Gary Wain

Gary uses a cou­ple of chronos to gather nu­mer­i­cal data from his pel­let-test­ing

Airgun World - - Contents -

Reg­u­lar read­ers will al­ready know that I am more than a lit­tle ob­sessed with pel­let test­ing, and in par­tic­u­lar the vari­ances we see be­tween cal­i­bres and pel­let shapes. By con­duct­ing tests on ter­ra­cotta wax last year, and tak­ing cav­ity moulds with plas­ter, I was able to demon­strate that given the same pel­let shape, the lighter .177 pel­lets pro­duced a greater wound cav­ity than the heav­ier, slower mov­ing .22 pel­lets. Hav­ing es­tab­lished that fact, and in the process up­set a few ap­ple carts, I wanted to gain a greater un­der­stand­ing of the physics in­volved, and to find a way of putting some ac­tual num­bers in place of the plas­ter-cast moulds. So, with the tem­per­a­tures start­ing to climb, if only by a lit­tle, it was time to break out the ter­ra­cotta wax and start gath­er­ing data.

Although ev­ery­thing I have done so far as­sures me that .177 leaves a big­ger wound track than .22, and there­fore pro­motes the the­ory that the .177 is giv­ing up more en­ergy, I still had at the back of my head the old be­lief that the .22 pel­let is ‘harder hit­ting’ than the .177, so I was more than a lit­tle ner­vous that my new ex­per­i­ments might prove me wrong.

I de­cided to keep things nice and sim­ple for the first of these new tests, so as much as I wanted to get into com­par­ing vary­ing pel­let weights, shapes, sizes and dis­tance to tar­get, I de­cided to com­pare sim­ple, domed pel­lets from the same man­u­fac­turer; in this case, the Di­abolo Field from Air Arms – the .177 and .22 weigh­ing in at 8.4gr and 16gr re­spec­tively. The ri­fles used were the match­ing pair of Daystate Pul­sars used in pre­vi­ous test­ing, one of which is mine, and the other kindly loaned to me by Ter­ence, at Daystate.

HOW THICK?

To con­duct the tests I fired pel­lets through a pair of R2a chrono­graphs, supplied by Lloyd, at Black­pool Airguns. I placed a sec­tion of the ter­ra­cotta wax, which I’ve come to rely on for its abil­ity to closely sim­u­late flesh, be­tween the chronos. Sim­ple? Yes, it ap­pears to be, but it’s not as sim­ple as it seems. For starters, how thick do you make the ter­ra­cotta wax? I had an ini­tial idea that 20mm would be about right, but it needs heat­ing up to 20°C to make it pli­able and us­able at this tem­per­a­ture,

“I was more than a lit­tle ner­vous that my new ex­per­i­ments might prove me wrong”

although the 20mm thick sec­tion wasn’t re­ally ca­pa­ble of sup­port­ing its own weight. Af­ter fur­ther ex­per­i­men­ta­tion, I set­tled on a thick­ness of 40mm be­cause it was thick enough to sup­port its weight, but wasn’t so thick that it would not be rep­re­sen­ta­tive of quarry.

SPLATTER THE­ORY

The sec­ond po­ten­tial prob­lem was that of par­tic­u­late dis­per­sion skew­ing the date. From pre­vi­ous use of the ter­ra­cotta wax, I was aware that there was a cer­tain amount of ‘splatter’ as the pel­let both en­tered and ex­ited the wax. The en­try splash wasn’t an is­sue be­cause, by def­i­ni­tion, the pel­let had al­ready passed through the chrono­graph and been read be­fore en­ter­ing the wax, so the splatter was se­condary to the read­ing. More of a con­cern was the spray cre­ated as the pel­let left the wax. I’d the­o­rised that the spray fol­low­ing the pel­let would be mov­ing slightly slower, and that the par­ti­cles would be too small to be picked up by the sec­ond chrono­graph. I needed to be sure, though,

be­cause if the sec­ond chrono­graph was ac­tu­ally read­ing the speed of the par­ti­cles, as op­posed to the pel­let, my data would be com­pro­mised.

Af­ter con­sid­er­ing both pa­per and tin­foil as in­con­ve­nient splatter catch­ers, I de­cided to wrap the sec­ond chrono­graph in Cling­film to act as a bar­rier to the many small par­ti­cles. I then set about test­ing with and with­out the Cling­film to see if there was any vari­ance in the data col­lected. I was just con­clud­ing the test­ing when my good wife, Mar­i­anne, ar­rived home from work. It’s fair to say that we have some odd con­ver­sa­tions in this house, but this pretty much takes the bis­cuit.

“Hi Honey, I’m home. What have you been up to to­day?”

“Oh, you know … the usual. Wrap­ping chrono­graphs in Cling­film and fir­ing pel­lets at them through a pre­cisely warmed bal­lis­tic clay ma­te­rial to see if par­tic­u­late dis­per­sion would skew my data. You?” Odd con­ver­sa­tions aside, I am happy to say

that although there was a sig­nif­i­cant amount of par­tic­u­late dis­per­sion, both for­ward of and be­hind the bal­lis­tic clay, none of this ap­peared to im­pact ad­versely on the data, so I wouldn’t need to keep wrap­ping the chrono­graphs in Cling­film ev­ery time I un­der­took a test fir­ing. Se­ri­ously, you have no idea how much of a re­lief that was! It’s hard enough heat­ing and main­tain­ing sev­eral ter­ra­cotta wax blocks to 20 de­grees, never mind hav­ing to wrap in­di­vid­u­ally R2a chrono­graphs in cling film for each test shot!

RE­SULTS RE­VEALED

Again, with an eye to keep­ing things sim­ple at this early stage, I de­cided to carry out ini­tial test­ing at around 15 me­tres. I hoped that

this would prove cor­rect the hy­poth­e­sis that .177 im­parts more en­ergy to the tar­get than .22, and so en­able me to move for­ward in the fu­ture by push­ing the dis­tance out, and test­ing vary­ing types of pel­let against each other.

A Lin­colnshire ‘fret, other­wise known as ‘a thick sea fog’, was mov­ing in so I thought I’d best get a move on with the test­ing. These frets can get so thick, I swear I’ve seen un­dead sailors with hooks for hands wan­der­ing out of them.

So, what did the day’s test­ing re­veal? First up was the 8.4gr .177 pel­let; this passed through the first chrono­graph at 717fps, reg­is­ter­ing 9.16 ft.lbs. It then passed through the 40mm-thick bal­lis­tic clay and ex­ited the ma­te­rial, pass­ing through the sec­ond chrono­graph, giv­ing read­ings of 420fps and 3.29 ft.lbs.

Next, it was the turn of the .22 weigh­ing in at 16gr, ba­si­cally dou­ble that of the .177. This pel­let passed though the en­try chrono­graph at a slower 543fps with 10.46 ft.lbs., and ex­ited with a speed of 377fps and 5.05 ft.lbs.

If you want to break out the cal­cu­la­tors, we can see that the .177 pel­let has lost 297fps in ve­loc­ity as it passed through the tar­get ma­te­rial, and 5.87 ft.lbs. On the other hand, the .22 lost 166fps of speed, and just 5.41ft.lbs.

In sim­ple terms, the .22 ar­rived with a heck of a lot more en­ergy than the .177, by virtue of its mass, but sur­ren­dered less of it to the sur­round­ing bal­lis­tic ma­te­rial. Con­versely, the .177 pel­let ar­rived much faster, and de­spite its en­ergy trans­fer­ence, departed hav­ing lost less of its ve­loc­ity to the sur­round­ings.

LEARN­ING CY­CLE

De­spite my ini­tial wor­ries that the data would prove my ini­tial find­ings wrong, I have been able to show that .177 ar­rives faster, but with slightly less en­ergy than the .22, but as a re­sult of its lower mass and dif­fer­ing bal­lis­tic co­ef­fi­cient, is able to im­part more of its en­ergy into the tar­get ma­te­rial as it passes though it, than the heav­ier, but slower, .22 pel­let.

LOOK­ING AHEAD

In fu­ture ar­ti­cles, I in­tend to push out the dis­tances and look again at the vary­ing types of pel­let on the mar­ket. I still have quite a stock of pel­lets from pre­vi­ous test­ing and in­tend to pitch the best of the best against each other. I’ve learned that what you think is go­ing to hap­pen, and what ac­tu­ally hap­pens are two to­tally dif­fer­ent things, and just be­cause I think I know which is the best sort of pel­let out there, that doesn’t nec­es­sar­ily mean I’m right. So, if you have a par­tic­u­lar favourite, a pel­let that you think will dom­i­nate the field re­gard­less of cal­i­bre, or per­haps one that you think will per­form bet­ter as a .22 than as a .177, then do please get in touch and let me know. For me, this is a jour­ney, a con­stant learn­ing cy­cle, and I’m aware that I have so much more to learn.

How thick should we make the wax sam­ples?

Spring! The time when a young man’s heart turns to thoughts of ter­ra­cotta wax. The spare tyre on my old Landy made a great rest. With the ‘fret’ mov­ing in from the sea, I knew vis­i­bil­ity would soon be limited. Whoops! Good job those guards are there. We’re all hu­man, af­ter all.

My thumb flat­tened the en­try splash, but you get the idea.

Would the ‘splatter’ from the wax skew the data?

Which gives the larger cav­ity – .177, or .22?

The proof of the pud­ding is in the eat­ing.

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