Su­per-power

Af­ter avoid­ing .25 airguns for over 50 years, Jim be­comes tem­po­rary cus­to­dian of a .25 BSA Su­per­star

Airgun World - - Contents -

Jim Tyler short­ens the spring on his friend’s ri­fle to make it a true ‘Su­per­star’

Let’s start off on a pos­i­tive note, and con­sider what .25 pel­lets have got go­ing for them, which is mo­men­tum. Mo­men­tum is de­fined as ‘quan­tity of mo­tion’, and is best thought of as how much more dif­fi­cult it is to ar­rest the mo­tion of what­ever it is that pos­sesses the mo­men­tum; in our case, a pel­let.

Mo­men­tum is the prod­uct of mass (think pel­let weight) and ve­loc­ity, and is usu­ally ex­pressed in kilo­gram me­tres per sec­ond, though to save a lot of con­ver­sion of the units we air­gun en­thu­si­asts are fa­mil­iar with, mo­men­tum can also be ex­pressed as grain feet per sec­ond (grain-feet/ sec). An 8.4 grain .177 pel­let trav­el­ing at 780fps, for in­stance, has mo­men­tum of 8.4 x 780, or 6552 (grain-feet/sec).

To put .177 pel­let mo­men­tum into con­text; a 15 grain .22 pel­let with the same muz­zle energy will be trav­el­ling at 583 fps, so will have 8745 grain-feet/sec, 133% that of the .177; and a 25 grain .25 pel­let with the same muz­zle energy would have 11300 grain-feet/sec. 129% that of the .22, and 172% that of the .177.

PRAC­TI­CAL TERMS

What does this mean in prac­ti­cal terms for air­gun pel­lets? It means that when the .25 pel­let hits a mov­able ob­ject, it tends to move the ob­ject more than the smaller calibres. A grey squir­rel in a tree, for in­stance, would not only be killed, but also usu­ally knocked clean off the branch. I have known .25 pel­lets to hit field tar­gets so hard that they are rat­tled enough to fall over from a hit to the face­plate. What hap­pens is that the face plate col­lides with the pad­dle, and the two bounce off each other, to the ex­tent that the pad­dle reaches its tip­ping point and pulls the face back.

Larger cal­i­bre pel­lets tend to have greater sec­tional den­sity, mak­ing them less prone to dis­tor­tion when hit­ting a tar­get. If we take 8.4 grains as a medium weight for .177 and scale that up by cross sec­tional area, a .22 would be 13.12 grains, and a .25 would be 16.94 grains, both of which would ac­tu­ally be con­sid­ered lightweights, with most .25 pel­lets tip­ping the scales at nearer 25 grains. Be­cause of this, the .25 uses less of its ki­netic energy in dis­tort­ing

it­self, leav­ing more to dis­place what­ever ma­te­rial is in front of it, and giv­ing a deeper wound chan­nel.

COST

The high mo­men­tum of .25 pel­lets makes them a good op­tion for those who pos­sess firearm cer­tifi­cates and are able to shoot well above 12 ft.lbs., but for those who shoot sub-12 ft.lbs. airguns, the mo­men­tum of the .25 comes at a cost, and that is the amount by which the pel­lets fall over dis­tance, in com­par­i­son with lighter, smaller calibres with equiv­a­lent muz­zle energy. The .25 pel­lets fall at the same rate as .177 and .22, but they take longer to reach the tar­get, and so fall more, which means that that the shooter has to aim off more.

A 25 grain .25 pel­let ex­it­ing the muz­zle with 11 ft. lbs., and ze­roed at 30 yards us­ing a scope in medium height mounts, would need around 1.5” of hold un­der at 18 yards; the apogee (the point in the tra­jec­tory when the pel­let is at its high­est above the sight line), and an inch of hold un­der at 12 and 25 yards. You’d need to be al­most yard per­fect in range es­ti­ma­tion to hit any­thing.

Bring the zero range down, and things start to change. With Bis­ley Su­per­field pel­lets at 450fps, zero at 19 yards and you will be re­warded with a con­tin­u­ous PBR (Point Blank Range) of 0.5” from around seven to 21 yards, so there will be no need to hold un­der at any range, only to hold over at ranges be­yond 21 yards; by one mil dot at 26 yards, two mil dots at 31 yards, three mil dots at 36 yards, four mil dots at 40 yards, and five mil dots at 45 yards, which is nice and easy to re­mem­ber. Those longer ranges need too much aim­ing off for pest con­trol for my lik­ing, and the lengthy flight time at longer ranges would in­crease the op­por­tu­nity for the tar­get to move be­fore the pel­let gets there so, if I used a .25, I’d prob­a­bly limit my pest con­trol to around 26 yards. This, to my mind, tells us where a .25 sub 12 ft.lbs. ri­fle reigns supreme, which is in short-range pest con­trol.

OTHER SHOOT­ING

Plink­ing is all about hav­ing fun safely. See­ing a re­ac­tion to a pel­let strike is part and par­cel of that fun, and .25 pel­lets ex­cel at knock­ing some types of plink­ing tar­gets about. How­ever, plink­ers gen­er­ally tend to shoot more pel­lets per ses­sion than hunters, or com­pet­i­tive tar­get shoot­ers, and the .25 is rather more ex­pen­sive than the smaller calibres, so the ex­tra fun comes at a price.

For outdoor tar­get shoot­ing, I have only ever known two peo­ple to use a .25 for HFT, nei­ther of them re­ally se­ri­ous com­peti­tors, and they did it for fun, rather than in any

ex­pec­ta­tion of a high score. The very marked pel­let drop at range, and the need to be al­most yard per­fect in range es­ti­ma­tion, would put any­one try­ing HFT with a .25 at a huge dis­ad­van­tage.

SHOT CY­CLE

Due to its low ve­loc­ity, the .25 pel­let takes longer to travel up the bar­rel, so it will gen­er­ally achieve higher muz­zle energy than smaller calibres in PCPs – pro­vided the valve re­leases enough air to ac­cel­er­ate the pel­let the full length of the bar­rel – but the sit­u­a­tion in spring/pis­ton airguns is not so clear cut.

Whilst the .22 is usu­ally more energy ef­fi­cient than .177, anec­do­tal ev­i­dence ap­pears to sug­gest that .25 does not nec­es­sar­ily con­tinue that trend and, if that is the case, the rea­son would prob­a­bly be that the ri­fles con­cerned lack suf­fi­cient swept vol­ume and/or pis­ton stroke. Too lit­tle air mass would mean that, even with high in­ter­nal ki­netic energy, the too few mol­e­cules would lack the mo­men­tum nec­es­sary to get the heavy­weight pel­let up to speed, whilst too lit­tle avail­able pis­ton stroke would fail to ac­cel­er­ate the pel­let for long enough to get it up to speed.

If the above thoughts on air mass and mo­men­tum are cor­rect, then lighter weight .25 pel­lets (in the re­gion of 19 grains) would give much higher energy lev­els than heav­ier ones if the swept vol­ume were too low. This the­ory cries out to be tested, be­cause it could help to re­veal more of the se­crets of the spring air­gun, but it would need to be based on one ac­tion tested with all four calibres, and a ri­fle with swept vol­ume to spare.

BSA SU­PER­STAR

The BSA Su­per­star is a real cu­rate’s egg of an air­gun, be­cause parts of it are ex­cel­lent, and then there’s the trig­ger unit, which sim­ply can’t live with mod­ern, four-lever groups. The ri­fle has been re­viewed many times, so I’ll con­cern my­self with an is­sue spe­cific to this ri­fle, but prob­a­bly fairly com­mon.

The only pel­let that fed eas­ily into the breech with­out need­ing some im­ple­ment to push it home was the Milbro Rhino which, at 19 grains, is a light­weight amongst .25 pel­lets, and was the owner’s pre­ferred pel­let. The tight­ness of the breech made me sus­pect that the bar­rel was ac­tu­ally .243, a cal­i­bre in which BSA made cen­tre­fire ri­fles like the beau­ti­ful Stutzen that I briefly had on loan from the com­pany in the 1980s. I’m in­formed that to­day’s BSA bar­rels are true .25, and that in­for­ma­tion came di­rect from the gen­tle­man who makes the man­drels on which the bar­rels are formed, so he should know.

The prob­lem with the ri­fle/pel­let combo was that the muz­zle energy was un­com­fort­ably high, and the owner asked me to take a look

“more energy ef­fi­cient, which in turn re­quires less spring force, for less re­coil”

in­side and make it safely le­gal with the Rhino. In­side, I found a crudely cropped, un­fin­ished and heav­ily greased, uniden­ti­fied af­ter­mar­ket main­spring, of 21mm OD, 3.25mm wire, 24.5 coils and 40mm of preload with no pack­ing wash­ers.

I did the sums – cal­cu­lat­ing how much energy the spring made avail­able to the pis­ton, the energy ef­fi­ciency, and what length of spring and preload would give the de­sired muz­zle energy – which sug­gested that re­duc­ing preload, by short­en­ing the spring, to 27mm would give around 11 ft. lb. Be­cause the end coil was un­fin­ished, cut­ting off just three­quar­ters of a coil and flat­ten­ing and dress­ing the end coil gave the 13mm re­duc­tion in preload, and the muz­zle energy du­ti­fully re­duced to 11 ft.lbs. to 11.2 ft.lbs. Per­fect.

SHOOT­ING IMPRESSIONS

Un­for­tu­nately, the trig­ger weight was so high that I was un­able to record the shot cy­cle with­out the ac­tion of pulling the trig­ger blade af­fect­ing it, so I had to set­tle for the old­fash­ioned way of shoot­ing the ri­fle and sub­jec­tively judg­ing the re­coil cy­cle by feel.

The shot cy­cle with the ri­fle set to 11 ft. lb. with Milbro Rhino pel­lets was, in a word, de­light­ful, so much so that I can at last ap­pre­ci­ate why some peo­ple choose to shoot sub-12 ft. lb. spring airguns. The best I can de­scribe it is like the dif­fer­ence be­tween .22 and .177 and, I sus­pect, for the same rea­sons.

The .22 vari­ant of a pel­let tends to have a lower start pres­sure than the .177, which means that it starts to move ear­lier in the stroke, and ben­e­fits from the energy of the pis­ton for a longer pe­riod of time. This makes it more energy ef­fi­cient, which in turn re­quires less spring force, for less re­coil. At the same time, the vol­ume of air be­hind the pel­let as it trav­els up the bar­rel is greater, leav­ing less in the cylin­der to drive pis­ton bounce, so the surge is less. The same prin­ci­ples ap­ply for .25 ver­sus .22 so that, for a given muz­zle energy, the re­coil cy­cle from the larger cal­i­bre is markedly bet­ter.

CON­CLU­SION

In my opin­ion, a light- to medium-weight spring air­gun in .25 would seem to have the mak­ings of a su­perb, short-range, pest­con­trol tool. I

Re­duc­ing spring preload by 13mm put the muz­zle energy right where I wanted it.

At 19 grains, the Milbro Rhino pel­let is a light­weight for a .25, but gives the best tra­jec­tory.

A length of Ac­etal rod is quite sturdy enough to take the preload force.

In­side, I found a crudely cropped, un­fin­ished and heav­ily greased uniden­ti­fied af­ter­mar­ket main­spring.

To re­move the main­spring, you need to make a tool to bridge the cross pin and take the force from the main­spring.

Af­ter fruit­lessly try­ing to lever it out, I used a neodymium mag­net to re­move the steel plug.

To the rear of the cross pin is a steel plug, held by the main stock screw and the scope rail, Allen head bolt.

With the stock off, pull off the plas­tic end cap, and the steel plug will be re­vealed.

I was un­able to re­duce trig­ger pull weight for se­ri­ous re­coil test­ing.

If I had to sum up .25 in a word, that word would be ‘clout’.

With its short­ened spring, the Su­per­star is run­ning sweetly at 11 ft. lbs.