Pressure does NOT equal velocity. Some powders produce higher pressure to achieve the same velocity as others.
There are a lot of things that can affect pressure.
Temperature, altitude, chambers, bores, and i'm sure there are others also.
Example of temp: Friend of mine had a near book max load he developed for a rifle in early spring, no pressure signs. In July he went out with same load to do some penetration testing, primers blew out and it was all he could do to open the bolt.
And yes there are slow and fast barrels.
2 of the Reminton 700 30-06's in my house shoot the same load of IMR 4350 in R-P cases, with the same primer, with a 165 gr Hornady. One of them gets 2720 fps the other right at 2900 fps, and they both have 22" barrels. The load shoots accurately out of both rifles.
Pressure = Velocity
There's no way around it. No one is talking about comparing different powders - this is about book powder charges not reaching book velocity. Loads that shoot fast in high heat do so because of high pressure (caused by the xs heat).
There's no such thing as a "fast barrel" or a "slow barrel". There are barrels with tight cut chambers and precison bores that make max velocity (and pressure) w/ a minimal powder charge and there are sloppy cut chambers and sloppy bores that make less pressure and less velocity with the same powder charge. The powder charge to reach the same velocity can vary significantly, the pressure will be close to the same.
David
True.
Although it would not be uncommon language to call the sloppy barrel "slow". The owner is going to be perplexed....looking for explanations as to "why",which is also pretty normal.
Although I frequently wonder about so many people making absolute statements about the pressures generated when so few have the means to accurately measure it unless they own the right equipment....which few do.
Most run to QuickLoad these days and make assumptions based on that.Most of the rest of us just "guess" and watch velocity,and case life.
I have seen more variation of this type due to differences in bore/groove diameter, throat length,barrel length etc. I say this having had quite a few barrels cut with over-sized groove diameter and/or longer throats. Friends have done the same. The results usually are pretty predictable. These conditions will even exist from one barrel to another,whether factory or custom.
Generally I think it relates back to the amount of resistance the bullet encounters traveling down the bore.
Anyway what I've come to learn after messing around with too much of this stuff is....if you want to hit a certain velocity level with a certain bullet, make sure you buy a big enough cartridge.Much tidier solution.
