A couple days ago RF posted a “Pro Tip” from Ammoland on measuring shot group size. I’m not a pro, but I took exception to some of the assertions in that write-up. After I did a little whining in the comments it was suggested that maybe this actually warranted a follow-up post. So… here she is. What was inaccurate in that Pro Tip piece and how can you measure your groups more easily, precisely, and quickly? Well first . . .
A Quick Primer:
Why measure center-to-center? Well, what we’re looking for is the largest spread between the precise points where the bullets impacted. We don’t want to count the diameter of the holes in that measurement, just the very center point of the holes. For instance, if you line up at 100 yards with your .50 BMG and you shoot 5 rounds precisely through the exact same hole, you have no spread. But, were you to measure edge-to-edge it would suggest you’re shooting 1/2 MOA (a half inch spread at 100 yards).
First and foremost, caliber designation cannot be used to determine bullet diameter as suggested in the Pro Tip:
In metric cartridges — actually, pretty much just non-U.S. calibers — the name is supposed to be the bore diameter of the barrel, not the diameter of the bullet. The bore diameter is the distance between the rifling lands, whereas the bullet diameter is typically the larger distance between the rifling grooves. For instance, 7.62×39 and 7.62x54R are actually a 7.92mm diameter projectile. .303 British (“standard/imperial” measurement but non-U.S. caliber) is a 0.311 inch bullet. 5.56×45 is actually a 5.7mm projectile. 10mm Auto is 10.17mm. 7.62×51 is 7.82mm. This confusion also came up in the 7N6 import ban thread, where folks said 5.45×39 should be exempt because it’s smaller than .22 caliber. Well, not really. Converting 5.45mm to inches doesn’t work, because the projectile is actually 5.60mm in diameter (0.22″). Now there are exceptions where the metric designation is actually the bullet diameter. 5.7×28, for example. 9×19 is close enough to call it rounded from 9.017. However, the exceptions are rare in this case. Much rarer than in standard calibers:
In standard calibers or, shall we say, U.S.-developed cartridges, the caliber designation is often the diameter of the bullet but could be based on chamber dimensions or could just be rounded at the whim of whoever developed the cartridge. Basically, you just can’t assume here. If you want to know the diameter of the bullet you’re shooting, you need to look it up. .357 is 0.357″, .308 is 0.308″. .44 special/magnum is 0.429″. .38 is 0.357″. .223 is actually 0.224″ and .22 LR is actually 0.223″. .380 is 0.355″. .300 BLK is 0.308″.
Second, much of the time none of this really matters at all anyway. The Pro Tip suggests subtracting the diameter of your projectile from the outside edges of the farthest apart bullet holes. Well who said the holes in the paper are actually the same as your bullet diameter? This is very often absolutely not the case. For instance, this photo (and the lead photo) is a 3-shot group I just shot with 147 grain 9mm FMJ:
Those holes are far from 9mm (or 9.017mm) in diameter! Naturally, today is the day I find my caliper dead and no spare button cells in sight, but these holes are actually coming in at about 4.3 to 4.5mm. If I measured from the left edge of one hole to the right edge of another hole and then subtracted 9mm from the total, I would come up with a group size that’s smaller than reality. If you want to measure hole outside edge to hole outside edge and subtract, you need to subtract the hole size not the bullet size. In the event that you should be so awesome as to shoot a group that is a single, ragged hole and you cannot tell how big an individual hole is, then “miss” on purpose somewhere else on the target and measure that for your hole diameter.
Alternative Measuring Methods:
If you don’t have a single-hole group, you can cut out all of the hole/bullet size estimation and the ensuing math by simply measuring from the inside edge of one hole to the outside edge of the other (using the two farthest-apart holes). This gets you exactly the same distance as center-to-center (which is the goal) and, assuming your math is correct, the exact same distance as outside edge to outside edge followed by subtracting “bullet diameter” from that result. The Pro Tip should really recommend this as the easiest, most preferable method in the vast majority of cases. At least when you have clean holes with clear edges. Like these “holes”:
As you can see in the pic above vs. the pic below, I can simply move the caliper over to line up in the centers of the holes also. If you’re dead-on centers and dead-on inside/outside edges, the measurement is identical.
Depending on target type, bullet type, and other variables you do not always get clean holes in the target, though. Case in point: my 9mm golf group pictured here. You’re already going to do a lot of estimating just picking the “edges” of the holes. So… my personal, non-pro “advice”: just measure center-to-center as best you can. In some cases eyeballing the center of the hole is every single bit as easy and accurate as trying to figure out where the edge actually is. Unless you’re shooting in a competition or something, getting to the closest like 1/16″ is plenty granular enough for load development or internet bragging or sighting in an optic or whatever you might be up to. On that thread, I don’t use a caliper when I’m out shooting. There’s a small tape measure in my range bag.
The exception to measuring inside edge of one hole to outside edge of another, or just going for center-to-center, is when your group is a single hole of any shape. One flyer and you can almost always go from the inside edge of that one to the outside edge of the ragged hole comprised by the rest of the shots. But if it’s truly a blob, then the Pro Tip applies. You have to measure outside to outside and then subtract… hole diameter, not bullet diameter. Assuming they’re different, which, again, you can figure out by shooting a single hole somewhere else on the target.
But you darn sure can’t figure out hole diameter by simply knowing the name of the caliber you’re shooting.
Oh yeah… the golf game target is from GunFun. Lots of games targets and other good stuff to be found there.