Range Report Ambient Temperature and Muzzle Velocity

[gstaylorg]

I had planned to determine and compare muzzle velocities for a number of different factory .308 loads over the weekend. Specifically, I wanted to compare MVs out of two different barrels that differ by 8" in length. Earlier in the week, the high was predicted to be in the upper 40's to low 50's, but that has now changed to a high in the upper teens to low 20's.

Obviously decreased temp can lower muzzle velocity, the question is how much? I have searched and found numbers for changes in MV per degree F that cover quite a wide range. So I thought I would ask if anyone here might have some personal data on decrease in MV as ambient temp. decreases for .308 rounds they'd be willing to share.

Even if the MV values I could expect might turn out to be of limited value (low) for preparing drop tables for use on warmer days, the trip would be worth it to me if the <span style="font-style: italic">relative differences</span> in MV between the two different length barrels (for the various loads) was even remotely comparable from low to warmer temps. Put another way, if the difference in MV from these two different length barrels was 200-250 fps at 70 degrees, would the difference be close to that value at 25 degrees, even though both MVs were lower?

Realistically, am I wasting my time and ammo trying to do this test when the temp is in the 20's? Thanks for any feedback you might have.

 

[Unsichtbar]

Re: Ambient Temperature and Muzzle Velocity

Link is to a temp. chart that was done over at practical rifle forum by Dan Newberry, works well for alot of powders http://practicalrifler.6.forumer.com/viewtopic.php?t=883

My chart for RL15 shows about .7 ft per degree increase heat or decrease for cold from 75F. Some powders more sensitive than others, I now shoot 8208xbr behind 155L. Most stable powder I have ever shot, they say it is stable over 165 degree range, its proved very stable, more than Varget

 

[desertHK]

Re: Ambient Temperature and Muzzle Velocity

I don't think it's a waste of time to try doing that. In fact, I have done it for my rifles. The point being that extreme temperature will affect the MV, even with the so call temperature insensitive powder.

Let just hypothetically speaking, and say that you have a temperature change of say 2 fps per degree temperature change, and you are going to engage a target within 500 yards, depending on how small your target, but you are more likely to hit your so call tactical MOA target size, with the regular ballistic data at that distance without worrying much about taking into account of the velocity difference due to temperature. Now, take the same temperature change and try to engage a target out at 1K or beyond, I assure you, your dope is going to be short. So, knowing the MV at different temperature will permit one to obtain the slope of the temperature curve to arrive at the delta V/degree. Factoring this into your ballistic software, with program permitting, will allow you to have a better chance of having a first shot hit at longer distances.

 

[Lindy]

Re: Ambient Temperature and Muzzle Velocity

Longer distances are the key. Variations of muzzle velocity with temperature aren't going to affect the POI much inside 600 yards, which a little experimentation with a ballistic program will quickly demonstrate.

A ballistic program is a great tool for determining the sensitivity of downrange results to variations in the things which affect it.

 

[gstaylorg]

Re: Ambient Temperature and Muzzle Velocity

Thanks a bunch everyone for the replies, they're very helpful. I will definitely get out.

To elaborate on one of the questions a little more, this is the first time out with this system (DTA SRS Covert). I have both the Covert 16" barrel/conversion kit and the 22" barrel that is standard on the full-sized SRS. What I had hoped to do on the first range trip was to get an idea of how much the MV will decrease using several different factory loads (8 different loads ranging from 2600 fps to 2900 fps factory MV values) going from the 22" barrel to the 16". My plan is to then use this info to select a couple that are best suited to each barrel, then use that info to generate some preliminary data so that I have something to work with to get dialed in at 600 and further.

I'm not so concerned about change in POI, as Lindy pointed out, at shorter distances it should be minimally affected. I was mainly wondering if I should expect the decrease in MV due to lower temp to remain constant for both barrel lengths, so that the difference between the two due to length would be about the same regardless of the lower ambient temp. Based on your replies, I'm happy to get out and simply find out by testing it. The primary reason I asked is that I will likely go through several hundred dollars worth of ammo doing these tests, if the resultant numbers were going to be useless, why waste the $$$? Again, thanks for the input, I appreciate it.

 

[TimK]

Re: Ambient Temperature and Muzzle Velocity

Why not just keep the ammo in an inside pocket until just before firing?

 

[gstaylorg]

Re: Ambient Temperature and Muzzle Velocity

That's a great suggestion and I've thought about how to do that. I'll probably keep it in a cooler with a warm water bottle and leave the bulk of it in my truck (I'll probably use about 20 boxes for what I want to get accomplished) and pull it out as I need it. Thanks.