A cronograph question

[beekeeper]

Sorry for not being around for a while but family takes first place.

Finally had a chance to go to the range for a little relaxation.
Was chronying some Noe 150 grain bore rider 7 MM cast boolits with my new magneto speed chrony.
Had as much as 100 FPS difference.
All loads were weighed and all bullets were within .5 grain of each other.
Brass was LC62 30/06 match brass converted to 7MM.
Powder was IMR 3031 29 grains , each charge weighed on a beam scale.
Bullets are bore riders and were set to have .015 of the bore riding section in the bore.

Shooting partner suggested it might be neck tension or crimp tension but with cast it was very good.
Would like your opinions!

beekeeper

[RBHarter]

That's not unusual, full cases will help. Sometimes primers will change the consistency also. I had a load that was all over with Winchester but settled in to 15 over under load with CCI.

[Chickenthief]

Case neck annealing!

[Ranch Dog]

Wow, 100 FPS difference. Any chance it is the new chronograph? You have any factory ammo that could be shot through it?

[beekeeper]

Gosh do they still make factory ammo?
Haven't fired any in about 10 years!

Will try to get a box and see.
Have had the chrony for about a year now but have only used it sparingly.
Had thought of the neck tension and have since annealed the bras as well as given it a complete inspection
Will give it a try the next time I have time to go to the range but that may be awhile like I said family comes first and right now things aren't the greatest!


beekeeper

[Chickenthief]

I asumed that it is a 7x57 you're shooting?

If so that powder is borderline slow!
SD tends to drop when burnspeed and/or pressure goes up.

[Fyodor]

Just another idea: Was the battery in your chrono fresh? 100fps seems a huge variation to me.

[Ranch Dog]

Gosh do they still make factory ammo?

When I need some I usually buy it from Ammunition To Go as this Texas based business seems to always have what I need available and at a better price than I can find from any other source including any shops within a reasonable drive.

I don't shoot much store bought stuff but use it sparingly when I face some kind of issue such as your experience here or as proof for pressure trace testing. I've actually come to prefer Prvi Partisan over any other brand. During the pressure trace setup, I've found both the velocity and pressure ES/SD to be extremely tight.