because the powder burn rate depends on pressure and heat. If the powder ignites in a smaller chamber, it's initial pressure will be higher, and the burn rate will reach a higher initial value, resulting higher peak pressure (it's a circular relationship). If the bullet is seated out farther (longer COAL), the initial pressure is lower and results in a lower initial burn rate and pressure spike.

Now consider what happens when the bullet encounters the riflings; which is, the bullet encounters a resistance to it's forward motion. This will cause the pressure behind the bullet to increase, thus increasing the burn rate, and in turn increasing the pressure even more. If you shoot the same ammo in 3 chambers, all having different lengths of free-bore, I would expect the chamber with the shortest free-bore to reach the highest max pressure. The reason is because if the bullet encounters resistance (hits the riflings) while the space behind the bullet is still small, then the burn rate and pressure will increase more rapidly and reach a higher value than if the bullet gets further down the barrel (leaving a greater, cooler, lower pressure volume behind).

What makes this so hard to correlate in reality is because we don't vary our throat length, we vary the seating depth, which is actually varying two variables at once. Then if you vary powder charge, it's a completely different dynamic. Seating a bullet deeper into the case (shorter COAL) increases initial pressure because the initial combustion chamber is smaller, HOWEVER, the shorter COAL also has the effect of reducing the chamber pressure because the bullet gets farther down the barrel before it encounters the riflings. Which of these offsetting factors is stronger? I don't know. Maybe some cartridge configurations and powder burn rates will cause one effect to be stronger than the other which would account for the conflicting observations above. I think the highest pressure situation you could have is a short freebore with the bullets loaded into the riflings with a near 100% fill ratio load. In that case, there's nothing to take the edge off the initial pressure spike.

Don't forget about the effect of neck tension.

Again the above is just my understanding of how it works and should not be considered 'authoritative'.

-Bryan

 

[Eaglet]

You've created a different scenario with dramatically differing case volumes, than the one I'm discussing. I agree with your opinion with respect to the example/illustration you have created/provided. But I don't agree that your example sheds any light on the scenario being discussed.

Here's the scenario I'm discussing: Take the reduced powder load from the 45-70 load and use it in the 458 Lott case. Now with that 45-70 powder charge held constant, vary the seating depth from having the bullet jammed into the lands (maximizing interior case volume) to the bullet seated deeper into the case (decreasing interior case volume). Then run these .458 Lott loads with every aspect of the load remaining constant other than the seating depths, over the chronograph. Record the MVs. MV will be highest with the bullet jammed into the lands, and MV will decrease as the interior casing volume is decreased by progressively seating the bullet further into the casing.

This is the scenario my comments and discussion apply to.

How can I agree with Phorwath and Eric when apparently they're both taking different positions? The truth is that I agree with both of them and here is why...

Phorwath post above is right on the money, no arguments there, what i have noticed though is that there is a "point of return" in other words, if we keep doing what phorwath is suggesting above, just little by little decreasing the OAL, we will reach a point that things will reverse and as the over all llength gets smaller now the pressure gets higher. It's amazing. So... depending on what side of that returning point you have been so your experience speaks of.

From a jammed bullet to a long jump somewhere in between pressures reverse.

Now Gents, easy does it, don't go cutting my head off!!!!!!!!!!!!!!!!

 

[Eaglet]

Eventually, is not hard to see. Bullet is jammed, explosion takes place ignited gasses desperately look for an exit, as the brass release the bullet some tries to escape around the bullet with some but not much success because the bullet is jammed into the lands preventing much of that scape.

As the bullet moves into the case, it allows a bit more of gasses to escape and those gasses now find some what less resistance and the pressure starts going down. This continues on... the pressure continues to decrease untill.... WHAT?... Yes... we reach that point where the pressure from very little room in the brass starts showing. Let's not sugar coat it, as the space decreases the pressure increases given the same amount of powder. That's a fact. Therefore we eventually reach the point where this fact starts showing and instead of continually seeing the pressure decrease we start seeing it increase. At this point the offsetting of the bullet jump is not preventing the pressure to increase and now we're able to measure it.

 

[Eaglet]

Gents, I was typing when Bryan posted...

 

[GMBshootingclubM60]

Check my math...

(f) in pounds / (m) mass in pounds = (a) in Gs, or 32 ft/sec/sec.


(f) = area x pressure
that is, area = pi x r x r x pressure
(f) = .169 x .169 x 3.14 x 65000
(f) = 5829 pounds on the base of a .338 bullet directed down the bore

(m) = 300 grains / 7000 grains / pound
(m) = .0429 pounds of mass

So...

5829 / 0.0429 = about 135880Gs.


This only works before the bullet starts to move fast and the gas laws come into play. The 5829 pounds will have the force required to engrave the bullet subtracted as the bullet enters the lands. Later, the force to push the bullet down the bore will need to be subtracted...but by then the peak pressure will be long gone.

 

[Pdvdh]

I agree with jamming the bullet into the lands, but that is another scenario!

Humorous or not - I contend your contention is incorrect - no matter what seating depth scenario you would like to stipulate, within the range of seating depths commonly employed with metallic cartridge reloading.

Jammed into the lands simply magnifies the pressure, compared to giving the bullet a running start, so that most every reloader will realize the significant jump in pressure, compared to seating the bullet off the lands. In other words, jamming versus not jamming does makes the largest difference in pressure and MV, compared to increased seating depths after the the bullet has been seated off the lands. As little as 0.008" off the lands reduces the MV and pressure enough that I already have to add 0.3 grains of powder to maintain the MVs obtained with the Hornady 162gr A-max bullet, when jammed in my .280 RCBS Improved.

Now what most reloaders don't understand, is that continuing to move the bullet farther from the lands (decreasing COAL) after the bullet is no longer in contact with the rifling continues to decrease MV and case pressure - at least within the range of seating depths reloaders commonly seat their bullets. Up to ~0.120" off the lands.

The instructions that come with the HAT bullets direct the user to load to maximum charge with the bullets 0.030" off the lands. So we're not talking about "jamming the bullet into the lands". And the instructions state "they will generate higher pressures when close to the lands." After ID'ing the maximum pressure with the bullets 0.030" off the lands, the instructions are to seat deeper (decrease COAL) while keeping the powder charge the same. The guy testing these bullets has done this with all of the differing caliber and weights of HATs, confirmed the reduced pressure/velocity effect with deeper seating depths over chronographs, and produced these tips/recommendation for fine tuning a load for a specific cartridge.

I've confirmed this cause and affect relationship over my chronographs with Bergers, A-maxs, and the HATs. So my comments are less opinion, and more a conclusion reached from the recorded MV data.

Not to say that if a fella took a sludge hammer and compacted the powder that some differing end result could never result. Take anything to an extreme and the pattern within normal ranges of operation could change. I couldn't say because I don't use the sludge hammer.

But within the realm of differing seating depths used by reloaders of metallic rifle cartridges, I believe my comments are correct and that my data could be repeated and substantiated by any other reloader with a reliable chronograph.

The link I provided in an earlier post to the other thread will provide another member's post, stating that he saw MV continue to decrease with bullets seated up to 0.200" off the lands, so in his cartridge/caliber/bullet/powder mix, the trend continued up to 0.200" off the lands. How much more deeply the bullet could be seated until the trend might reverse, I have no data to know, because I've never seen the trend reversed. It may not reverse, short of employment of the sludge hammer to seat the bullet.

The freebore affect taken advantage of by Weatherby, and a slightly modified variation tested by Kirby Allen with his modified throat design, confirm that the affect of increasing the bullet-to-lands jump distance simply continues to reduce peak pressure and reduce MV, to the point that additional powder must be added to reach a maximum powder/pressure charge. After maximum pressure is re-established with the additional powder charge, the consequence of the freebore chambering is an additional ~75-100 fps MV. All because the freebore bullet jump prior to the bullet engaging the rifling has been rather dramatically increased.

 

[Eaglet]

Happy now?????????​

 

[BryanLitz]

Check my math...

(f) in pounds / (m) mass in pounds = (a) in Gs, or 32 ft/sec/sec.
.
.
.
(f) = .169 x .169 x 3.14 x 65000
(f) = 1856 pounds on the base of a .338 bullet directed down the bore

Your approach is correct (f=ma; Newtons second law)

However,

.169*.169*3.14*65000 = 5829 lb, not 1856 lb
(1856 results from .169*.169*65000, in other words, you left out pi).

So then;

5829/.0429 = 135,881 G's.

This number is higher than the range I gave (80,000 to 100,000 G's) for two reasons:
1) I based my range on lower pressure.
2) The above calculation (f=ma) ignores friction. I've estimated (using Quickload) that you need to scale down the 'a' implied by the simple f=ma force balance by about .86 in order to get close to the actual bullet acceleration, accounting for friction. This is a rough estimate, but is better than ignoring it.

Thanks for running the numbers, always good to have someone checking the math.

-Bryan

 

[GMBshootingclubM60]

Fixed it.. can't type with one finger and calculate.

One could get a rough idea of the engraving force with a press and a rod and a scale. Same for the barrel friction.

What is needed is a pressure vs position plot. I would think that the bullet would have hardly moved before the pressure was past its peak.

Internal ballistics, another hobby for a rainy day!

 

[royinidaho]

The RSI Shooting Lab does this.

What is needed is a pressure vs position plot.

 

[groper]

Bryan, you may have answered this already, but as yet i have not found a response from you to kirby allens issues with this pressure theory...

From some of his testing, he stated that he tried some very light loads of H50BMG in the 338 cheytac varient cases to the point where he was getting hang fires etc... he estimiated that the pressures would have been 50,000psi or less, but obviously with long barrels and such a large case, velocity was still quite high.... However, he was still getting the accuracy problems and suggested it was more the velocity and more specifically the RPM`s rather than pressure deforming the bullets. He could not get the bullets to perform well at any velocity above 3000fps regardless of pressure, and tested the bullets to the point where they came apart not far above this point.

Im simply curious, but how are you determining that it is infact 'nose slump' rather than some other form of deformation, and that it is caused from pressure and not excess velocity or RPM`s?