De­signer Pel­lets

Jim stud­ies pellet de­sign to ex­plain why they be­have the way they do in dif­fer­ent air ri­fles

Airgun World - - Contents -

Is there a sim­ple an­swer to the muz­zle en­ergy/ pellet weight ques­tion? Ask Jim Tyler

The big prob­lem with try­ing to make sense of the work­ing of dif­fer­ent types of air­guns, and es­pe­cially in fault find­ing, is that every­one is look­ing for sim­ple an­swers to very com­plex is­sues. A prime ex­am­ple is cold weather pellet point of im­pact (POI) shift, which many seem to as­cribe to the grease on the spring and ham­mer of a PCP, or the spring, piston and seal of a springer, thick­en­ing and rob­bing the ham­mer or piston of en­ergy. Sim­ple, easy for any­one to un­der­stand, and wrong. The in­creased vis­cos­ity of cold grease will un­doubt­edly have a small ef­fect on the air ri­fle, but what ef­fect there is will be dwarfed in both the PCP and springer, and for dif­fer­ent rea­sons.

The po­ten­tial en­ergy (to drive the pellet) in a PCP’s air de­pends on its pres­sure and vol­ume, and its tem­per­a­ture. For any given pres­sure and vol­ume, higher tem­per­a­ture means higher muz­zle en­ergy, and vice versa, and that will move the pellet POI; down in win­ter, up in sum­mer.

The spring ri­fle is the com­plete op­po­site, and the POI is most likely to rise in win­ter tem­per­a­tures, fall in sum­mer, which it does be­cause the piston seal shrinks in cold con­di­tions, swells in hot, hugely af­fect­ing the fric­tion be­tween it and the cylin­der wall, not only chang­ing the muz­zle ve­loc­ity but, in the worst cases, the point in the re­coil cy­cle that the pellet ex­its the muz­zle, which can cause huge changes in POI, of­ten re­ported as in the re­gion of an inch at 30 yards.

“… The re­la­tion­ship be­tween pel­lets and spring air ri­fles truly is fiendishly com­pli­cated …”

Pellet POI shift is but one ex­am­ple of why a sim­ple ‘one size fits all’’ so­lu­tion rarely ap­plies to any as­pect of the work­ings to any air gun

PCPS AND PEL­LETS

It is widely ac­cepted wis­dom that PCPs achieve higher muz­zle en­er­gies with heav­ier pel­lets, and that the rea­son is that the heav­ier pel­lets travel more slowly up the bar­rel, and have more time to gain en­ergy from the air. Un­for­tu­nately. It isn’t quite that sim­ple, as some lighter pel­lets can give higher muz­zle en­er­gies than heav­ier ones.

We can, how­ever, qual­ify the state­ment that PCPs achieve higher muz­zle en­ergy with heav­ier pel­lets by pre­ced­ing it with ‘All other things be­ing equal’. That means pel­lets hav­ing the same head and skirt di­am­e­ters, the same skirt thick­ness, and the same hard­ness, be­cause all of these (which com­bine to dic­tate pellet start pres­sure) can af­fect muz­zle en­ergy ev­ery bit as much as pellet weight.

I tested a range of pel­lets through my S510, the re­sults of which can be seen in Ta­ble One.

IN­FLU­ENCES

Ar­ranged by pellet weight, it’s clear that the muz­zle en­ergy is in­flu­enced by far more than mere weight, and we find the same if we ar­range the pel­lets by start pres­sures, size, hard­ness or any other sin­gle char­ac­ter­is­tic, be­cause the muz­zle en­ergy de­pends on all the char­ac­ter­is­tics, and not all of them al­ways as­sum­ing the same de­gree of im­por­tance.

So, how do we read those fig­ures? The light­est pellet does in­deed give the low­est muz­zle en­ergy, and the heav­i­est the high­est muz­zle en­ergy, but in be­tween the two ex­tremes it’s a jumble, so let’s try to make some sense of it.

If we sep­a­rate out the three low start pres­sure pel­lets (Air Arms Field and Ex­press, and the Fal­con Ac­cu­racy Plus), then we find that their muz­zle en­ergy and mass do line up. If we sep­a­rate out the two medium weight, high start pres­sure pel­lets (Su­per­dome and H&N Field Tar­get Tro­phy), then their mass and muz­zle en­ergy fig­ures also line up, and the same goes for the two heavy­weight high start pres­sure pel­lets (Su­per­mag and Bis­ley Mag­num).

All other things be­ing equal’, heav­ier pel­lets do in­deed give higher muz­zle en­er­gies in PCPs.

SPRINGERS AND PEL­LETS

If the re­la­tion­ship be­tween PCPs and pel­lets is com­plex, that be­tween springers and PCPs is fiendishly so. In the 2017 Sum­mer edi­tion of Air­gun World, I de­scribed a se­ries of ex­per­i­ments in which I swapped the ‘power plants’ (cylin­der, piston, spring, guides and preload wash­ers be­tween my TX200 and TX200HC, in an ef­fort to discover how much en­ergy pel­lets gained in the ex­tra 3” of bar­rel of the full length ri­fle, and un­ex­pect­edly found that not only did each pellet have its own in­di­vid­ual ac­cel­er­a­tion pro­file, but that each of the strokes of the two ri­fles (96mm and 85mm) also had their own pellet ac­cel­er­a­tion pro­files. The re­la­tion­ship be­tween pel­lets and spring air ri­fles truly is fiendishly com­pli­cated.

In an at­tempt to sim­plify the re­la­tion­ship be­tween pellet and springer, many be­lieve that lighter pel­lets tend to give higher muz­zle en­er­gies but, even more than in the case of the PCP, that des­per­ately needs qual­i­fy­ing with the pref­ace ‘All other things be­ing equal’.

In an ef­fort to throw some light onto the re­la­tion­ship be­tween the springer and pellet, I recorded the shot cy­cles of my .177 TX200 with nine pel­lets, and the re­sults can be seen in Ta­ble Two.

START PRES­SURE?

Un­like my re­sults with the PCP, the light­est pellet does not give the high­est muz­zle en­ergy, nor the heav­i­est the low­est, and in fact the only thing in com­mon be­tween the PCP and springer re­sults is that the pel­lets be­tween the two ex­tremes of weight are a jumble! Clearly fac­tors other than pellet weight are at work, and the prime sus­pect is pellet start pres­sure.

In order to make sense of the re­sults, I ar­ranged them by muz­zle en­ergy, and cre­ated the bar chart ‘Chart One’.

Now, per­haps, it all be­gins to make sense. Of the four high­est muz­zle en­ergy pel­lets, three (AA Ex­press and Field, and Fal­con Ac­cu­racy Plus) have low start pres­sures, so the piston ac­cel­er­ates them in the bar­rel for longer. The fly in the oint­ment of the four high­est en­ergy pel­lets is the Su­per-H-Point, for which I don’t have a start pres­sure, but a close ex­am­i­na­tion of its skirt re­veals that it is markedly thin­ner than that of its sta­ble­mates the Hobby and Su­per­dome, so it will def­i­nitely seal at a lower pres­sure, and I strongly sus­pect that its muz­zle en­ergy is due to a com­bi­na­tion of low start pres­sure and very light weight.

Mov­ing on to the HW Field Tar­get Spe­cial, the Hobby and Su­per­dome, the FTS is the heav­i­est but has the low­est start pres­sure and the high­est en­ergy, the Hobby and Su­per­dome have sim­i­lar start pres­sures but the Hobby is lighter, so has higher muz­zle en­ergy.

Fi­nally, the heavy­weight Bis­ley Mag­num and Su­per­Mag have the low­est en­ergy, with the lower start pres­sure of the Bis­ley Mag­num edg­ing it over the lighter weight of the Su­per­Mag.

WHERE DOES THE ‘LOST’ EN­ERGY GO?

In record­ing the re­coil and surge of the TX200 shot cy­cles with the nine pel­lets, I was able to get a han­dle on the piston bounce of each shot, which al­lowed me to com­pare the muz­zle en­ergy and piston bounce for each pellet. The re­sult can be seen in Chart Two.

The piston bounce ap­pears to be dic­tated not only by the pellet weight and start pres­sure, but also by the pellet ac­cel­er­a­tion, and pos­si­bly the en­ergy vented through the muz­zle fol­low­ing pellet exit, or its tim­ing. .

The three low­est start pres­sure pel­lets gen­er­ate the least piston bounce, fol­lowed by the two light­est pel­lets (Su­per-H-Point and Hobby), with the re­main­ing four lin­ing up more by start pres­sure more than mass.

I al­ways se­lect pel­lets on the ba­sis of ac­cu­racy ahead of any other con­sid­er­a­tion.

I use Air Arms Ex­press in my TX200 for the flat­ter tra­jec­tory rather than muz­zle en­ergy con­sid­er­a­tions.

Anec­do­tal ev­i­dence sug­gests springers re­form mildly dam­aged pellet skirts and shoot them OK, but not PCPs.

‘All other things be­ing equal’, heav­ier pel­lets do in­deed give higher muz­zle en­er­gies in PCPs.

Re­sults from my TX200 show that there is any­thing but a sim­ple re­la­tion­ship be­tween pellet weight and muz­zle en­ergy.

Re­sults from my S510 show that there is no sim­ple re­la­tion­ship be­tween pellet weight and muz­zle en­ergy.

The main des­ti­na­tion for lost air en­ergy is piston bounce.

Ar­ranged in order of de­scend­ing muz­zle en­ergy, these re­sults from my TX200 sug­gest that far more than pellet weight is at work.

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