With competition season in full swing, and the fact we had a mechanical offset stage at the Sniper’s Hide Cup. I thought we should revisit the concept of clearing the muzzle when shooting through a port or over an objective like a car or barricade.
Mechanical offset refers to the height of the scope over the barrel. Inside 100 yards, this offset determines the hold for shots that are presented and fall inside the shooter’s zero range. Also it matters when it comes to clearing obstacles in front of the barrel.
This value is also used with ballistic computers. It’s referred to Sight Height or Bore Height depending on the program. It is measured from the center of the scope tube or objective to the center of the bore.
Often, the shooter will see a clear sight picture in the scope and then have the bullet impact the obstacle in front of them because they failed to account for the mechanical offset. Even in the field this can be an issue with rocks or trees depending on your shooting position.
During the Sniper’s Hide Cup, our USMC Scout Sniper Stage honoring Sgt. Robinson saw 3 shooters impact the barricade because they failed to account for the mechanical offset. This surely resulted in a miss. The bullet passing through the wood will not fly true the remaining 600 yards if you give it two layers of plywood to penetrate first.
For the AR15 crowd in a CQB context, this means that aiming at the threat dead on will result in the round hitting low. At distances inside a room, mechanical offset is a major factor. You need to aim about 2.5” high in order to hit in the desired location.
During competitions you’ll often find shots inside 100 yards, or shots that require you to clear a hold or barricade. Knowing your dope inside 100 yards is a key factor if you want to be successful at these stages.
The best way I know to determine your inside 100 yard dope is to first run the numbers in a ballistic calculator like JBM and then to go out and confirm this data on the range. Below I have included a chart made up in JBM for this very purpose. I set the range increments to 2 yards, and then set the minimum range to 0 and the maximum range to 100 yards. Any more and you’ll have a very long dope table to work with. One note, because the rifles 100 yard zero will work from about 78 yards to 110 yards, you can further reduce the range from 100 to 75, but for demonstration purposes I have used 100.
The final factor to discuss is your scope. Not all scopes will zero down that far. Some will require you to power the magnification down a significant amount. So instead of trying to use your scope 25x you may have to drop to 10x. You’re still multiplying the view by 10x, you just can’t zoom in 25x. There are various scopes that double as air rifle scopes and will focus inside 10 yards, but you have to check with the specs. The S&B 5-25x PMII has a very close focus as it was designed as a cross over scope. But still a little less power will go a long way.
It’s important to practice the things we are not good at, or don’t find common place. If all you do is shoot prone, or groups at 100 yards, you are missing out on several big components to driving the rifle effectively. If you compete in a PRS type competition, you’ll certainly see this again.