velocity fluctuations

[Dougfir]

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]

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]

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]

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]

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]

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...

 

[Ridge_Runner]

seating depth, the group within the group is your rifle asking for a seating depth change.
RR

 

[lifetimeclams]

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 Melon Playground 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?

If one shot is 100+ fps off, first suspect inconsistent powder charge or a bad primer/case (weigh a sample of charges, inspect primers, sort brass), but also rule out shooter flinch, barrel temp/fouling, or a bad scale — a chronograph reading that large usually means a real load/ignition inconsistency, so verify charges and components and repeat.

 

[NYDAN]

Another factor could be the present of the "dreaded donut" at the junction of the shoulder and neck. Brass that has been shot in rifles with larger chambers and then sized down will force brass into the shoulder/neck junction. This can significantly increase neck tension and release force resulting increased pressure and velocities.

I recently processed a batch of brass which had been shot in two different 7mm RM rifles. One rifle had a significantly longer shoulder headspace reference (as measured with a Hornady headspace tool). The brass was being processed to fit the rifle with the smaller chamber. I kept all the brass segregated by which rifles it had been fired in. I found the brass fired in the longer chamber all formed significant donuts which had to be reamed out.

Wishing you the best in getting this sorted out.

 

[Idaho53]

This can be frustrating to figure out. You have received great advice already. Please consider the powder as a possible problem. If you are using powder that is not temp. sensitive rated, then 100 fps variation can be very common. Also, neck tension (and/or seating depth) can also be the culprit. Good luck and keep us posted!

 

[Charlie-NY]

The original post was 7 years ago. Hopefully, he's got it figured out by now.

I would occasionally see something like the OP experienced and always wondered if my RCBS Chargemaster was at fault. Recently I bought a FX-120i to verify the charges dropped by the Chargemaster. To my surprise the Chargemaster was throwing very consistent charges and never deviated more than a few tenths of a grain from the display, and even that was rare. Now that I've been loading charges that are right to the exact kernel (not grain), I still see an occasional round that is out of range velocity wise. I think that it might have to do with neck tension but it sure isn't the powder charge.

 

[NYDAN]

The original post was 7 years ago. Hopefully, he's got it figured out by now.

Yeah, I noticed that AFTER I made my post.