Range Report Question about honest group size

[MJY65]

I know that the two typical ways to measure group size are center to center on the widest holes (finding exact center can be a challenge when the holes overlap) or edge to edge and subtracting one bullet diameter.

I've used both and try to be as accurate as possible. I use a draftsman's circle template on the group to make sure I'm really including all of the edges of the holes. The problem I see is with subtracting a bullet diameter. I typically shoot .308 Win. On standard paper targets, the hole left behind is not really .308". If I measure a single bullet hole right to the edges of the gray streaks, it's about .250. Are we really giving ourselves credit for .05 that we don't deserve when using the edge to edge method?

 

[walley2960]

Yes. .250 hole then subtract that from your out to out measurement.

 

[kraigWY]

I'm different if I'm shooting for groups (which I don't often do).

Most people shoot a series of groups, say 10 5 shot groups, they take the smallest group and say their rifle shoots X groups, (X being the smallest group).

Me, if I shoot a series of groups, using the 10 5 shot groups, I take the largest group and use that to measure my rifle's ability.

What would be better then 10 5 shot groups would be "1" fifty shot group and use that as a true measure. If I had to shoot something, I want to base my chances of a hit on the worse case scenario not the best.

As a side not that 50 shot group will give you a more accurate zero. Lets say you are sighting in your gun. Shoot 50 rounds on a target. Then draw a line from 12 go 6 o'clock and another line from 6-9 o'clock. This gives you 4 quarters. Count the shots in each quarter and see what you have. Then adjust your sights until you get an equal number of shots in each quarter. Now you have a true zero, regardless of the size of the group.

The size of this 50 shot group will also give you an idea of the range of you, your rifle, and ammo is capable of. If you can't keep all the shots in a 10 inch circle at 500 yards then your gun/ammo/shooter combination is NOT a 500 yards gun if you're using 10 inch targets.

I stress this when I teach LE sniper classes. LE snipers are different then military. They have to take head shots. The vital area of a human head is 4 inches. Meaning you are limited to the range that keeps your shot in 4 inch groups. So that is the limit of your shot. You can't count on the smallest group, you have to plan on the largest group, the life of the hostage is at stake. A true 2 MOA rifle/ammo/shooter group would be limited to 200 yards, which is well with in the range of urban LE sniper shots.

What happens if you're taking a 300 yard shot? Lets say you think you're group is a 1 moa rifle because you're basing this on your best group, but your critical shot is actually from your 2 MOA group......OOOPS dead hostage. The worse words a LE sniper can hear from his spotter is "OH SHIT".

 

[Just Macca]

I'm different if I'm shooting for groups (which I don't often do).

Most people shoot a series of groups, say 10 5 shot groups, they take the smallest group and say their rifle shoots X groups, (X being the smallest group).

Me, if I shoot a series of groups, using the 10 5 shot groups, I take the largest group and use that to measure my rifle's ability.

What would be better then 10 5 shot groups would be "1" fifty shot group and use that as a true measure. If I had to shoot something, I want to base my chances of a hit on the worse case scenario not the best.

As a side not that 50 shot group will give you a more accurate zero. Lets say you are sighting in your gun. Shoot 50 rounds on a target. Then draw a line from 12 go 6 o'clock and another line from 6-9 o'clock. This gives you 4 quarters. Count the shots in each quarter and see what you have. Then adjust your sights until you get an equal number of shots in each quarter. Now you have a true zero, regardless of the size of the group.

The size of this 50 shot group will also give you an idea of the range of you, your rifle, and ammo is capable of. If you can't keep all the shots in a 10 inch circle at 500 yards then your gun/ammo/shooter combination is NOT a 500 yards gun if you're using 10 inch targets.

I stress this when I teach LE sniper classes. LE snipers are different then military. They have to take head shots. The vital area of a human head is 4 inches. Meaning you are limited to the range that keeps your shot in 4 inch groups. So that is the limit of your shot. You can't count on the smallest group, you have to plan on the largest group, the life of the hostage is at stake. A true 2 MOA rifle/ammo/shooter group would be limited to 200 yards, which is well with in the range of urban LE sniper shots.

What happens if you're taking a 300 yard shot? Lets say you think you're group is a 1 moa rifle because you're basing this on your best group, but your critical shot is actually from your 2 MOA group......OOOPS dead hostage. The worse words a LE sniper can hear from his spotter is "OH SHIT".

Kraig,
In your experience; what's the most accurate LE rifle/shooter combo you've seen perform consistently well? Are we talking a shooter and rifle combo who shoots 1" groups reliably? Or better? I only ask as it is something I want to do one day, and a benchmark would be interesting to establish.
Cheers for your time, Lachlan.

 

[kraigWY]

I'm going to get flamed for this. But in Civilian LE a 2 MOA gun will work quite well. Good for 200 yards and the average LE sniper/counter sniper shot is 80 yards or less.

There are exceptions but they are as rare as hens teeth. I did make a shot on a dog once that was measured at 237 yards. This was the exception. If I missed my shot would hit the bank in a gravel pit, not a hostage. The down side is if I would have missed, a little boy would have to gone through some painful rabies shots.

What happen was this dog bit a kid. After a couple days of not being able to corner the dog, the doctor said if the dog wasn't captured by mid night, he would have to start a series of the rabies shots.

The Patrol LT called and told me to go get the dog. As luck would have it as I pulled up in the area and asked dispatch where the last place the dog was seen, I saw the dog running along the bank across a large gravel pit. I didn't have time to get out of the car. I shut off the engine, stuck the rifle out the window and shot just before it hit the wood line. I got the dog and saved the kid from the shots (it wasn't rabid). This was a no lose situation for me. I'd have never taken such a shot if a hostage or bi-standers were around.

When I say a two MOA rifle, I'm not talking bench rest. I'm talking practical, shooting from the window as above, shooting over the hood of a car, shooting kneeling unsupported. People practice from the bench. Never seen a LE sniper pack a bench around. Sure there are times you may have the time to get a good solid position using a rest. In such cases you can get good groups. But then, more often you only have time to jump from the car, throw the rifle over the hood, or maybe not even get out of the car. The groups are going to be different but that's the groups you have to count on.

I only ask as it is something I want to do one day

PM me with your e-mail address and I'll send you a .pdf copy of the USAMU LE Sniper/Counter Sniper Guide. Its old but the most honest, realistic material you'll find on the subject.

Most dept LE snipers are not SWAT 'n such. Most are smaller departments with now SWAT at all, and the sniper/counter sniper is a additional duty, (as mine was). The guide is for them, it presents realistic data not the SWAT type para-military stuff you find on the internet.

 

[Just Macca]

Cheers Kraig,
I'll get that email to you now.
Regards, Lachlan.

 

[bluto51]

Hmmmmm. Using electronic calipers, I just measured about a dozen .308 bullet holes in several different groups and most of them came out to .308 at the edges of the
grease rings. On some irregular holes I could "fudge" the calipers up to .320 or down to .299 but I couldn't get anywhere near .250! Most could only be "fudged"
by one or two thousandths. It seems safe to say that careful measurement of group diameters will bring you to within a couple of thousandths give or take. If you're
interested, there are a couple of computer programs that will probably reduce the guesswork even more.

 

[Aimsmall55]

I just measure the outside diameter with a pair of calipers and subtract .264 or .308" for example.

Like this:

 

[RobertB]

Hmmmmm. Using electronic calipers, I just measured about a dozen .308 bullet holes in several different groups and most of them came out to .308 at the edges of the
grease rings. On some irregular holes I could "fudge" the calipers up to .320 or down to .299 but I couldn't get anywhere near .250! Most could only be "fudged"
by one or two thousandths. It seems safe to say that careful measurement of group diameters will bring you to within a couple of thousandths give or take. If you're
interested, there are a couple of computer programs that will probably reduce the guesswork even more.

I imagine the stock of paper you shoot through would make a difference. BR matches doesn't use thin paper and this being one of the reasons. Wind is another.

 

[damoncali]

Subtract the actual hole size from edge to edge. Your goal is to measure center to center of the actual holes. That said, unless you're shooting benchrest competitions, this is way more precision than is necessary. The only reason it matters for benchrest is because they're not measuring rifle accuracy. They're measuring actual groups. There's a subtle difference there. Group size is a discreet value (my group measured .502"). Rifle accuracy is more of a probability ("my rifle will shoot .5 MOA with 5 shots 95% of the time"). If you want to know how accurate your rifle is, measurements to .001" are going to get lost in the uncertainty due to a generally too-small sample size. If you're measuring groups, you can get pretty damn close to +/-.001".

Pro tip:
If your groups are big enough, you can measure from outside edge to inside edge and not subtract anything. That won't work if the inside edge has been mangled by other bullet holes, though.