Home Forums Thread Like Summary - What causes certain barrels to hit pressure before others

I know barrels vary in speed and some seem to have earlier pressure signs. Some obvious variables include bore diameter, type of rifling, throat and lead angle but what are some lesser known reasons some won't achieve the speed before pressure? I just rechambered a 6.5cm to 6.5x284 and I hit pressure signs with 47 gr h4831 and 140 at 2600 with a 22" barrel. That's creedmoor speed. I'm not sure what's wrong. New lapua brass and 1x fired in this rifle. Tried RL 22 with same result hit pressure at like 2625 with 140s. The chamber is tight but bolt closes on new lapua brass smoothly. The chamber has been polished so after reading about the need for brass to grip the walls as it expands I'm going to add some cross hatching thoroughly clean and try again. Any other thoughts on this issue? I know I'm not going to get super speed from a 22" barrel but has to do better than a creedmoor.

The brass gripping the chamber walls is a myth. I tested even dipping a cartridge in oil. Same pressure on bolt face.
Charlie

Barrel length
Bore diameter
Rifling. Depth, style.
Bore smoothness
Throat length
Chamber dimensions

There are surely more that aren't here.


Bullets. How they interact with that bore.
How they are seated to interact with the throat.


All of those are factors, to varying degrees.

Length is the one folks jump on.
It's easy to measure, the rifle is often shorter that the one used in testing.

Every one of the listed factor come into play. Let them stack in your favor and you
might beat listed velocities with no issue. If they stack the other direction, you
have a slug.




Oh yeah.
Soft brass.
If you have a batch of too soft brass it will show pressure when your pressures
are below max.

So pressures are too high. For that brass.
Brass being the weakest link, if soft brass is showing pressure you are heading
toward a case failure. And, pressure numbers be damned, you don't want
case failure.