MV deviation between reloading sessions

[carter33]

I have been getting very different MV readings between reloading sessions. I use an RCBS chargemaster and calibrate prior to each use. The rifle that has most recently shown this is a 300 winmag with 196 hammer hunters. In my first session after loading some rounds I averaged 3020fps. I reloaded a second group and averaged 3080fps. Today, after reloading a new set of rounds, I averaged about 2980. This has all been with virgin norma brass ran through a mandrel/chamfered&deburred and CCI250 primers. My CBTO measurements have also been very consistent.

Each time shooting was a separate reload process. I don’t believe it’s my chronograph as I haven’t seen this with some unknown munitions rounds I have. And each batch has an extreme spread of no greater than 40.

My best guess right now is that my RCBS chargemaster is off each time I do a new batch. The next logical step is to get another scale to verify this. I had a secondary scale that broke in a recent move but will be getting another this week.

Has anyone else had an issue like this? Is their something else that could be causing this? If it is my chargemaster what should I replace it with? I have looked at the A&D setups but don’t really feel like spending that much cash but if theirs no middle ground I’m not completely opposed to it.

 

[WKR]

Maybe temperature and air density but probably your scale.

 

[wind gypsy]

They make cheap check weights you can use as well. My chargemaster has never wandered that much that I’m aware of but I’ve heard of folks having issues. I’ve checked it with beam scales, check weights, and lately an A&d 120.

What powder are you using, how have weather conditions changed, and has ammo baked in the sun on any trip more than others? Chrono perfectly parallel to path of bullet (what kind of chrono)? Powder and bullets from the same lot?

 

[sneaky]

What kind of chrono are you using? Find a different chrono and verify. Check one thing at a time. Then, you using a temp stable powder? Check one thing at a time

Sent from my SM-G955U using Tapatalk

 

[Tod osier]

You have the calibration weights sitting right there if you want to check it. Just weigh them in grains every time and record. If you want to check in the range of what you are throwing, cut a piece of copper wire to the length that gives you the approximate charge weight and weigh it every time.

 

[rayporter]

welcome to the real world.

over the course of a day you can watch that much change if you start in the morning and shoot till evening. it is so common that many benchrest shooters load for the day when they reach the range.

just last weekend my target velocity was 3360 and from friday evening to saturday morning i had to adjust .6 grains to keep that velocity. and as saturday wore on i had to lower my charge .5 to keep it at 3360.

 

[carter33]

They make cheap check weights you can use as well. My chargemaster has never wandered that much that I’m aware of but I’ve heard of folks having issues. I’ve checked it with beam scales, check weights, and lately an A&d 120.

What powder are you using, how have weather conditions changed, and has ammo baked in the sun on any trip more than others? Chrono perfectly parallel to path of bullet (what kind of chrono)? Powder and bullets from the same lot?

I am using H1000 and powder and bullets are from the same lot. Weather has changed pretty drastically between range trips. Probably 40 degrees or so but oddly the most recent trip when it was the warmest resulted in the slowest MV average. The ammo was never really exposed to high heat of the sun on any of the trips.

 

[carter33]

welcome to the real world.

over the course of a day you can watch that much change if you start in the morning and shoot till evening. it is so common that many benchrest shooters load for the day when they reach the range.

just last weekend my target velocity was 3360 and from friday evening to saturday morning i had to adjust .6 grains to keep that velocity. and as saturday wore on i had to lower my charge .5 to keep it at 3360.

That is interesting to hear. I would assume all these fantastic MV stats of single digit ES and such are all point in time strings then not data collected over multiple days. I will post back up when I get a new scale to bounce off the chargemaster. If the scale is accurate and my MV really does change that drastically from day to day and I can’t adjust for it I’ll have to really look at the numbers and consider limiting the range even more at which I feel comfortable shooting game.

 

[wind gypsy]

That is interesting to hear. I would assume all these fantastic MV stats of single digit ES and such are all point in time strings then not data collected over multiple days.

Yeah, most internet ES/SD talk is statistically insignificant one group numbers far as I can tell.

A lot of things happen as atmospheric conditions and bore conditions change.

 

[Rob5589]

I warm up my CM lite for 30 mins; wipe down with a used Bounce sheet; then calibrate. I drop a few charges and verify on my 10-10 beam scale. Extreme weather variances like you experienced can change things as well.

 

[rayporter]

yes it was one day. they called 'shooters to the line ' at 9 and we shot. 30 min later it was 'shooters to the line'.
and so it went for 10 targets. and we done it again the next day. practice Thursday and Friday allowed me to find the velocity that worked. the same velocity as last month. if i had not dropped the charge my velocity would have went over 3400, but i have no idea what the es/sd was.
a labradar was always on but i just watched velocity averages.

i keep a sliver of metal tucked in with my check weights on the CM to make a quick check as the day or days go on. the CM runs off a battery and i dont notice any change from a cold start to a warmed up machine, but i never noticed any difference when it was plugged up, either.

the Aand D is definitely an awesome unit.

a serious question, are you touching the table the CM is setting on while it is running? i know a fellow that sent in 3 CMs for repair and it was him resting his elbows on the table as the CM ran.

 

[carter33]

@rayporter

Yes I do touch the table quite a bit. I have another bench next to me that I will put it on this next go around that will isolate it better.

 

[Rob5589]

@rayporter

Yes I do touch the table quite a bit. I have another bench next to me that I will put it on this next go around that will isolate it better.

I don't touch my bench at all while it's throwing. You can breathe on it and induce variation.

 

[Elkdreamer]

I don't touch my bench at all while it's throwing. You can breathe on it and induce variation.

The CM shows weight thrown? Correct? You just have to look and pour back and re-throw to fix this issue?

 

[rayporter]

The CM shows weight thrown? Correct? You just have to look and pour back and re-throw to fix this issue?

you are basically correct except that the CM has a wide range of what it accepts at correct or accurate when you have a target weight.

movement even a slight tilt of the table if you rest an arm on it changes this.

try this,
lets say you are loading 45 gr . watch the unit trickle up to 44.9 and gently blow on the powder pan. my CM will shut off and read 45.0 gr. it will do it at 44,8 too.

i have been speed loading ammo with the CM recently by using 2 pans i cut and notched to make them the same wt .

as soon as one pan is full i take it off and set the other on and it fills as i put the powder from the first pan in a case. so with the cover off any air currents cause a shut off and i have to dump in a dish and restart that pan.

you have to watch the display carefully.

 

[rayporter]

here is a quote by frank green, barrel maker, made on rokslide,
here he is indicating environmental conditions change impact.
this is to show that there is a lot of variables out there that can cause problems, not just internal ballistics.

I've been in plenty of matches where you shoot several times thru out the day. Let's for example say 3 relays at a 1k yards. No cleaning the gun etc...as you basically have no time. You shoot the first relay at 9am and your second relay at 12pm. Temp and environment conditions change and even though it's the same ammo and same dope on your scope from 3 hours ago....and the bullet impacts differently!

 

[Mule3006Elk]

Assuming case control is perfect as well as bullet seating depth as well as ambient temperature when shot....powder weight would be my first thought.

I use RCBS deluxe scale check weights. Very helpful with my reloading consistency.

 

[prm]

Are you using a ball powder and varying the time the cartridge sits in the chamber? Leaving it in a hot chamber will increase velocity. The temp stable powders help with that. I agree a CM can be off a .1 here or there, but that won‘t result in that great of a change in velocity.

I was loading with my CM just yesterday and checked every load on the balance beam and they were all consistent.