Extreme Velocity Spreads

[M7025-06]

I was out at ENGC this morning with some test loads for my 280 Rem and had some really weird results.  I loaded up 4 sets of 3 rounds all with the same powder charge but at different seating depths.  The sets were shot from deepest seating to shallowest with the first 3 sets showing fairly consistent velocities.  The chrony was set 12' from the muzzle.  The 4th set (bullet seated closest to the rifling) had the following velocities:

#1. 2864 fps
#2. 3116 fps
#3. 2592 fps

All brass was prepped the same way, same brand, trimmed to the same length.  The brass showed no signs of pressure.  The group on this set was horrible.  Shot #1 was about dead-center, shot #2 was about 6'' high, and shot #3 was about 4'' low (all at 100 yards).

Does anyone have any thoughts as to what would cause a 500 fps velocity spread?  A guy at the range thought turning my case necks would make a difference.  I don't know...I'm stumped.

[RLMoeller]

I would question the accuracy of the reading.  Did you have a cover on the Chrono?  Chrono too close to get accurate reading?  12 feet seems plenty far enough away to not be affected by muzzle flash. Is the chrono new, or do you have some experience with it?  Seating depth alone would not cause a wide variance.  Maybe the sun peaked out during that set and it was cloudier during the others?

[AWick]

With the target hits correlating with the chrono readings somewhat I don't think the chrono is really the culprit. Could seating them too far forward (towards the rifling) affect the contact pressure and distribution of forces between the case and bullet?

How did they compare to the next level down? What were your velocities and target locations?

[M7025-06]

I would question the accuracy of the reading.  Did you have a cover on the Chrono?  Chrono too close to get accurate reading?  12 feet seems plenty far enough away to not be affected by muzzle flash. Is the chrono new, or do you have some experience with it?  Seating depth alone would not cause a wide variance.  Maybe the sun peaked out during that set and it was cloudier during the others?

I had the sunshades on.  I've had this chrony for about a year haven't had any issues with it.  I ran some loads across it that I had worked up last fall and the readings this morning were almost identical to when I first developed that load.

Could seating them too far forward (towards the rifling) affect the contact pressure and distribution of forces between the case and bullet?

How did they compare to the next level down? What were your velocities and target locations?

This was my first thought.  It seems to make sense.  The velocities are increasing as the distance to the rifling closes down.  I'm still not sure why the readings are so erratic though.  My jug of powder was almost empty when I loaded these...would that cause any inconsistencies?


#1:  2789, 2818, 2800 / 0.50'' group (one ragged hole)
#2:  2694, 2804, 2982 / 0.50'' group (one ragged hole)
#3:  3006, 2857, 2838 / 1.25'' group

The velocities are in the order they were shot. 



 

[RLMoeller]

Sounds like you know what you are doing with chrono.   

I misread the original post, I thought it was the 3rd group that was varying for some reason.

Since you asked about powder,   Did you weigh each charge?

[AWick]

What were your 4 steps of lengths of the overall cartridge? Do you think the last length was feeding into the chamber differently or contacting the front of the rifling and screwing things up?

It looks like your first group seemed spot on! Maybe try another set with that set up and see if you can replicate your success. But I guess then you wouldn't get the joy of still searching for that perfect load

[M7025-06]

Sounds like you know what you are doing with chrono.   

I misread the original post, I thought it was the 3rd group that was varying for some reason.

Since you asked about powder,   Did you weigh each charge?

Yep...each charge is weighed before going in.

What were your 4 steps of lengths of the overall cartridge? Do you think the last length was feeding into the chamber differently or contacting the front of the rifling and screwing things up?

It looks like your first group seemed spot on! Maybe try another set with that set up and see if you can replicate your success. But I guess then you wouldn't get the joy of still searching for that perfect load

I usually neck size all of my once-fired brass, but since these were the fourth firing, I full length sized this brass, measured for length, and trimmed to 2.530'' (+/- 0.003''). 

I might try 2 sets of the same load & seating depth as set #1, neck sizing one set and full length sizing the other to see if that makes a difference.

As much as I'd love to have consistent 3000-3100 fps out of this load, out of the 3 manuals I have with data for a 140 gr. soft point (Sierra Gameking is what I'm using), not a one shows 3000 fps with a full dose of IMR-4350.  In fact I'm 0.2 grains over max according to Sierra's manual, but 0.3 grains under max per Lyman's 49th Edition.  That's what has me thinking something's not quite right when my chrony's showing me these high velocities (+3000 fps).