OAL and velocity

[UclaPeon44]

Been working up RL7 for 7.62x39mm and I've been noticing that if I seat OAL to mag length, I get 50-100fps faster than when the loads are at 0.050" shorter. This result is very counterintuitive. as I assumed shorter OAL equals high pressures and in turn more velocity. At first I thought I mixed my reloads, but I repeated reloads and sure enough longer OAL resulted in more velocity. someone explain this to me. pretty sure i'm not going crazy.

[reverser]

Compressing powder sometimes decreases velocity. How much RL7 are you throwing?

[bsumoba]

you could be touching/loading into the lands also. that will also increase pressure.

[kcstott]

Pressure = velocity simple as that. Figure out what is causing you pressure rise and there’s your answer

[UclaPeon44]

you could be touching/loading into the lands also. that will also increase pressure.

How is this possible loading to mag length.

[silvertriple]

How is this possible loading to mag length.

Some chambers have tight specs so it's easier to hit the lands if you load longer than SAAMI spec. Do you notice marks on the projectile after ejecting unfired rounds? If so, more than likely you're in the lands.

You can load a dummy round longer than mag length with slightly lose neck tension, measure the coal, chamber it, and then measure again. The lands will push your projectile into the case. This is a quick way to check the max coal of your chamber.

[JagerDog]

Actually, it would be pressure over time. Chamber pressure is rather inconsequential to velocity by itself.

Now...could the effective burn rate be modified by seating depth?

[bsumoba]

Actually, it would be pressure over time. Chamber pressure is rather inconsequential to velocity by itself.

Now...could the effective burn rate be modified by seating depth?

The higher the pressure, given the same load and barrel length, the higher the velocity, generally speaking.

Increase barrel length and you will get higher velocity as well.

[17+1]

Maybe being closer the the rifling causes the pressure to build/spike differently, creating higher pressure in the case. The higher pressure may seal the case in the chamber better so less gas leaking back towards the bolt and more pushing the bullet down the bore.

IE: hitting a door at a full run to knock it down, rather than standing in front of it trying to push through it. You will really have to strain to push through it without the momentum from running.

[waveslayer]

It's most likely your chamber is short and your loading into the lands. If you're not getting signs of pressure, lucky you, shoot it and enjoy the velocity. It will slow once you wear the barrel lands down.

My wife thinks I only have 3 guns

[UclaPeon44]

checked out where the lands are. No, I'm not touching the lands. my OAL is 0.020" from contact.

[Whiterabbit]

Having studied the effect on OAL on velocity, my bet is on one of two phenomena at play:

a) the load is not anywhere near the node, allowing for drastically different internal ballistics with minor changes in the remaining inputs we vary while loading, or

b) the natural variance of the load is quite variable, and that a 10 round collection of data for each OAL put into a student's t-test will reveal that they are in fact the same population of data.

That's just a guess on my part, but my bet is that a consistent 50-100 fps faster is based on a small sample size, or data collected based on an aggregate of many small sample sizes collected over a long period of time to allow other variables to come into play.

Oh, or:

c) the shooting is not random, and so the first, colder shots are the short OAL and the later, hot shots are the longer OAL. (which is a variant of another factor affecting the velocity, not OAL)

[kcstott]

The higher the pressure, given the same load and barrel length, the higher the velocity, generally speaking.

Increase barrel length and you will get higher velocity as well.

And since there is no accurate, repeatable way for a home reloader to measure chamber pressure we are all looking and primer flattening and brass flow anyways.

[kcstott]

Actually, it would be pressure over time. Chamber pressure is rather inconsequential to velocity by itself.

Now...could the effective burn rate be modified by seating depth?

At what point did I say Chamber pressure. I simply stated pressure = velocity

Broken down to the most simple aspects of what we have going on in a firearm barrel, that is the equation.

Now since we have no way to properly measure chamber absolute pressure and we read brass flow and primer flattening we are all just guessing.

Someone will post that strain gauge pressure trace crap. All I have to say is calibration & repeatability?

[fguffey]

If what your are saying is true: I am the fan of the jump start, I want my bullets to hit the rifling 'a-running', all other reloaders want their bullets setting at the rifling.

F. Guffey