Crazy velocity spreads

[gunmutt]

So I started load development on a new rifle. I will list specifics and then get into my problem.

Rifle:
Custom Action - GA Precision
Barrel - Hawk Hill 8" twist 22"

Cartridge:
.243 Improved - 30° shoulder
Berger Hybrids 105gr Moly
Lapua 243 case
Fed 210M
H4350

Reloading tools:
RCBS Chargemaster verified with a GemPro
Whidden custom bushing die
Redding competition seater

So with my normal routine I have been able to load very consistent loads. Very little runout and ES in the single digits. I have used this same lot of power, primers, and even bullets for my old rifle and cartridge to get very consistent loads. With this new rifle and cartridge my ES have been ridiculous. For a single charge I have gotten as much a 76FPS variance. With all things consistent with my loads what would be the reason for this. Can a barrel or cartridge just not like a powder that is known for low ES?

 

[jerseyjoe]

What chronograph are you using?

With everything else that you listed, it sounds like you are experienced at reloading.

I would suspect the chrono and try to verify your results with a different one.

 

[gunmutt]

I was using a MagnetoSpeed Sporter. I have used it before and had no issue. Now I am trying to save up to buy a LabRadar but that is only to keep my round count low by getting velocity data while performing other tests.

 

[dminn1]

It could be a combination of a new barrel and moly coating on the bullets. I have no experience with moly coated bullets but do recall some who use them suggesting it can take a while to season the bore with moly before things settle down some. Just something to consider.

 

[gunmutt]

It could be a combination of a new barrel and moly coating on the bullets. I have no experience with moly coated bullets but do recall some who use them suggesting it can take a while to season the bore with moly before things settle down some. Just something to consider.

That is a possiblity as I only have about 200 rds down the tube. I broke the barrel in with my 60 round recommended.

 

[ab_bentley]

In all honesty, until you brake the 250rd mark, maybe more depending on pressure, you can expect velocity change. I've been running an exceptionally slow powder in my creedmoor and I'm at 600+rds and am still seeing an increase of 10-20fps every 200rds or so. Once the throat gets softened up and a good layer of treated steel is formed, you'll see a plateau, but every barrel is different.

Adam

 

[gunmutt]

In all honesty, until you brake the 250rd mark, maybe more depending on pressure, you can expect velocity change. I've been running an exceptionally slow powder in my creedmoor and I'm at 600+rds and am still seeing an increase of 10-20fps every 200rds or so. Once the throat gets softened up and a good layer of treated steel is formed, you'll see a plateau, but every barrel is different.

Adam

I don't think I want any more velocity as I am in the 3250 range at the moment. Bergers can't take much more than that

 

[seymour fish]

It could be a combination of a new barrel and moly coating on the bullets. I have no experience with moly coated bullets but do recall some who use them suggesting it can take a while to season the bore with moly before things settle down some. Just something to consider.

15 rounds of moly coated is max required for settling

 

[JohnKielly]

My first call when figures are bad is to check out ignition & what can send it to hell in a bucket:

Firing assembly - adequate protrusion, adequate spring power, no hang ups or rubbing with pin fall, trigger correctly hung
Primer - consistent seating, properly bottomed in pocket
Case - snug in chamber, uniform small clearance for all cases

 

[ebb]

What John Kielly said. I had one that drove me crazy for over a year, it was firing pin protrusion.

 

[fyrewall]

Some time ago I found the attached link about primers. According the link, as I understand it, blast variations were primarily a function of explosive weight variations from primer to primer. Temperature differences would have an effect.

I found this to be an interesting read as I am curious about how primers work.

I will read the link carefully again looking for effects primer protrusion differences might have on primer/explosive detonation.

My experience has been with approximately equal temperatures, identical lots of primers, identical lots of powders, and a history of low velocity variations using identical components firing pin protrusion differences haveno effect on velocity or any resulting velocity variations.

A factor discussed in the link was, "...possible that some loads are more sensitive to primer uniformity than others". This might mean that differences in firing pin protrusion might affect pre detonation primer uniformity resulting in load variations, or "sensitivity".

Explosive blast velocities are huge and detonation times are tiny making me think that it either goes bang or does not. Then there are "hang fires".

btgresearch.org/High-speed%20measurement%20of%20rifle%20primer%20blast%20waves.pdf

Taking a stab or guess about the cause of this would be variations of pressures for any of a number of reasons,

 

[Texas10]

New barrel....are you certain you had the Magnetospeed adjusted properly for correct gap? With too big of gap on a bull barrel, I got the same results you are getting.

 

[C.E.Smith043]

So I started load development on a new rifle. I will list specifics and then get into my problem.

Rifle:
Custom Action - GA Precision
Barrel - Hawk Hill 8" twist 22"

Cartridge:
.243 Improved - 30° shoulder
Berger Hybrids 105gr Moly
Lapua 243 case
Fed 210M
H4350

Reloading tools:
RCBS Chargemaster verified with a GemPro
Whidden custom bushing die
Redding competition seater

So with my normal routine I have been able to load very consistent loads. Very little runout and ES in the single digits. I have used this same lot of power, primers, and even bullets for my old rifle and cartridge to get very consistent loads. With this new rifle and cartridge my ES have been ridiculous. For a single charge I have gotten as much a 76FPS variance. With all things consistent with my loads what would be the reason for this. Can a barrel or cartridge just not like a powder that is known for low ES?

Does this barrel happen to have a muzzle break that the other gun didn't have? That could be giving the different readings?

 

[Alex Wheeler]

What is you actual neck clearance?

 

[gunmutt]

New barrel....are you certain you had the Magnetospeed adjusted properly for correct gap? With too big of gap on a bull barrel, I got the same results you are getting.

I think that is what I am seeing. I am using a different mount

 

[fyrewall]

Was there a velocity for the .243 AI shooting 105's? A guess of 3,100 fps and a velocity spread of 76 fps would be 2.5 %. Reading the info on the Magneto Speed Chrony , it works by using magnetic sensors that detect changes in the magnetic field that caused by passage of the bullet over the sensors that are spaced about .005 inches apart. My guess is that the Magneto Speed Chrony did just as it was expected to do because a failure to detect a change in the magnetic field caused by device positioning would result in a gross discrepancy - like a velocity of 300 or null reading.

 

[gunmutt]

Was there a velocity for the .243 AI shooting 105's? A guess of 3,100 fps and a velocity spread of 76 fps would be 2.5 %. Reading the info on the Magneto Speed Chrony , it works by using magnetic sensors that detect changes in the magnetic field that caused by passage of the bullet over the sensors that are spaced about .005 inches apart. My guess is that the Magneto Speed Chrony did just as it was expected to do because a failure to detect a change in the magnetic field caused by device positioning would result in a gross discrepancy - like a velocity of 300 or null reading.

You have a point there.