Like a lot of us in the shooting community I am a sponge and always looking to soak up more relevant information where ever I can. Phil and Caylan over at Modern Day Sniper are an excellent resource for learning and just keeping appraised of what’s going on in the precision rifle community.
Anyway…… One of the things they discussed early on in their podcast series was determining your “gun number”. Since then I have had quite a few conversations with people both online and in person who aren’t exactly sure what they meant by this terminology or how to calculate their own “gun number”. They generally just plug their details into their chosen ballistic app and it spits out a chart with elevation and windage data. Job done for most people. We aren’t most people. Well I assume you aren’t since you are reading my website.
None of this is voodoo black magic stuff. It is just plain and simple primary school math and looking for patterns. I will attempt to break it down for those of you who are more visual learners. If you still have questions after please feel free to comment below or reach out on socials. I am more than happy to help you on your precision rifle journey.
Determining Your “Gun Number”
First of all you need to have a zeroed rifle with verified data. This means your Velocity and BC for your chosen projectile need to be correct.
If you haven’t already get yourself a ballistic calculator for your smart phone. There are a bunch of free versions out there if you don’t want to pay as well.
Step 1: Load up your Ballistic Calculator and load your profile. For this Example I will be using the Applied Ballistics mobile app. For this example I’ll be using my AX308 with my 175gr SMK load going at 2720fps with the AB custom drag curve enabled for BC.
Make sure you turn off spin drift and coriolis effect if you have them present in whatever app you are using.
Step 2: Next you will need to manipulate your wind speed for a full value wind until it looks something like the table below. I recommend you set your table up to be in 25m increments out to 1000m or whatever range you have access too.
For this to work I concentrated on the 500m mark and manipulated my wind speed until the drift was 0.5mils. For this table it just happened to be 4mph. Now you will notice it doesn’t line up exactly for the chosen cartridge I was using; however, it is close and there is a pattern to it.
As a shooter on the clock at a PRS match do you think you could tell the difference between a 4mph wind and a 5mph wind at 600m? Probably not is my guess. I know I can’t. With the size of the targets using this method can be very forgiving too (if you have a nice match director).
Realistically for me and most likely you, your quick wind will look something like this in 100m increments using this gun and ammo combination.
Using This Information In A Practical Way: Now I know I have a 4mph gun but I have a target at 500m. But I have a 8mph wind. What do I do? Easy. Double the hold of course. So a 4mph wind at 500m is a 0.5mil hold. A 8mph wind at 500m must be 1.0mils.
Lets look at another example using the bigger table above
Target Range: 850m
Gun Number: 4mph at this distance = 1.0mils
Actual Wind Speed: 12mph
12 divided by 4 = 3
3 x 1.0mils = 3.0mils
Dial or hold your elevation and hold your wind of 3.0 mils at 850m for a 12mph wind and SEND IT!
Conclusion: Using this math your drift per 4mph should be about roughly your range divided by 1000 out to 700m when you just add an extra 0.1mil until 900m when you add and extra 0.2mil and 1000m you add 0.3mil. All pretty easy to remember on the fly or with a quick glance at an arm board during the match.
Just remember this is a Quick Wind method. If you have the time and are shooting at live game at extended distances take the time to get our your kestrel or other weather device and get the actual number. Nobody wants a wounded Sambar Deer you now have to track for hours through the bush.