velocity fluctuations

Dougfir · Nov 8, 2018

I've always gotten those groups, at times, when testing a load, where you get two rounds right next to each other, then another one 2 inches away. Since getting a chronograph, I've noticed that sometimes when this happens, the errant shot had a much different velocity than the others.
Yesterday, I shot a 3 round group and had two print 5/8" apart, with velocities within 11fps of each other. Then, the third was 2" away, but was moving over 100fps faster.

My question is: I'm pretty sure I measured them all the same, so when you see this, do you start to doubt your scale, or is there another explanation that doesn't involve faulty equipment?

AK7AN · Nov 8, 2018

I would weigh the Brass and see if there is a big difference in the weight of the 3 that you referred to. Could be that faster one has a lot less capacity than the other 2.. That could be one explanation. I have found that since I started weighing each powder charge on a balance scale I have greatly reduced my ES. Every once in while still get a fast or slow one and usually chalk up the difference to the brass fluctuations.. I am sure there could be a host of other reasons that I am missing and ithers will chime in too.
Take care
Ed

AK7AN · Nov 8, 2018

I would weigh the Brass and see if there is a big difference in the weight of the 3 that you referred to. Could be that faster one has a lot less capacity than the other 2.. That could be one explanation. I have found that since I started weighing each powder charge on a balance scale I have greatly reduced my ES. Every once in while still get a fast or slow one and usually chalk up the difference to the brass fluctuations.. I am sure there could be a host of other reasons that I am missing and others will chime in too.
Take care
Ed

Nimrod84 · Nov 8, 2018

Dougfir,

When this happens to me the first place I look is the chronograph. My optical chronographs can show up to 100 fps difference due to environmental / light conditions. Usually a 50 fps variation between sunny and cloudy, but some times I see 100 fps. The magnetospeeds can move around on the barrel, causing POI shifts and display odd velocities as well.

As AK7AN noted, brass water capacity can make a difference along with bullet seating depth... If your powder charge is on a book end of an accuracy node, than a small variation in powder charge can push you out of the node. This can happen as well with bullet seating depth (for example a little long and you might be in the rifling), and when both powder and bullet seating depths are out of the node you might see a significant increase in velocity and POI change.

If you are crimping and have an extra heavy crimp (common with the LEE Crimp die) or longer brass that gets crimped more; that can increase pressure leading to velocity variations and POI shifts. Long brass can also contact the crimp ring in the bullet seating die, or go into the rifles throat achieving the same higher pressure and POI shift as an extra heavy crimp.

FYI and this is my personal approach... I personally always doubt my scales until proven otherwise. I use three scales to verify my charge weights, an RCBS Chargemaster 1500 which can be up to .2grns off of the display (very sensitive to calibration conditions), a gempro 250 and a balance beam scale which is currently a Lyman M5. I have found that all three can wander a bit due to temperature & humidity changes along with static. When things are stable they all agree, when things go weird I have seen up to .6grn difference between them. If the gempro 250 isn't agreeing with the M5 or the RCBS 1500 won't settle, than I stop loading.

I don't think this is your issue; but marginal primers comes to mind as well. This would include using too strong or too weak of a primer for the powder charge type and weight, creating an inconsistent powder burn.

Hope those thoughts help.

hunter24605 · Nov 8, 2018

If your powder charge is on a book end of an accuracy node, than a small variation in powder charge can push you out of the node.

I've also noticed this. But usually in the 50 fps change, not the 100 you're mentioning. But I suppose to what degree it changes is cartridge dependant .

Aside from what's already been mentioned, Some other things to consider is when in the string does the errant shot occur? cold/hot barrel, clean/fouled

Nimrod84 · Nov 8, 2018

Hunter,

Good call-out and I agree, usually I see 30-50 fps when I jump .1 grain outside of a node. That is for 30-06, 280, 280 AI using IMR and H 4831 & RL-22. Different cartridges and powders will act differently. I did mention bullet seating depth possibly playing a role with the powder charge; "...and when both powder and bullet seating depths are out of the node you might see a significant increase in velocity and POI change."

On some of my rifles they are much more dependent on bullet seating depth for staying in a given node than they are on the powder charge. When both charge and cartridge length go out the bottom of the node, the velocity drops like a rock. I can't speak from experience on the high-side though as I tend to not push my stuff real hard, so I could be off base when it comes to gaining velocity on the high side of a node.

Without knowing more, I would personally put money on it being a crimp / long piece of brass. Or a chronograph issue, as I consistently get 50 fps variation based on cloud cover and another 50 fps or so depending on where I shot over the chronograph. So if he was using an optical chronograph I would suspect that the fast shot was pulled or the rifle re-positioned between shots and that would affect POI and velocity reading...