If a short barrel produces less velocity with the same load as a long barrel, then is the pressure also less in the shorter barrel? I guess my question is, does barrel length have an effect on pressure as well as velocity, and are they related?

Peak pressure normally occurs before the bullet has traveled very far so you would have to drastically shorten a rifle barrel to reduce it.

Steve-

Peak pressure should be the same, a longer barrel just gives the expanding powder gasses more time to "push" the bullet. (Once it leaves the muzzle it starts slowing down, obviously, so the longer barrel gives it a better start due to "time" of pushing, rather than higher "pressure").

The longer barrel gives the gas a longer TIME to accelerate the bullet. Peak chamber pressure is still the same however, as is the pressure at any given point along the barrel.
Crude analogy would be to accelerate your car as fast as possiable for 1/10 of a mile and look at the speedometer, do it again but go 2/10 and look again. The acceleration rate for the first 100' (peak pressure) is the same for both, isn't it?.

The converse of this is that if you hold a short barrel velocity constant, and work up a long barrel load to equal that velocity with the same or slower powder, the pressure will drop.

OK, next question, I have a Browning Bar 300WSM with BOSS. The barrel is 23" with BOSS and 20" without. Would this be considered a 20" barrel? If so, then would the BOSS CR (Conventional Recoil- No muzzle brake) still be 20" or would the fact that it has no muzzle brake bring it back to 23"? I am asking this because my load development is giving me velocities much lower than listed. Browning is way behind on the delivery of the BOSS CR so I can't compare the two.

The short answer is, both short and long barrel guns will see the same peak pressure, barrel lenght is irrelevant to maximum pressure. Smokeless powder is fuel. It will only generate energy until it is consumed, and the smaller the space for burning, the greater the powder's ability to pressurize the area.

It functions as a 20 inch bbl, since the pressure drops dramatically when you enter the brake.

You are wrong, there is a PEAK PRESSURE in both short and long barrel lenghts. You take the 44 mag for example in a pistol. And you say you have about 38,000 PSI when the bullet has traveled 4/10 ths of an inch. At 1 inch of travel, pressure has dropped to 32,000PSI, at 2"-22,000 PSI ect. Muzzle pressure with a shorter barrel, the pressure at the crown is higher with a short barrel; 14,000-15,000, half as much for a 7.5" barrel and one of the primary reasons short barreled guns have unpleasant muzzle blast and muzzle flip.

There are 3 signicant pressure levels at work when a cartridge is fired; Maximum, Average and Muzzle

So why are you saying eldeguello's wrong? He said: "There is NO relationship between barrel length and peak pressure!" At any practical barrel length, that's perfectly true. I don't think we're talking about cutting our rifles down to snubbies with 1" of bullet travel to the muzzle here.

Like I said, both short and long barrel guns will see the same Peak pressure Than I mentioned the 3 levels of pressure. As soon as you ignite the primer and the powder charge, the bullet begins to move forward, as the bullet moves forward, it leaves an increasing volume behind it for burning powder to expand. As the increase in volume grows at a rate faster than gases expanding from burning powder, pressure in the chamber and bore diminish. That 1 inch or so of barrel was an example

I understood all that. And it in no way disagrees with what eldeguello said. He only said that the peak pressure was unaffected by the barrel length. That's why I asked.

I always thought that revolvers achieved peak pressure before the bullet base left the cylinder and for most it drops like a lead anvil after that. Maybe the Casull as an exception due to small gap dimension? True/False? Anybody seen pressure curves from a SB/RH or Smith .44 Mag?

Never cared that much for them anyway.