Home Forums Hunting & Shooting Ask The Gunwriters Substituteing primers

Is the time the pressure acts on the bullet also a factor in the pressure/velocity relationship? Like follow through when a batter hits a ball, if the same force (pressure) is applied for a longer time, wouldn't the velocity be higher as there is more time for acceleration?

Indirectly.

I use SR in pistols all the time

I use WLP's in 30-30 cast bullet load's.

You're correct, but the point stands - you cannot increase pressure on the same basic burn curve and not increase velocity. The two go together.

That is what a longer barrel does

Velocity IS the relationship between time and distance. So you can think about it either way around you want, but it's cleaner if you integrate through the distance, because that's a constant whereas the time in the barrel goes down as pressure & velocity go up.

You do realize, don't you, that you typed this inquiry on a forum wherein over 1/2 of us, in order to explain that we "saw" something, will start our sentence with the phrase "I seen"?

LOL

That is true, and it may be the only thing keeping that firing pin from piercing the primer.

Now that’s funny right there.

Height difference with large but not with small. That wouldn’t help the firing pin piercing because pressures shove the primer back against the breech face. Makes it worse. I seen it.

Your eloquence is matched by your understanding of the problem. Let's work with lab data.

Consider typical shot to shot variations, where everything is nominally the same. Lyman 47 shows a 4900 psi change came with less than 60 fps more speed. That was a 10% rise in pressure for a 2% increase in speed. If by "basic burn curve" you're implying the shape of pressure/time or pressure/distance curve must be about the same for a given powder and bullet, then 10% more pressure should get you 10% more bullet energy or 5% more speed. This doesn't happen and for an obvious reason: There is in the cartridge case no extra chemical energy. Only the conversion efficiency can change, and the big factor in the conversion from chemical energy to projectile kinetic energy is the expansion ratio, which didn't change. Raising peak pressure increases efficiency at a given expansion ratio, but not as much as a like change in expansion ratio. In Lyman's test, an increase in the pressure early on clearly must be matched by less increase (and perhaps even a decrease) in pressure further down the barrel.

In the 06/2004 issue of Handloader, a writer who posts in these forums reported changing primers in one .300 Win load increased the pressure 14,300 psi (a 26% increase) with a change of only 71 fps (a 2% increase). In A-Square's old reloading manual, they also give an example of what might go wrong when you change primers, this time with the 7 Rem Mag. Here, a 12,800 psi increase (23%) in pressure delivered but 34 fps (1%) more speed. (In both of these two tests, I didn't record if the stated numbers were averages over strings, but if not, the results may include effects of shot to shot variations.)

The peak pressure in a rifle is very sensitive. As powder burns, the pressure rises, and as the pressure rises, the powder burns ever faster. Also, if the bullet doesn't get out of the way fast enough, things will get out of hand rapidly, which is why bullet construction and bore erosion can cause big problems. The primer likewise will have big effects. If its flame lights up more powder and deeper into the charge volume, that exponential growth in burn/pressure gets a bigger head start. If the primer manages to light up the charge with little increase in initial case pressure, the bullet may not get moving proportionally as much as the burn, and the pressure grows unexpectedly. The last time I looked through research reports from the Army, even they can't model (and thus predict) these effects (but in cannons they can).

I'll end with a quote from Mic McPherson, who has worked in ballistics labs and also edited the user guide to the QuickLoad simulator:

With very little experience compared to most of the above posters, Here is my only contribution:

When loading my 1895 .405 Winchester with 400 grain Woodleigh bullets for a Cape Buffalo hunt, I had reached around 2050 fps with TAC powder and the recommended magnum primer.
A buddy observed that I was just below the regulation velocity of his 450/400 DR and and suggested that I switch to Federal 215 Match Magnum primers, (he is also a gunmaker and very experienced hand loader).

I switched to Federals and the Chrony measured velocity went up to 2076 fps. He suggested we stop there as his 450/400 had killed multiple ele and cape buff with that load.
In Africa, the load was effective, shooting through 40+ inches of buff from behind the rear left rib, through the heart, and out between the front legs; bullet not recovered.

Upon return to Texas, the TAC load measured 48,800 psi , so I changed powders to VV N133 and achieved the same velocity with a peak pressure of less than 40,000 psi.
For those not familiar with N133, the pressure curve is smooth with no spike and generally has a peak pressure of 10,000 -12,000 psi less than most other powders at the same muzzle velocity.

That is all that I know about such matters and cannot explain further things beyond my experience. Yes, I do use N133 for most of my handloading now.

Ah yes, physics works differently in your gun than anywhere else in the world. You push harder on the bullets (higher PSI) but instead of that causing them to go faster like the uninitiated might think, all that extra kinetic energy goes to the land of magical unicorns, where it makes the rainbows dance. Got it.

More pressure causes more velocity. The bullet literally is a pressure gauge. Measurement errors don't change that, and publishing those measurement errors in Handloader speaks to the ignorance of the editors there rather than suddenly making a lab mistake correct.

The bullet is a gauge of the pressure across its entire run in the barrel. It does not measure peak pressure. Ultimately, it's a measure of the conversion efficiency of the powder's chemical energy. If the pressure peaks higher early in the run, the chemical energy used to get to that pressure will not be there to keep up pressure further down the barrel.

You seem confident that three separate ballistics labs and their technicians have it all wrong. From where do you think load book data comes? Your intuition? Good luck!

Not exactly, use a powder with too fast of burn rate and you can be way over pressure and under velocity.

I was thinking the same thing myself JWP.

That's irrelevant as we're talking about holding the powder constant. If you change BOTH the primer and the powder, then you can get radical changes in pressure without a change in velocity. Of course you could get that by just changing the powder and ignoring the primer too.

I use large pistol primers in all my rifle cast bullet plinking loads. Pressures are kept low, but large rifle primers have more margin for safety.