The Effects Of Crown Damage On Accuracy And Point Of Aim

After we got some surprising results of doing a similar test to this on a 22 rim fire rifle, we decided to try it with a 308 Winchester. We took a brand new BRNO CZ550 rifle, installed a Nikon Monarch 6.5-20 scope on it. We decided to shoot 5 5-shot groups, then damage the crown with a hammer - to simulate something that one might do on a rock. Shoot another 5 5-shot groups. Then cut the barrel with a hacksaw, use a knife to deburr it, and shoot it some more.

The ammo we used for our test

Measurement of the above groups with the rifles as it came from the factory

1. 2.246

2. 2.342

3. 0.847

4. 1.327

5. 1.010

From top left. The first target shows a 5 shot group in the middle, with two shots one above this group and one below. These two were fired to adjust the scope - the first one went too high, the second too low. We fired the first groups on the same target. A total of 5 5-shot groups were fired, as shown above.

A hammer was used to hit the muzzle, as you can see by the flattened side, at the 3 o"clock position.  The photo also shows the copper fouling sticking to the metal pushed into the bore - with s light depression probably caused by the displaced metal.

We fired a shot after the hammer damage, aiming for target number 6. The bullet went about 18 inches to the left, and about 6 inches low - landing between target 3 and 4 as you can see in the photo above. We placed an additional target half way between target 6 and 9, and used that as our aiming point. The groups formed is shown on target 7.

Results of groups shot after the damage

1. 3.299

2. 2.253

3. 2.181

4. 1.529

5. 2.019

The measuring tape shows the distance and direction of the change of our point of impact.

We used a carbide deburring tool to clean up the damage caused by the hammer blow.

The point of impact moved back to where it was before the damage was caused - that is 18 inches to the right, and about 4 inches high.

Results after the damage was repaired with a carbide de-burring tool

1. 3.097

2. 1.421

3. 1.852

4. 0.852

5. 2.608

We fired one group without doing anything to the barrel as seen above.

This group was 2.823

We then used a knife to remove the burrs at the end of the muzzle, and fired five 5-shot groups

1. 1.097

2. 1.766

3. 0.879

4. 0.592

5. 2.647

We then tried a box of Norma's 168 Match ammo, and got the following results

1. 0.758

2. 1.960

3. 1.301

4. 0.941

It seems if the crown of a barrel gets damaged in the field, and a hacksaw is available, one can chop the end off and continue his hunt