What raises pressure, 'soft' or 'hard' bullets?

[Silverado]

If all else is equal between 2 loads, as in same brass, primer, powder charge and bullet weight, which will have the higher pressure; a soft, thin jacketed bullet, or a hard, thick jacketed/bonded/partition type bullet?

I've read both theories. One says that the softer bullet will 'slug up' under load, raising pressures.

The other states that the harder bullet will not conform to the bore as readily, raising pressures.

What say you?

 

[SandRoad]

Peak pressure is generated very soon after ignition. For most handgun cartridges peak pressure is generated at 1/4" to 1" of bullet travel. Larger rifle cartridges generate peak pressure at 2" to 5" of bullet travel.

The only difference the composition of a bullet will make a difference is when it engages the rifling. The tougher the bullet, the more force required to engage the rifling.

For many handgun cartridges that use a fast powder and are set back from the rifling, or have a long throat, the composition of the bullet becomes irrelevant because peak pressure occurs before the bullet ever touches any restriction.

Also, don't confuse peak pressure with ultimate velocity. They are related, but not the same. Which ever bullet goes down the barrel with the least resistance will (usually) have the highest muzzle velocity (but not always the case because of the pressure curve generated by the burning powder). This is typically a lead bullet.

Think of it this way. If you want to find out the diameter of the bore of your barrel, how would you prefer to slug the bore ? With a soft lead slug or a solid copper slug? Which one would take more force to push thought the barrel? <= There's your answer.

EDIT: But that is not the entire answer, it is actually over simplified, see more in posts below.

 

[44Bore]

Jacketed bullets used with smokeless poweder do not "bump up". Pressures are rased by harder jackets/partition or solids due to force required to "engrave" the lands. There was much to do about this way back when nosler partition bullets first came in to general useage.

 

[H4831]

The shooting fraternity had this bullet and bore fit figured out 100 years ago.
Prior to WW 1 the British did a lot of experimenting with their 303 service rifle and ammo. They kept cutting down the length of the barrel and testing speed, (meaured by using a swinging pendulum and great calculations) to see the effect of the shorter barrel.
Finally, they cut the SMLE barrel so short that part of the bullet protruded, when it was loaded in the chamber. When they retrieved the bullet they discovered the portion stiking out had increased in diameter! Thus they concluded that the base of the bullet starts before the front of the bullet, and this completely fills the bore, seals it shut. Remember, this was with their quite hard military bullets.
P.O. Ackley, in his book, Volume 2, Handbook for Shooters and Reloaders, lists a lot of pressure testing he did.
Probably his most radical test, don't try this at home, was shooting an 8mm bullet in a 30-06 barrel! He reamed out a 30-06 chamber to accept an 8mm bullet. He then attached a pressure barrel to the action.
He then pulled the bullets from two Remington factory cartridges, and replaced the 150 grain bullets with 150 grain 8mm bullets.
With the 8mm bullets the velocity av. 2901 and pressure averaged 40,700 psi.
Somewhere he states that the reason for no great pressure increase, is that the bullets are sized, well before peak pressure is raised, but I didn't see that quote as I wrote this. He does say here that his test results created great confusion in the industry. He further states that those writers who claim an oversized bullet will blow up the rifle, are on the wrong track.
Now, please don't pile on the messinger, me. I only wrote down here what is in the Ackley books.

 

[SandRoad]

Somewhere he states that the reason for no great pressure increase, is that the bullets are sized, well before peak pressure is raised, but I didn't see that quote as I wrote this. He does say here that his test results created great confusion in the industry. He further states that those writers who claim an oversized bullet will blow up the rifle, are on the wrong track.
Now, please don't pile on the messinger, me. I only wrote down here what is in the Ackley books.

You (and Ackley) are absolutely correct. As I mentioned, peak pressure for rifle rounds occurs after the bullet has travelled 2" to 5" from rest, this is obviously well after the bullet has been swaged into the rifling. This can now easily be shown with electronic instrumentation of barrels.

The effect of a harder bullet is very small, on the order of a couple thousand psi peak pressure. In part is because it "delays" the bullet moving forward until enough pressure is built up behind the bullet to swage it. It is this delay that allows pressure to build a little more quickly, and can create a MINOR increase in peak pressure that occurs a couple inches or so LATER/FARTHER down the barrel.

 

[tjhaile]

The harder projectile will resist being engraved by the rifling to a greater extent, thus raising pressure.

This is why cast bullets in handguns, which are softer than any bullet jacket, will give a lower pressure for any given muzzle velocity or, alternately, a higher velocity for any given pressure.

 

[RT]

Is there a way to calculate pressure? If Im working up a aload is there any way to figure out what pressure Im working at? What should be looked at for signs of pressure...other than a broken gun?

 

[notsorichguy]

No learned opinon here ...... I just guessed...... I went with the "harder" jacketed ammo. In my mind that is the case........ But I don't "know" that.......

Some very interesting reading here after I voted tho.......

Cheers!

 

[tjhaile]

Is there a way to calculate pressure? If Im working up a aload is there any way to figure out what pressure Im working at? What should be looked at for signs of pressure...other than a broken gun?

There is an internal ballistics program called Quickload, which can estimate pressure. There are also pressure measuring devices which use a strain gauge glued to gun's chamber.

Generally, by the time you encounter obvious pressure signs, such as severely flattened primers, difficult extraction, or loose primer pockets, chamber pressure is really excessive.

 

[Silverado]

Is there a way to calculate pressure? If Im working up a aload is there any way to figure out what pressure Im working at? What should be looked at for signs of pressure...other than a broken gun?

Have a search through the Reloading forum... there are countless threads on pressure signs, safe reloading, etc.