The reason this happens is well known. Because this is why we free float our barrels in the first place. Pressure on the rifle’s barrel, especially on a longer barrel, actually flexes the barrel and changes its point of impact. But I was very surprised at how much it influenced my shot at such a short distance.
When resting my rifle’s free floated rail against the barricade at just 50 yards my shots were perfectly centered on target. However when I rested the rifle’s barrel on the barricade my shots were consistently 10 inches higher and a few inches to the right due to the fact that I was also pulling pressure to the left with my support hand to stabilize the rifle. The shots were surprisingly consistent and my group was only a tad bit larger than my original. But a 10-inch shift at only 50 yards is once again a very significant change in point of impact and can mean the difference between life or death in a defensive scenario.
Since I was shooting a suppressed rifle, I decided to test out a nearly stock AR15 in the same scenario. I know that rifles have a point of impact shift when using suppressors so I wanted to eliminate that variable. I also wanted to use an AR15 with a standard handguard as opposed to the free floated rail that I have on my primary rifle. I figured there were two possibilities: first, the results would mimic the earlier tests. Or second, the results wouldn’t be as drastic because perhaps the suppressor POI shift or because that a factory handguard mounted to the barrel might give more support.
The rifle I used in the second test was a lightweight Bushmaster Carbon 15 that I keep around for my wife. While I’ve seen these composite receivers crack, I figure the overall lightweight helps my wife for short training sessions. So duplicating the exact test from earlier, I had the exact same results as the previous rifle, with over a 10” point of impact change when fired from 50 yards.
The moral of this story is to not rest your barrel on anything when shooting. This is why we use free floated stocks and handguards that do not contact the barrel in precision and competition shooting. I knew the theory was sound, but I figured at close distances on a moderate sized target it wouldn’t matter. It does. Know your equipment, train hard, and shoot straight.