Flat light

Do lighter pel­lets al­ways mean faster ve­loc­ity, and flat­ter tra­jec­tory? It ain’t nec­es­sar­ily so, says Jim

Airgun World - - Contents -

Peo­ple who use light­weight pel­lets gen­er­ally seem to do so be­cause they want the fastest muz­zle ve­loc­ity, in the ex­pec­ta­tion of it giv­ing the flat­test tra­jec­tory, but one does not nec­es­sar­ily guar­an­tee the other. Pel­let mass (weight) is one of the fac­tors that con­trib­utes to the pel­let’s Bal­lis­tic Co­ef­fi­cient (BC), which is a mea­sure of how well the pel­let main­tains ve­loc­ity in flight and, all other things be­ing equal, greater pel­let mass equals higher BC, and hence higher ve­loc­ity re­ten­tion. Light­weight pel­lets might have the high­est muz­zle ve­loc­ity, but if they shed ve­loc­ity in flight too quickly, they might not give the flat­test tra­jec­tory out to any great range.

Pel­let drop is largely – al­though not ex­clu­sively – a func­tion of flight time. Over short ranges, high ve­loc­ity light­weight pel­lets with poor BC may well get to the tar­get ahead of heav­ier ones with bet­ter BC, but at some point, the heav­ier pel­let with bet­ter ve­loc­ity re­ten­tion can catch the light­weight up, and then over­take it, so it ar­rives at the tar­get sooner, and drops less.

RE­COIL

If you missed last month’s Air­gun World, you might be for­given for think­ing that light­weight pel­lets will cre­ate less re­coil. In the case of the PCP then yes, it’s true, but the dif­fer­ence be­tween very light and very heavy pel­lets is so lit­tle, in the or­der of un­der one tenth of a mil­lime­tre, as to be in­con­se­quen­tial for the vast ma­jor­ity of peo­ple and pur­poses. In the case of re­coil­ing spring air­guns, though, the ef­fect of pel­let mass on the re­coil cy­cle is tiny com­pared to that of the start pres­sure of the pel­let. The only point we can safely draw re­gard­ing pel­let mass and spring air­gun re­coil is that, for two pel­lets with sim­i­lar start pres­sure, the lighter will cre­ate MORE re­coil, not less, and the heav­ier pel­let will cre­ate less re­coil, at the ex­pense of more surge.

SPRING AIR­GUNS

My ex­per­i­ments strongly sug­gest that spring air­guns are far more af­fected by pel­let start pres­sure than they are by pel­let mass, not only in the ef­fect on the re­coil cy­cle, but also in how pel­let mass and start pres­sure help to de­ter­mine muz­zle en­ergy.

There is a gen­eral ten­dency for pel­lets with low start pres­sures (typ­i­cally char­ac­terised by very thin skirts and soft lead al­loy) to pro­duce higher muz­zle en­ergy than pel­lets with thick and/or hard skirts, and this is due to the pel­let skirt de­form­ing, con­form­ing to the ri­fling in the breech, and hence seal­ing, at lower pres­sure, so that the pel­let and pis­ton travel in the same di­rec­tion – the ‘cylin­der pulse’ – when most en­ergy is gained by the pel­let, for longer.

Pel­lets with thick and/or hard skirts will not form a seal un­til the cylin­der air pres­sure is much higher, so they start to move later in the com­pres­sion stroke, re­duc­ing the time that they and the pis­ton are trav­el­ling in the same di­rec­tion. The re­duc­tion in the cylin­der pulse with hard pel­lets is coun­ter­acted to a de­gree by the higher air pres­sure and in­ter­nal en­ergy when they do start to move, but on the whole, the longer cylin­der pulse usu­ally gives the higher muz­zle en­ergy.

To my knowl­edge, the only way

“In the UK, we tend to try to run our air­guns to give the high­est ve­loc­ity that’s safely within the UK muz­zle en­ergy limit”

for a high start-pres­sure pel­let to de­liver higher muz­zle en­ergy than a low start-pres­sure pel­let of sim­i­lar mass is for the air en­ergy and pres­sure to be boosted by au­toigni­tion (diesel­ing), which oc­curs at some­thing over 200psi, at around 85% into the com­pres­sion stroke, and pro­vides a pres­sure ‘spike’ suf­fi­cient to start the pel­let ear­lier in the com­pres­sion stroke. This is what was hap­pen­ing in many spring air­guns in the 1980s, when peo­ple were de­lib­er­ately get­ting lu­bri­cant into the com­pres­sion vol­ume to pro­mote diesel­ing, and the pel­let that con­se­quently gave the high­est muz­zle en­ergy was the high start pres­sure, 7 grain (gn) RWS Hobby.

Com­bine a low start pres­sure with low weight, and the pel­let exit can be so early that it re­duces cylin­der air pres­sure greatly dur­ing the ini­tial stage of pis­ton bounce, lead­ing to re­duced pis­ton bounce, and hence re­coil surge, and a very gen­tle pis­ton fi­nal land­ing. The pel­let that does this best is the 7.3gn Fal­con Ac­cu­racy Plus, as can be seen in the il­lus­tra­tion ‘Pis­ton bounce’.

PCPS

Ex­per­i­ments with PCPs sug­gest that they are far less sus­cep­ti­ble to pel­let start pres­sure than spring air­guns, but more af­fected by pel­let mass, to the ex­tent that PCPs have a gen­eral ten­dency to pro­duce higher muz­zle en­ergy from heav­ier pel­lets, which is the op­po­site of spring air­guns.

The re­la­tion­ship be­tween pel­let mass and PCP muz­zle en­ergy is not in­fal­li­ble, as a test of ten pel­lets shot through my Air Arms S510 re­vealed. True, the two heav­i­est pel­lets pro­duced the two high­est muz­zle en­er­gies, but the third heav­i­est pro­duced the sec­ond low­est muz­zle en­ergy.

Pel­let start pres­sure may or may not play a part, but it’s not dom­i­nant as in a spring air­gun, be­cause the pel­lets with the high­est start pres­sures pro­duced the two high­est and two low­est muz­zle en­er­gies. Some other fac­tor must be at work, and the most likely cul­prit seems to be blow-by. The il­lus­tra­tion PCP, pel­let mass and muz­zle en­ergy shows the masses of the ten pel­lets in the test, but not the hard­ness of the pel­let skirt, so let’s put that right by recog­nis­ing that the three low­est muz­zle en­er­gies were recorded by the three pel­lets with the hard­est skirts. The hard­ness of the skirt may be al­low­ing a de­gree of blow-by, but it is matched, of course, by the hard­ness of the rest of the pel­let, which may in­crease fric­tion in the bar­rel, adding to en­ergy loss.

The il­lus­tra­tion ‘PCP ve­loc­ity and pel­let mass’ shows that, whilst us­ing light­weight pel­lets in a PCP might cost the user muz­zle en­ergy,

it usu­ally (not al­ways) re­sults in a higher muz­zle ve­loc­ity. The av­er­age weight of the five pel­lets that gave the high­est ve­loc­ity was 7.8 grains, against the av­er­age 10.34 grains of the five that gave the low­est muz­zle ve­loc­ity.

So, for a PCP, is it bet­ter to go for a light­weight pel­let that gives higher muz­zle ve­loc­ity in the cause of a flat­ter tra­jec­tory, or the pel­let that gives the high­est muz­zle en­ergy? The an­swer is, of course, nei­ther, but to se­lect pel­lets ac­cord­ing to their ac­cu­racy for the in­tended pur­pose.

Of the pel­lets in the test, one of the two 8.44 grain pel­lets – the Air Arms Di­ablo Field – gave the third high­est muz­zle en­ergy, and third fastest muz­zle ve­loc­ity and, the im­por­tant point, it also hap­pens to give the best outdoor ac­cu­racy, and that’s the pel­let I use.

WIND

It is com­monly imag­ined that light pel­lets will be af­fected more by the wind than heavy ones, but the truth is a lit­tle more com­pli­cated than that.

A pel­let’s sus­cep­ti­bil­ity to wind drift is, in essence, a func­tion of the length of time the pel­let takes to reach the tar­get, which in turn is a func­tion of the pel­let’s ini­tial, or muz­zle, ve­loc­ity and how well it main­tains ve­loc­ity in flight, given by its bal­lis­tic co­ef­fi­cient (BC). For a given muz­zle ve­loc­ity, wind drift hence be­comes a sim­ple func­tion of the BC.

In the UK, we tend to try to run our air­guns to give the high­est ve­loc­ity that’s safely within the UK muz­zle en­ergy limit, usu­ally a lit­tle over 11 ft. lb., with our cho­sen pel­let, and so a for­mula to pre­dict wind drift sus­cep­ti­bil­ity at an equiv­a­lent muz­zle en­ergy would be ex­tremely use­ful. Mike Wright came up with a for­mula he called the Wind Sus­cep­ti­bil­ity Fac­tor (WSF), which was the square root of the pel­let mass in grains, di­vided by the pel­let’s BC, which gave a three (some­times two) digit num­ber, and the lower the num­ber, the less sus­cep­ti­ble the pel­let to wind drift.

With very lim­ited fa­cil­i­ties, we tested Mike’s equa­tion and found that it was good at pre­dict­ing which of two pel­lets would suf­fer less from wind drift, and gave a fairly ac­cu­rate idea of by how much, at the same muz­zle en­ergy. The WSF was in­cor­po­rated into Hawke Chair­gun Pro, which lists WSF val­ues along­side BC val­ues in the pel­let se­lec­tion ta­bles.

UL­TRA­LIGHT

To pro­mote the sale of spring air­guns in coun­tries with no le­gal en­ergy limit, some man­u­fac­tur­ers vie to be able to claim the high­est muz­zle ve­loc­ity fig­ures, which are achieved in part with big swept vol­ume, long stroke and hefty pis­tons and springs, and in part by us­ing ul­tra­light non-lead pel­lets.

The me­tals in the al­loys used to make non-lead pel­lets; nickel, tin and zinc, are much harder than lead, de­spite be­ing an­nealed dur­ing man­u­fac­ture to in­crease duc­til­ity, and so they will not seal at low air pres­sure, re­duc­ing the cylin­der pulse. Once they have started to move, though, their very light mass (around 5 grains) en­sures rapid ac­cel­er­a­tion, mak­ing the most of what cylin­der pulse there is.

The high muz­zle ve­loc­ity might sound at­trac­tive, but the low sectional den­sity of non-lead pel­lets means their ve­loc­ity re­ten­tion in flight will be markedly less than it would were they made of lead, pos­si­bly nearer that of a flat-head lead pel­let than a domed-head lead pel­let, so not only will they lose ve­loc­ity rapidly, but they will be more sus­cep­ti­ble to wind drift af­ter they’ve lost that ex­tra ve­loc­ity.

TO SUM­MARISE:

Light pel­lets tend to cause slightly more spring air­gun re­coil, with less surge,

Light pel­lets can give higher muz­zle ve­loc­ity and en­ergy in spring air­guns, depend­ing on pel­let hard­ness.

Light pel­lets can give higher ve­loc­ity but lower en­ergy in PCPs, depend­ing on pel­let hard­ness.

Light pel­lets are not ‘blown around’ by the wind, as some sup­pose.

Above: In pel­let se­lec­tion, ac­cu­racy trumps flat­ter tra­jec­tory and bet­ter im­pact force.

Above: At just 13.4gn and with a low start pres­sure, the .22 Fal­con Ac­cu­racy Plus could suit older, shorter-stroke spring air­guns.

Above: The light­weight Fal­con Ac­cu­racy Plus gen­er­ates the least pis­ton bounce of all the pel­lets I’ve tested.

Above: I shot .22 11.9gn RWS Hobby and 13.4gn Fal­con Ac­cu­racy Plus into mod­el­ling clay to see which of th­ese lightweights pen­e­trated more.

Above: Even a cur­sory glance re­veals that the Hobby pen­e­trated less, in fact only two-thirds as far.

Above: The Hobby (left) hits with 1.5 times the im­pact force.

Above: The light­est pel­let does not au­to­mat­i­cally pro­duce the high­est muz­zle ve­loc­ity from a PCP.

Above: PCP muz­zle en­ergy de­pends on far more than just pel­let weight.

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