Strange Velocity Changes

[mps5949]

After diligenty trying to reduce extreme spreads and work up some long range loads for woodchuck hunting, I noticed some inconsistencies in my velocities and was looking for some advice. Here are details on the gun and load:

6mmbr, 1-14' twist
Norma brass, CCI 450 primers
65g V-max, 34.6 gr Varget
0.010' jam
all powder from same lot

I know this is a stout load. Occasionally i see some light ejector hole marks on some of the brass, but have not any pierced primers or difficulty removing the fired cases.

My problem is that after several shooting sessions with the exact same loads, my velocities seem to be all over the place. At first I thought it was temperature related, but look at this average data:

Date Temp,F) Avg V,fps)
26-Apr 80 3517
25-May 70 3504
21-Jun 72 3485
27-Jun 70 3416
28-Jun 67 3498
5-Jul 63 3453
25-Jul 66 3389

These are all averages of at least groups of 5 or more shots with foulers not in the data. I am open to recommendations as to what could be causing these dramatic changes!

Thanks

Mike

 

[2506]

Most likely it could be lighting conditions above your chrno. I get better readings by always installing the shades. One of the gun magazines had an article on this a while back. The eye of the chrno has to see the bullet to trip the timer. With different backgrounds, bright sun, clouds etc. it sees the bullet differently and will trip the timer differently thus you get different readings. This is my best guess.

 

[mps5949]

Thanks for the reply. I always use the sunshades on the chrony. I did go back and look at the data and some highs were taken on cloudy days as well as sunny days. This really has me puzzled. I believe my barrel is clean and i have been very careful at sorting brass, bullets, and I weigh every charge....

 

[CPorter]

How often do you anneal the necks? Do you burnish the inside of the necks? Where do you store the powder?

Double base powders seem to evaporate some nitro or whatever out of the kernel with time and this seems to fit your data.

 

[mps5949]

I have never annealed the necks on this brass. They have been fired maybe 5-6 times. I generally store the power in the original container in my reloading area. I have an area in my basement that is dedicated to reloading and it is cool and dehumidified. Lately I have left some powder in the powder measure dispenser bottle as I have been reloading a couple times a week. Maybe the Nitro is evaporating? I do not burnish the inside of the necks, but I do brush them with a stiff brush. Interesting......

 

[jb1000br]

If you are using a Shooting Chrony, don't sweat it.

Unless the setup and lighting are the same every time, your numbers will be all over. Even then, they are only +-~15fps on a 3000fps load...

Put it on paper. Get a good zero and shoot at distance to see if the drops are the same,or similar) regardless what the velocities are. If your drops are consistent, your chrony is the weak link...

I have seen bigger swings than that in my chrony when running it simultaneously with my Oehler...

The necks and powder are not your problem...

JB

 

[mps5949]

Thanks for the tip. I plan on shooting at 500yds this weekend so i will see how it goes!

 

[MJC]

The ejector tattoos are indicative of high pressure, so I would back off the powder charge or back the bullets out of the lands or both. Hopefully your accuracy will not suffer from either change.

Removing all the carbon from inside the case neck with a bore brush can cause bullets to stick to the case necks causing inconsistent tensions. I have experienced wild velocity spreads from known good ammo going from <30 fps ES to 100+ fps ES because of this. I discovered the cause when attempting to pull bullets to check weigh powder charges. The bullets had to be deep seated a tad to break the seal and some resulted in an audible pop when they broke loose. They had only been loaded for a couple months.

Since you didn't mention unacceptable velocity variations within a string when shooting over the chrony, this may not be something you care about, but - the trouble I have with jamming ~.010 is that ogives vary and some may be jammed more than others unless you measure every round. Some bullets may not be touching at all.

 

[lpreddick]

MJC: thank you for confirming my suspicion...bullet sticking in necks that i actually polished with flitz,thought the slickness would allow bullet to slip out quickly). i also noted large ES and pulled a few with my kinetic bullet puller and worked up a sweat! i knew something was amiss since neck tension was light. now i only lightly brush inside neck with nylon brush. i do,however, polish any bullets that show oxidation. you can feel the slickness afterwards. such a small tip that makes a big difference...thanks again.if i can see you, i can touch you!BANG!

 

[jd12]

Does anyone lube the inside of neck right before bullet seating? Just wondering if this would help with the sticking bullets.

 

[jb1000br]

Yep, with NECO's dry moly neck lube. Took OAL variations from +-0.002 to +-0.0005'

JB