Asking the right question

Monday, June 1, 2009

Attending the NRA Show in Phoenix was an interesting and educational experience.

I didn’t spend a huge amount of time hanging around the Airguns of Arizona booth, but when I did, one of the more enlightening aspects was the kind of questions that visitors to the booth asked.

Frequently I would hear a comment like: “I bet this will take care of rabbits in the garden, yeah?” And sometimes someone would eye a beautiful airgun and inquire about the cost.

Curiously, no one within my hearing ever asked about accuracy. Most visitors to the booth (except those who were already high precision airgun enthusiasts) were unaware that high quality precharged pneumatics will easily product sub-one-inch groups at 50 yards and sometimes at much longer ranges.

The one question that I did hear most often was: “How many feet per second?”

Since that particular question was asked so many times, it told me a couple of things. First, it told me that the average non-airgunner is woefully ignorant when it comes to the real questions to ask about airguns. Second, it showed me that the companies that are marketing on the basis of feet-per-second claims are winning the marketing battle, for now, at least.

It wasn’t until I had returned from the show, was sitting in the comfort of my office and meditating on the experience, that the correct response to the “How many feet per second?” question came to me.

The proper response is another question: “Do you want to be fast or do you want to be accurate?” Tony Belas, Director of Daystate, hit the nail on the head: “You can take the analogy of the WWII Spitfires. When they broke the sound barrier, they used to fall out of the sky. We can shoot a pellet out of an 80 foot-pound Air Ranger and it will go 1380 fps. And at 20 yards, it will go through the same hole, day in and day out. But at 40 yards, you won’t find the hole, because the pellet goes from supersonic to subsonic and goes its merry way. The problem is that if you crack the sound barrier, the pellet is going to be out of the sound barrier long before you hit the target.”

I saw this demonstrated in spades when I tested the Gamo Hunter Extreme in .177. “Hunter Extreme, 1600 fps!” the box read, adding, “The fastest spring airgun on earth.” This claim was made based on shooting Gamo’s new Raptor Performance Ballistic Alloy which are very light (under 5 gr., if I remember correctly)

So I tested the Hunter Extreme at 50 yards with the Raptor pellets. Velocities – which were loudly supersonic — ranged from 1477 to 1525, averaging 1491 (this isn’t the 1600 fps that Gamo promised, perhaps because we were keeping the chronograph a couple of feet from the muzzle), but the accuracy simply wasn’t there. Group size at 50 yards was 3.5 to 5 inches, depending upon whether you called one shot a shooter-produced “flyer” or not.

But if you slowed the velocities down by shooting a much heavier pellet, the accuracy improved substantially. The Hunter Extreme “liked” Crosman Premier 10.5 gr. Heavies (CPHs, for short). After dieseling for a couple of shots, it settled down, launching them at around 1021 fps, average (high was 1026, low was 1015). That’s over 24 foot-pounds of energy. Our first 5-shot group measured just 1.25 inches edge to edge.

So I have a modest proposal for the good folks at Airguns of Arizona. The next time they go to a show, they should put up a BIG poster with two targets, both shot at 50 yards with the same gun and the same pellets. The first would show a tiny little group with the velocity prominently displayed below: 960 fps. The next target would show a much bigger raggedy group with the velocity: 1500 fps.

Then the poster would ask the right question: Do you want to be fast, or do you want to be accurate?

Til next time, aim true, stay subsonic, and shoot straight.

- Jock Elliott

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2 Comments

  1. Michael Nager says:

    Mr. Elliot,

    as Scotty used to say to Captain Kirk, “Ye canna change the laws of physics”.

    The problem with alloy pellets being inaccurate is not because of their supersonic velocity, but rather that after about 10 yards or less they enter what is called the “transonic region”.

    To be truly supersonic an object has to move at 1339 ft/s (Mach 1.2) at sea level, to be truly subsonic the object will move at 803 ft/s (Mach 0.72) or below.

    When I say “truly supersonic” I mean the velocity where ALL of the airflow around the pellet is supersonic (Mach 1.2).

    When a pellet leaves the air rifle at truly supersonic velocity it will very quickly – due to drag – decelerate and enter the transonic region and this is where the fun starts.

    At transonic speed the drag on the object increases dramatically, but it gets worse; the centre of pressure (CP) of the pellet will move forward and this will cause an erratic and sudden CP shift and (temporary) decrease of dynamic stability.

    What this means is that you can basically forget the accuracy of any air rifle pellet leaving the muzzle of an air rifle at truly supersonic velocity beyond the range of 5 – 10 yards. When the pellet decelerates into the transonic region it will go “cuckoo for coco puffs”.

    Any company claiming any kind of accuracy for their pellets leaving the barrel of an air rifle at truly supersonic or very high transonic (Mach 1.0+ or greater than 1116 ft/s) beyond 10 yards maximum are – how can I put this politely? – lying.

    The catastrophic effect of velocities in the transonic region obviously decline as the pellet decelerates towards the subsonic region where the pellet will stabilise, but by that time the damage will have been done to the accuracy. They can be ignored for pellets that leave the muzzle in the range of velocities between Mach 0.72 and 0.8 or so – truly subsonic to just entering into the transonic – or 804 – 893 ft/s.

    What makes me really angry however is recommending alloy pellets for springers.

    In a springer the piston depends on the pellet offering enough resistance to the blast of air so that the crown of the piston is cushioned on a layer of air in the cylinder before it makes contact with the cylinder itself.

    With the very light alloy pellets this is not assured which will result what is known as “piston slam”, and if, on top of this, the alloy pellet fits loosely into the barrel of a magnum power springer air rifle then you can basically measure the lifespan of the piston crown in tens of pellets and not tins of pellets.

    I also have concerns with regard to using alloy pellets in choked air rifle barrels. The choke has a beneficial effect on the accuracy of lead pellets because they are malleable and the choke homogenises the pellets to assure that the pellets out of the same tin will leave the barrel the same shape every time.

    When pellets are produced there are tolerances which will be deemed acceptable for the diameter of the head of the pellet. The choke has no tolerance whatsoever and the pellet will come out of the barrel at EXACTLY the diameter of the choke. Also pellets, the head of which may not engage the rifling of the barrel properly as it moves up it will be forced to leave the barrel straight by the choke.

    Anyone who has looked at a lead pellet they have fired at a soup can will see how deformed the pellet has become. Fire an alloy pellet at that same soup can then it is almost like you could put the pellet back into the rifle and fire it again as far as the deformity of the head is concerned.

    This makes me suspicious with regard to the effect of firing alloy pellets through a choked air rifle barrel and the damage to that choke which will accrue over time thus making the rifle intrinsically less accurate.

    Another thing that drives me absolutely NUTS is the penetration claim for alloy pellets with regard to hunting.

    Now to put “driving me NUTS” into perspective is the age old marketdroid claim of “New and Improved”. Look, if something is “new” then it is something which has not been there before, and if something is improved then it is something which has been around before and it has been enhanced. There is no way that something can be new AND improved at the same time.

    With regard to hunting, penetration is a good thing right? I mean if the pellet bounces off the target then it pretty much defeats the point of shooting the pellet off in the first place.

    But hunting is not just about the pellet penetrating the target, but also delivering traumatic damage to the quarry. Lead pellets do this by penetrating the target, deforming and through this deformation causing a large wound channel destroying a lot of tissue as it enters the quarry’s body and if it stays in the quarry’s body then it will have transferred all of the energy of that pellet to the target causing even more trauma – so called “stopping power”.

    Of course alloy pellets have a higher penetrating power than lead pellets – the bloody things are armour piercing – however they will not deform (thus less tissue damage once it has penetrated) and for the kind of quarry one can typically hunt with an air rifle, it will exit out of the other side (thus not transferring the energy of the pellet to the animal).

    Therefore I would suggest that the danger of just wounding the more highly penetrated animal (and thus leaving it to suffer) instead of killing it outright at the instant of impact is a lot higher with alloy pellets.

    I can thus not in any way shape or form (puns intended) recommend alloy pellets for hunting.

    I started this post with a Star Trek quote, but my favourite Star Trek joke has to be:

    Question: What did Mr. Spock find in the toilet of the USS Enterprise?

    Answer: The Captain’s log.

    1. Jock Elliott says:

      Michael,

      Thanks for your excellent comments. You and I are basically in agreement. Powder burning varminters shoot bullets at supersonic speeds and KEEP them at supersonic speeds for precisely the reasons you state. If you search the blog, you will see that I have shot some non-lead pellets that were decently accurate, but they were heavy enough to keep the velocity below the trans-sonic region. Thanks again for your comments.

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