Velocity Changes Over Time

[Skigolfmike]

Back in Feb I got an 8 lb can of Varget that I have been loading with until last week. The original loads out of that lot were running 2900 fps with a 155 Scenar in Lapua brass with 45.4 gr of Varget and CCI 200 primers. By the end of that 8# can that load had slowed to 2800 fps and I worked up a new load of 46.0gr to get back up to 2860.

Well I used the last of the 1st can loading 50 rounds for this last weekend. I broke open a new 8# can and loaded 50 more rounds with everything identical. These rounds from the 2nd can ran at 2970.

Why would I lose 100 fps while loading from the same can of powder? The lid is always on and tight except to pour powder into the ChargeMaster and to drain the excess after a loading session back in the can. Could it be picking up moisture somehow? I thought it was weird.

 

[Break Bad]

Re: Velocity Changes Over Time

I managed a consistent 2850 with 155 scenars, lapua brass, and 47gr of varget through the life of an 8lbs jug. The fact you are getting 2900 with only 45.4 gr raises flags. Talked with a gentlemen at a tournament recently running 48.5'ish gr to get 2900 with scenars, not sure which brass.

Maybe you were jammed in the lands, causing a bit more tension/pressure, and as the chamber wore down, you loose tension. Recheck your oal to lands, 8lbs of powder is enough rounds to cause wear the chamber.

I doubt its the power if you are storing it in low-humidity / room temperature. I've heard of different lots of varget having different results, but not the same jug.

 

[Break Bad]

Re: Velocity Changes Over Time

I was going to ask if you annealed your brass every 2-3 firings, but if you didn't you would be gaining tension/pressure, someone correct me if I'm wrong here.

 

[Skigolfmike]

Re: Velocity Changes Over Time

I anneal every 1 - 2 firings.

Initially I was seating .010 in the lands. I now seat touching the lands to .001 off the lands.

My understanding is the highest neck tension is after anealing. The more the brass is work hardened, the less neck tension it has.

Initial chrono numbers with the 1st lot were 2860 fps, but the dope I got at TVP said 2910 on 45.4. That was using Shooter with the Litz G7 BC, tuned using the MV Tool in table mode to best match the dope.

I came up with the 2970 number running the Applied Balistics calculator with the custom curve for the 155 Scenar. I used dope on a 950 target with balistic calibration to get the MV. I san that the rest of the day and it was pretty accurate.

Powder is stored in the garage I'd like to move to the basement where it's better environmentally, but I don't have a bench I can use down there ATM.

 

[MALLARD]

Re: Velocity Changes Over Time

powder lot to lot variations could explain the change in velocity.

 

[Skigolfmike]

Re: Velocity Changes Over Time

OK, why did the MV drop 100 fps from the beginning of the 8# jug to the end?

 

[Break Bad]

Re: Velocity Changes Over Time

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: skigolfmike</div><div class="ubbcode-body">
Initially I was seating .010 in the lands. I now seat touching the lands to .001 off the lands.</div></div>

Frequently verify your OAL's? Maybe the adjustment ring on your die is loose and slips over time. Just taking stabs now...


<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MALLARD</div><div class="ubbcode-body">powder lot to lot variations could explain the change in velocity. </div></div>
Are you saying there can be 2 different lots mixed in the same 8lbs jug? I wouldn't think so...

 

[Skigolfmike]

Re: Velocity Changes Over Time

I check the ogive with a Hornaday bullet comparitor. At most, I see about .001 variance in ogive length.

I noticed a drop in velocity at the TVP sniper match. At the time, I thought it might be the brake we just put on the gun. However the last load ran even slower.

I originally loaded 175 SMKS, those still ran at 2710. Granted, they were loaded from the beginning of the 1st jug. The 175 Berger OTMs, loaded with poder from the top of the 2nd jug ran 2750 with the same exact load. These are loaded 2.800 OAL and about .020" off the lands.

I'm thinking the powder in Jug 1 either picked up moisture or got contaminated somehow for it to change that much. Also, I think Jug 2 is running a little hotter than Jug 1.

I run a RCBS Chargemaster to weigh my loads. I leave it on all the time and I calibrate it before each loading session. I have reprogrammed it to dispense a little faster and I have done the McD's straw mod. At the end of each loading session, I drain the powder back in the jug and cycle the unit to get all the powder out of the drop tube.

 

[Break Bad]

Re: Velocity Changes Over Time

Out of ideas other than possible contaminates during case prep/seating. Change anything about your reloading practices/equipment in that timeframe? New case lube? Maybe didn't rinse brass after sonic cleanng with something reactive?

 

[Skigolfmike]

Re: Velocity Changes Over Time

I'm thinking possible contamination. It's the only thing that makes sense.

I have run Trailboss through the Chargemaster when loading Sub-sonics, but I clean it out good afterwards. It can get pretty humid out in the garage; however, the jug is sealed up tight.

That said, this batch seems to be a little faster than the last one.

 

[Mike Sobr]

Re: Velocity Changes Over Time

That's a big difference I would be checking the chronograph or the way it is set up. If it's a Chrony brand there are a few tricks to getting them to repeat better. Just thinking out loud.
If your vertical is still tight all the more reason to doubt the chrono.

 

[Unknown]

Re: Velocity Changes Over Time

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: skigolfmike</div><div class="ubbcode-body">

I came up with the 2970 number running the Applied Balistics calculator with the custom curve for the 155 Scenar. I used dope on a 950 target with balistic calibration to get the MV. I san that the rest of the day and it was pretty accurate.

</div></div>

I wouldn't rely on numbers generated like that as there are too many variables that can change like humidity, wind to or from 6 or 12 o'clock, temperature, and most importantly a very slight incline (either up or down) to the target.

I had a 300 win mag with a velocity of 2960 over a chrono that was consistent. At the 1000 yard range, my dope showed that my velocity had changed to around 3200-3300 FPS for a 190 sierra Match King. It turned out that the range had a very slight uphill angle to the 1000 yard targets that accounted for the changes.

I would check velocity only over a chrono to make sure of the velocity, and make sure you are measuring the same way each time, and also record meteorological and environmental conditions during testing. Changes in temperature of the ammo, or the air can easily account for the changes in velocity you are getting. So can changes in humidity, altitude and other factors.

I think the best suggestion offered was throat erosion accounting for a drop in pressure, and velocity. If you started out with higher pressure, you would get more velocity. Over time, the throat erodes, thereby lowering pressure and velocity.

Changing the seating depth by going either closer, or further from the lands kinda throws all your factors screwy. Unless you hold the other variables constant, you will never find out which one of the factors made the change.

Sort of like if your rifle doesn't shoot well, and you change the scope, tighten bases, rings, glass bed the action, and free float the barrel, you will never know which factor had the biggest impact on your improvement in accuracy. If you only change one variable at a time, you can easily figure out which variable made the biggest difference.

 

[SPAK]

Re: Velocity Changes Over Time

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Unknown</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: skigolfmike</div><div class="ubbcode-body">

I came up with the 2970 number running the Applied Balistics calculator with the custom curve for the 155 Scenar. I used dope on a 950 target with balistic calibration to get the MV. I san that the rest of the day and it was pretty accurate.

</div></div>

I wouldn't rely on numbers generated like that as there are too many variables that can change like humidity, wind to or from 6 or 12 o'clock, temperature, and most importantly a very slight incline (either up or down) to the target.

I had a 300 win mag with a velocity of 2960 over a chrono that was consistent. At the 1000 yard range, my dope showed that my velocity had changed to around 3200-3300 FPS for a 190 sierra Match King. It turned out that the range had a very slight uphill angle to the 1000 yard targets that accounted for the changes.

I would check velocity only over a chrono to make sure of the velocity, and make sure you are measuring the same way each time, and also record meteorological and environmental conditions during testing. Changes in temperature of the ammo, or the air can easily account for the changes in velocity you are getting. So can changes in humidity, altitude and other factors.

I think the best suggestion offered was throat erosion accounting for a drop in pressure, and velocity. If you started out with higher pressure, you would get more velocity. Over time, the throat erodes, thereby lowering pressure and velocity.

Changing the seating depth by going either closer, or further from the lands kinda throws all your factors screwy. Unless you hold the other variables constant, you will never find out which one of the factors made the change.

Sort of like if your rifle doesn't shoot well, and you change the scope, tighten bases, rings, glass bed the action, and free float the barrel, you will never know which factor had the biggest impact on your improvement in accuracy. If you only change one variable at a time, you can easily figure out which variable made the biggest difference.

</div></div>

Agree with Unk.... There's too many variables that can affect your down range results to try and calculate your muzzle velocity from your drop based on a single outing.

That being said, powder will gain and lose moisture over time based on the humidity. If you're outside your optimal charge weight, temperature swings have more of an effect on your velocity. Granted Varget should be more temp stable than some of the other powders but that's not a guarantee.

What really needs to be done is shoot over a reliable chrony and get your speed. Go over your system again and make sure everything is tight and squared away.

 

[6.5x47]

Re: Velocity Changes Over Time

I lost 30 fps with throat wear over about 600 rds (round 300 to 900) with H4350. After about round 900, throat wear has ceased and velocity has stabilized. This in my 6.5x47.

If your chrono is not set up exactly the same distance from the muzzle, each time you shoot over it, it WILL give you higher/lower velocity.

Living in Ohio, you live in a humid state. Even if you keep it tightly sealed, it can gain a bit of moisture weight with each opening. A heavier powder charge will actually be less in volume. Meaning the bulk density per cubic centimeter will increase in a humid environment. If unclear, read the section of Glen Zediker's book, "Handloading for Competition", that deals with throwing vs weighing. It's the volume/amount of powder, not the weight, that determines the pressure created for any given amount of propellant. When I open an 8lb jug of H4350/Varget/etc, I decant it to a 1lb container and use the 1lb container to pour in to my powder hopper. This way I'm opening the 8lb container fewer times than the 1lb bottle and thus slowing the amount of moisture increase/decrease with the 8lb'er.

Changing your seating depth, even though the powder charge stays the same, will increase/decrease velocity.

Basically, I'd say it's not any one of the above mentioned items that contributed to your 100 fps loss, but an accumulation of the bunch.

6.5x47