• Brian

Birdshot for Distant Steel

A somewhat niche subject that spurred my interest: For shooting long steel in an action-style shotgun competition (3-Gun, Practical Shotgun), is there a benefit to using a larger or smaller shot size?


To test, I printed out 6" square targets and shot three different 1oz birdshot loads. Test shotgun was a Benelli M2 3-Gun model with a 21" barrel. Targets were shot with a MOD (.020" constriction) and CYL (no constriction) choke tube at 25 yards (a long birdshot poke). The intent was to determine which shot size had the best balance of pattern size to KO likelihood on steel - Muzzle velocity of all loads are not the same (which has a minor effect on pattern size), however all loads will be treated as having an MV of 1180fps (a 1oz 1180fps shell being my preferred speed-shooting birdshot).


Ammunition used:

- Winchester AA "Xtra-Lite Target: 1oz 7.5 shot

- Winchester Super X "Game Load": 1oz 6 shot

- Brian's Test Load: 1oz 4 shot


Testing Procedure


Each load was fired at 25 yards - usable pattern size was measured (the total pattern was not measured, only the area where there was sufficient density to expect a KO). Pellet strikes within the 6" target area counted and Power Factor calculated based on pellet weight multiplied by muzzle velocity, divided by 1000. I could not find any data or calculator to estimate data for the larger shot sizes. I was able to find an NRA Range Services paper on the ballistics of common target shells which indicated 1200fps load had slowed to 874fps at 20 yards, 742 fps at 30 yards - so let's call a 25 yard velocity 789fps (right in the middle). Let's also assume the larger pellets slow down at the same rate - they almost certainly do not, considering that they're heavier- but I have no data to determine how differently they'll perform.


CYL Testing:


7.5 Shot:

- Pattern Size: 18"

- 6" Square Hits: 47

- PF/MV: 71

- PF/DV: 47


6 Shot:

- Pattern Size: 18"

- 6" Square Hits: 29

- PF/MV: 68

-PF/DV: 45


4 Shot:

- Pattern Size: 17"

- 6" Square Hits: 12

-PF/MV: 46

- PF/DV: 31 MOD Testing:


7.5 Shot

- Pattern Size: 18"

- 6" Square Hits: 67

- PF/MV: 101

- PF/DV: 68


6 Shot

- Pattern Size: 15"

- 6" Square Hits: 71

- PF/MV: 167

- PF/MV: 112


4 Shot

- Pattern Size: 15"

- 6" Square Hits: 40

- PF/MV: 155

- PF/DV: 104


Conclusion:


It's hard to draw a hard and fast conclusion from this data as I still cannot account for the following variables:

- Velocity degradation of larger pellet sizes

- Whether that velocity degradation is relevant to knocking over plates

- What the actual minimum PF to knock over a 6" plate is


However, based on the data represented combined with anecdotal experience, I will say:


- If using a Cylinder choke, the 7.5 shot has the highest probability of knocking over a 25 yard plate - more hits = more momentum, the larger shot patterns just aren't dense enough to connect enough pellets on target.


- For certainty of a KO, some sort of choke is required at 25 yards.


- If using a choke, the larger shot sizes transfer significantly more momentum to the target as the pattern becomes slightly smaller, but more dense.


Let's assume the power factor standard to knock over a 6" plate is 100 - per USPSA/IPSC rules, the PF for a pepper popper is between 115-120 (using muzzle velocity for a 9mm projectile), KO plates are not subject to calibration but are easier to knock over. 100 PF allows for differences in plate thickness and base design. Based on the data gathered, none of these loads are sufficient to guarantee a KO at 25 yards with a Cylinder choke, though the 7.5 shot outperforms the larger two sizes. This reflects my experience attempting to knock over distant steel with insufficient choke - it just ain't gonna fall.


With a Modified choke, all three loads are sufficient to knock the plate over if using muzzle velocity to calculate power factor. The standard for calibration ammo is to go by velocity at the muzzle, however handgun bullets will not lose velocity at the same rate as a tiny birdshot pellet. If using the downrange velocity, 7.5 shot falls short while the 6 and 4 shot are both OK.

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I'll Shoot It Fast. 

I've been involved in practical shooting sports for most of my life. I've achieved the highest rankings in several shooting sports and applied that knowledge and skill to a wide variety of shooting-related topics. 

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