Loss of MV due to humidity

[gvanhyning]

I had some loads for my 6.5x47 Lapua that I had developed in Colorado where the humidity is relatively low and stable. I moved out to Virginia and was checking loads prior to a match and noted that my muzzle velocities were low by as much as 160 fps. The loads were still very accurate but had nowhere near the muzzle velocity I had when shooting in Colorado. While in Virginia I didn't have time to really explore the cause of the low velocities. I recently moved to Texas and began researching what the problem was. I did have some high extreme spreads in velocity and wanted to address that issue too. I began with going to stainless tumbling media to thoroughly clean the brass and then annealed everything. All was FL sized and trimmed and loaded. While I did lower the ES the velocities were still nowhere close to what I expected so I began doing some searches. I found one article that stated the powder can absorb moisture and increase in weight thus when loaded by weight, reduce the volume of powder. With that knowledge I went back to the drawing board and began load development all over again. Today I tested some loads and got the velocities back to within 45 fps of the original loads. Today's loads shot good and there were no signs of pressure with powder weights 1.2 grains over the maximum predicted with QuickLoad. I'm going to load up some more rounds and head back to the range again. One reason I bring this up is to help anyone else that may have the same experience as well as get feedback from folks who already have this Tshirt. Your thoughts and input would be appreciated.

 

[Drifter™]

... I found one article that stated the powder can absorb moisture and increase in weight thus when loaded by weight, reduce the volume of powder. ...

I was already thinking that before I read down that far. I've heard of some people purposely drying out powder (spread out on a cookie sheet during low humidity) in an effort to get consistent results. I just try to keep powder sealed and keep exposure times to a minimum.

 

[Jrb572]

What was the elevation in Colorado compared to Virginia ?

 

[gvanhyning]

Elevation in Colorado was 6000' and Virginia was 1100'.

 

[gvanhyning]

I was already thinking that before I read down that far. I've heard of some people purposely drying out powder (spread out on a cookie sheet during low humidity) in an effort to get consistent results. I just try to keep powder sealed and keep exposure times to a minimum.

I have thought about ordering some silicate gel packs and putting one in each powder container.

 

[Jrb572]

I'm not sure if that makes a difference on muzzel velocity being at different altitude.

 

[r1rn04]

Elevation in Colorado was 6000' and Virginia was 1100'.

the elevation change is the likely culprit, along with any temperature variation. Rember higher altitude = less dense air. I recently read an article at 8541tactical on altitude density calculation, was really interesting. might be wrong here but I think for every 1000' of elevation you lose approx 1" of mercury for barometric pressure. my Altitude here is about 2550' and the std pressure is about 27 inHg.

 

[67rschev]

All about altitude air densities . Home range is 5800 feet above sea level , but add in some temperature , and low humidity and I've seen corrected altitude air densities approaching 8300 feet on a hot day . Have had to make dope adjustments from the cool morning , shooting into the heat of the afternoon . Being a pilot can very hazardous up here in the heat of summer . People call me a liar when I tell them I can chuck a 168 SMK in the summer , averaging 2840 FPS with my match load out of a 22 inch barrel . Thin air helps

 

[waste_knot]

I feel kinda inadequate at 22 feet. The OP is talking about MV so density should not come into play.
I'd do a water absorption test by weighing out couple or three batches of say 45gr and put one in the freezer and leave one out in the open and se if there is a weight diff.

 

[gvanhyning]

Altitude is definitely a factor "down range" but I'm not inclined to believe altitude accounts for a loss of 160 fps with the chronograph 10' from the muzzle. The top load was 37.4 grains of IMR 8208XBR under a 123 grain Lapua Scenar. QuickLoad predicted pressure spike was over 66,000 psi which is roughly 3,000 psi over max pressure, yet I had zero signs of pressure, not even a flat primer.

 

[67rschev]

I feel kinda inadequate at 22 feet. The OP is talking about MV so density should not come into play.
I'd do a water absorption test by weighing out couple or three batches of say 45gr and put one in the freezer and leave one out in the open and se if there is a weight diff.

You are right , as I did not read the OP correctly . Thank you WasteKnot

 

[KYpatriot]

Whenever I am seeing MV discrepancies with my own reloads I ALWAYS suspect the chronograph first. There can be a lot of error in a chronograph reading, for a variety of reasons. You are not getting 160fps spreads based on humidity.

So are your velocities derived from actual dope? If so, have you track tested your scope so that you know the clicks are consistent and accurate across the range of adjustment?

If your velocities are measured, I would start there. Is the dope your need in VA consistent with a 160fps MV loss?

There just isnt enough info in your post to figure out the problem.

 

[SporterII]

Tag for later.

 

[Bob 964]

It is my understanding that, as humidity increases, air density decreases. This is because the molecular weight of dry air is greater than the molecular weight of water. The significance for us is that higher humidity creates less resistance, causing our bullets to fly more efficiently and with a higher POI. So... if humidity goes up, POI goes up; if humidity goes down, POI goes down. Everything I have read suggests that, of all the environmental conditions that affect the trajectory of a bullet after it leaves the muzzle, change in the humidity level generally has the least effect.

 

[gvanhyning]

Whenever I am seeing MV discrepancies with my own reloads I ALWAYS suspect the chronograph first. There can be a lot of error in a chronograph reading, for a variety of reasons. You are not getting 160fps spreads based on humidity.

So are your velocities derived from actual dope? If so, have you track tested your scope so that you know the clicks are consistent and accurate across the range of adjustment?

If your velocities are measured, I would start there. Is the dope your need in VA consistent with a 160fps MV loss?

There just isnt enough info in your post to figure out the problem.

I had suspected it's possibly the chronograph, which is a CED Millenium 2 with IR screens. I replaced/charged batteries and got some more shots with the same result, 160 fps low. All the shot velocities were low, it's not a spread. I then took actual shots to 980 yds and used the Shooter app Velocity Calibration feature to arrive at a velocity average which was 160 fps below the original load testing data, which had been verified out to 1020 yds.

I use QuickLoad as a tool in load development and it has always been reliable with it's predictions of pressure and velocities. In particular, IMR 8208XBR and the Lapua 123 Scenar, at 37.4 grains of powder it predicted pressure well above maximum and a velocity which I was still over 100 fps shy of attaining. With that in mind, the theory that moisture absorption by the powder increases the weight of the powder may explain how some folks are using loads that I once thought was just forum BS may in fact be true and attributable to this phenomenon.

 

[Fuzzball]

KYpatriot has your answer, a change in the temp of our cheep electronics changes the clock rates. Powder is not very hydroscobic and neither humidity nor altitude will matter very much when measuring muzzle velocity.

 

[jeffersonv]

The title of your thread says loss of MV due to humidity.

As Bob 964 said above, humid air is less dense.

There have been some good suggestions here. I would look at humidity last.