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Air rifle 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?

Exact Heavy Diabolo air rifle pellets

JSB Exact Heavy Diabolo .177

Overall Rating: 91%

Price as reviewed: £10

Airgun Shooter awarded the JSB Exact Heavy Diabolo .177 pellets the Gold Award… Chris Wheeler explains why.

In Airgun Shooter  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 Exact Heavy Diabolo (4.52mm, 10.34 grains).

JSB Exact Heavy Diabolo .177

There’s no head expansion upon impact

JSB Exact Heavy Diabolo .177

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.  (Read our list of best airgun pellets.) 

Test conditions

 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. (Read how to pick the right pellets for your air rifle.) 

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 kill zone 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

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