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What’s the ‘sweet spot’? Does it af­fect ac­cu­racy? Our guru knows

Dear Guru

I shoot an Air Arms S510 PCP ri­fle and I’ve heard peo­ple talk­ing about the sweet spot, but I don’t know what it means. What is it and how does it make a gun more ac­cu­rate? My gun will shoot ¾” groups at 35 yards all day long, but if I could make it bet­ter, then I want to.


Hello Phil Un­less your ri­fle has a reg­u­la­tor, the ve­loc­ity of your pel­let at the muz­zle will be af­fected by the pres­sure of the air in the reser­voir, and only the lat­est S510s have one fit­ted. The way most PCP ri­fles work is as fol­lows: As you cock them, you com­press the ham­mer spring so that when you press the trig­ger, the ham­mer flies for­ward and hits the knock- open valve in the end of the air reser­voir. This opens for a tiny frac­tion of a se­cond, al­low­ing air to flow through the ac­tion and up into the bar­rel to drive the pel­let. As quickly as it opens, the re­main­ing pres­sure in the reser­voir closes it again.

When the reser­voir is at its max­i­mum fill pres­sure, the ham­mer can’t open the valve to the max­i­mum, so the gun shoots a few per cent slower. As the reser­voir pres­sure re­duces, the force of the ham­mer can open the valve for longer and ve­loc­ity in­creases. Fi­nally, as the reser­voir pres­sure drops below the op­ti­mum amount, ve­loc­ity ta­pers off.

Pa­tiently shoot­ing your ri­fle through a chrono­graph, from max­i­mum to min­i­mum pres­sures, and log­ging the fig­ures will show this clearly. The sweet spot is the part of this curve where the dif­fer­ence be­tween the high­est and low­est ve­loc­ity is so small that it makes lit­tle dif­fer­ence to the tra­jec­tory. I ran that test on my Air Arms S410 and found that be­tween 180 and 150bar the ve­loc­ity change was very small and I got some 45 near- per­fect shots, which is more than I’ll ever need from a hunt­ing gun.

ABOVE: Learn­ing which fill pres­sure works best is an easy job

RIGHT: 45 pre­ci­sion shots is enough for any­body

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