Anyone care to explain these velocities?

[Ruger]

Was looking over velocity recordings fired from my .270 (22" barrell) using H4831SC. With the exception of the amount of powder, all other components were the same. This is what I saw.

57.5 GR averaged 2843 fps
58 GR averaged 2826 fps
58.5 GR averaged 2882 fps
59 GR averaged 2803 fps
59.5 GR averaged 2838 fps
60 GR averaged 2920 fps

Seeing a that few powder increases don't neccessarily give velocity increases. For instance, 57.5 grains is showing me faster speeds than 59. Perhaps barrell temps were a little different throughout the shooting of each load or is this something normal to see?

 

[Antelope_Sniper]

How many shots in each string, and what were your standard deviations?

 

[Ruger]

First three were 3 shot strings. The others were 4 shot strings. I did not record the standard deviation from the chrony when I was shooting these. I was just noting the velocity. Now that you have asked me this, I am thinking this may not be enough data to come to any conclusions.

 

[Antelope_Sniper]

I'd be interested to see the rest of you components....brass, primers bullets ect.

I usually shoot 5 when chronographing, and pay close attention to the consistancy. One or two flyers can really affect your average velocity. With a 22" bbl on my .270 Win I usually start loosing velocity if I exceed 59.5gr of H4831 and a 130gr bullet. This will very some depending on the rest of the components.

 

[Ruger]

Sierra 130Gr Prohunter
Win. Case
CCI LR primer

 

[Antelope_Sniper]

That's my load, except I use Nosler BT's.
I had some problems getting the 130 gr Sierra's to shoot in my gun, so I switched to the old Solid base, way back when. Problem eliminated.

Here's what can happen with a sharp boattail bullet such as the Sierra. As the bullet exits the barrel, it is followed by a ball of expanding gas. When the ball of gas impacts the back of a flat base bullet, it breaks up the ball of gas. With the sharp boattail, the ball of expanding gas can wash over, and surround the bullet. The bullet must must now break out of this ball of gas, which can effect accuracy. Now some loads can be on the edge of wether this gas will will envelope the bullet or not. This can create erratic velocites, wide standard deviation, and an all round miserable day at the range.

I guess the next quesiton is, how were your groups? If groups were also erratic, I suggest switching to Nosler BT, or Speer flat base bullet.

 

[taylorce1]

I'd agree with you if the Prohunter wasn't a flat based bullet as well.

 

[Antelope_Sniper]

I stand corrected. I was thinking of the Game King.

 

[hardpan]

I would be certain to start from a cold barrel and watch the std dev. Hot barrels have been the cause of a lot of poor groups here in Ca.
Hardpan

 

[DrMike]

Have you verified your chronograph with a load that delivers a known velocity? It is always possible that you have a bad canister of powder. Have you checked this canister with other known loads?

 

[Ruger]

This is the only canister of H4831SC that I had. All loads came out of it.

 

[old #7]

I have noticed that my S.D will increase if I am shooting a warm barrel and especially when the chamber gets warm.
The round will tend to "cook" in the chamber while adjusting the sight picture and squeezing the trigger.
I try to take three firearms to the range if I am doing any kind of load developement. I will shoot the 22's, hand guns or the ak-47 and ar-15 while letting the rifle cool completly.

 

[Antelope_Sniper]

Or chronograph with 3 rifles and rotate.

 

[old #7]

Or chronograph with 3 rifles and rotate.

Exactly.
Some people have a very confused look on their face when they notice me doing that. Meanwhile they are blasting away shooting 40 rounds in ten minutes and having problems zeroing the rifle in.