Pellet Test: JSB Exact Heavy Diabolo .177

Chris Wheeler takes a look at the JSB Exact Heavy Diabolo in .177. Is it a match for its hard-hitting rivals?

We’ve awarded these pellets our Gold Award… Chris Wheeler explains why

Key Specs
Pellet name: JSB Exact Heavy Diabolo .177
Distributor: John Rothery Wholesale ( 
Type: Domed diabolo
Calibre Tested: .177 [4.52mm head size]
Advertised weight: 10.34gr
Average weight: 10.35gr
Use: Hunting and HFT
Supplied in: Tin of 500
SRP: £9.50

In Airgun Shooter 100 we looked at the performance of the Bisley Magnum (4.50mm, 10.65 grains), an established favourite at the heavier end of the .177 weight scale; so it seems only fair to investigate its nearest rival, the JSB Heavy (4.52mm, 10.34 grains).

There’s no head expansion upon impact

Since its establishment in 1991, JSB has forged a reputation for quality and consistency that is recognised by shooters worldwide. My tin of 502 Heavies arrived in excellent condition, free of any production crud and without a single unusable pellet. Fifty were weighed individually and in groups to discover weight variations, and they averaged 10.35gr. (See the weight table chart opposite.)

My tin arrived in excellent condition, free of any production crud and without a single unusable pellet


Weather at this time of year is very changeable, but I was lucky to have a day of fair conditions, offering a blustery breeze within shooting tolerances. As with the Bisley Magnum test, I used my Brocock Compatto and fed it several magazines’ worth of lead to bed it in and adjust POA. Making around 11.6 ft lb with the Heavies, it took only a few clicks of the scope turrets to find the mark.

This pellet made a 30mm wound channel

In order to make the most of the test conditions, I set a ‘windicator’ by my target, and also one on the shooting line to take shots only when the wind settled. The test protocol was 10 shots per target, aimed at the dead centre of the 20mm killzone at all ranges.

The Heavy is a well made pellet


During the 40-yard target test, the breeze was pressing left to right fairly consistently, but I kept my eyes on the wind flags and shot accordingly. The outcome of the next magazine was a surprise: the Heavies dropped by an average of 7mm more than the Magnums, clustering around 35mm low, with one high flyer. As the JSB Heavy is marginally lighter than its opposition, it should be expected to drop slightly less.

The Heavy is great for hunting and for HFT

More worryingly, the group was 20mm to the right of the vertical axis, so either that right-sided looping was exaggerated at the longer range or the breeze still caught them. A calmer day would settle this.

The JSB Heavy 4.52 is a well-made and competitively priced choice for the hunter wanting a little more heft to ensure a lethal outcome with vermin and small game. This test indicates that shooters must still exercise skill – and that the extra weight is no guarantee of a making a long shot.

Test results analysis

Verdict? 91/100

Quality: 15
Weight: 17
Muzzle Velocity: 18
Accuracy: 18
Suitability: 18

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