My air rifle is under/over powered!
I get a number of calls, emails and messages a week saying “my new airgun is underpowered!”. I also often see people slagging off airgun manufacturers and tuners on forums and social media because their rifle is under powered. More often than not the rifle is not actually underpowered. Usually it is because the owner does not know enough about their rifle, fill pressures, different pellets or how to use their chronograph. I will try and clear up some of the common misconceptions in this article:
One very common problem is the chronograph, well more accurately the user not knowing how to set the chrono up correctly. The chronograph measures the time it takes the pellet to pass 2 sensors, which it then reads out on the screen as fps (feet per second), mps (meters per second), ftlbs (foot pounds) or joules etc. Most people in the UK use ftlbs (foot pounds) to see if their rifle is under the 12ftlbs rule, mps and joules is commonly used in Europe. What they tend to forget/ignore is the chronograph needs to know how heavy the pellet is so that it can calculate the foot pounds. If you have left the pellet weight setting in the chronograph on the default pellet weight of say 10 grains (grams and grains are not the same thing!) and you shoot a .22 Air Arms Field @ 16grains through the chrono, it will measure the speed at say 570fps but tell you that the ftlbs is only 7.2ftlbs, when in fact it is actually 11.5ftlbs. The same applies if you have the chrono set on 10grain and you use a 8.44g JSB Exact that is doing 775fps, your chrono will tell you that your rifle is doing 13.3ftlbs, which is over the UK legal limit but in reality it is doing 11.3ftlbs.
Another common chronograph problem is the batteries. If you use a chronograph like the Combro, you need to make sure the batteries are fresh. When the batteries start going flat they can give you wild readings that will make you think your rifle is broken/overpowered/under powered. Change the batteries and try again before getting on the phone or forums to kick up a fuss.
Some chronographs are very sensitive to the type and amount of ambient light. Some will only work well in bright light (sunlight) and other will not work well in bright light. The limitations will depend on the type of chronograph you have, the instructions should point you in the right direction. Some benefit from battery powered LED push/touch lights being mounted above the sensors. Google your chrono model for some ideas.
The distance of the muzzle (end of barrel) from the chronograph can make a difference to your chrono readings. Always make sure you follow your chronograph manufacturers guidelines.
The angle you shoot through the chrono can also make a difference. For the best results use a guide or rest to ensure you are always shooting through the chrono at the same angle and in the same place.
This is a simple instructional video for using a Combro >>here<<
This is a very simple feet per second to foot pound converter www.reloadammo.com/footpound
Not all pellets are the same, some are light, some heavy, some are soft lead, some are a hard alloy, some plastic, some copper etc. There are hundreds of different types of pellets available on the market and new types are released all the time. The pellet the manufacturer/tuner uses to set your power may not be the most accurate pellet in your rifle but it is usually used because it is a pellet known to be very efficient, to try and make sure your rifle will be under the legal limit with any other pellet that may be used in it. If the manufacturer states “full power” in their specifications that usually (not always) means around 11.5ftlbs with the most efficient pellet they can find. It does not mean it will be doing 11.5ftlbs with every make/type of pellet on the market. It also does not mean that your rifle is under powered if it is doing 10.5ftlbs with your chosen pellet, it may be that it will be around 11.5ftlbs with an efficient pellet and the pellet you are using is not as efficient. An example of this is a .177 JSB Exact Express @ 7.9g may be doing 800fps, which is 11.2ftlbs but a Crosman Premier that is also 7.9g may only be doing 760fps, which is 10.1ftlbs. 2 different pellets but both the same weight and over a foot pound difference between them n the same rilfe. This is down to the different lead alloys used to make them, the JSB is a softer lead that seals in the barrel well and is very efficient, where the Crosman is a harder lead alloy which dos not seal as well and has more friction, which saps the power. This does not mean your rifle is under powered or that Crosman Premiers are a bad pellet, they may well be more accurate than the JSBs in your particular rifle.
Bear the above in mind when setting your rifle's power, if your barrel happens to like a particular pellet, let's use the Crosmans as an example again, you want the power set at 11.5ftlbs, so you set the pellet speed at 810fps. If you or the police then put a JSB Exact Heavy @ 10.3g through your rifle it may well be doing well over the legal limit, in fact it would be well over the limit with pretty much any pellet.
You also need to watch for pellet head size. You may find your rifle is perfectly legal with a particular head size of a particular pellet but it may be over the limit with the smaller or larger head size of the same type of pellet.
Something that a lot of people don't realise that is there is a lot of re-branding of pellets. There are a handful of pellet manufacturers but if you believe the labels on the tins, you would think there were hundreds of pellet manufacturers. You may unknowingly be buying the same pellets but with a different label on the tin. A quick Google should let you know who actually makes the pellets you buy labelled up as something else.
Which pellets should I test with?
Which pellet is best to use for checking and setting the power? There is no hard and fast, set in stone rule for this sadly, the only way to know for sure is to check with every type of pellet available in your calibre. This is obviously not practical because you would have to buy hundreds of tins of pellets and it would take ages. I think you are going to be fairly safe if you test with an efficient pellet (soft lead compound, thin skirt etc) in heavy, medium and light weight to cover you as much as possible. I tend to use the following:
.177 – JSB Exact Heavy 10.3g, JSB Exact Diablo 8.44g and JSB Exact Express 7.9g.
.22 – JSB Express RS 13.4g, JSB Express 14.3g, JSB Jumbo 15.9g and JSB Jumbo Heavy 18g
You also need to watch for pellet head size. You may find your rifle is perfectly legal with a particular head size of a particular pellet but it may be over with the smaller or larger head size of the same pellet.
As mentioned above, these are the pellets I use for testing, there may well be more efficient pellets on the market, so use them at your own risk.
Some manufacturers are now setting the power with their own branded pellets and telling people to only use that brand in their rifles but sadly the police can test your rifle with any pellet they like.
I think manufacturers and tuners long for the day when an industry standard pellet can be used for setting/testing the power of rifles. It will make life easier for everybody, we would not be constantly worrying that our rifles may be over the limit with a pellet that we have not tested our rifles with.
Accessories like silencers and shrouds can make a difference to the power of your rifle. Some designs of silencer/shroud will sap the power of your rifle. This is fine if you set the power with a power sapping silencer/shroud fitted but if you change the silencer or shroud you should check the power again because it may well be different. Same applies, if you think your rifle is under powered, try it without the silencer or shroud fitted before getting up in arms about your rifle being under powered.
Some chronos can have read errors or give strange results if you use an air stripper, I suspect it has something to do with the air blast. It may be worth checking without the stripper fitted or shoot from further away.
Quoted Power vs Reality
Often the power quoted by manufacturers is misleading, in the sense that the figures have been achieved by using a type of pellet that may not actually be any good to use in the rifle in question. For example they may say a riffle does 1000fps but when you test it with all the different pellets you can find it only does a maximum of 850fps. Did they lie, probably not, they more than likely used a non-lead, lightweight pellet that does 1000fps when it comes out the muzzle but could not hit a barn door from 20 yards away.
The same sort of thing applies to the quoted power of rifles, usually FAC rifles. They quote that the rifle does 30ftlbs but you can only get a maximum of 27ftlbs with all the pellets you have tested in the rifle. They more than likely use a really heavy pellet to get the power figure they quote, that has a very loopy trajectory, that nobody would ever actually shoot in the real world.
Basically take the quoted figures with a pinch of salt and accept that they were probably achieved with a pellet that you would not consider using.
This is a big one! If you have an unregulated rifle, which most are, you will have a power curve. Within the power curve you will have a “sweet spot”, where the shot to shot consistency is pretty good and near full power. Within the sweet spot there will be a peak, where the power is at it's highest, which is ideally where you want to chrono your rifle. Chronoing it either side of the peak will give you lower power readings, which may lead you to believe your power is low. If you adjust the power when it is not at it's peak, it may go over the limit at the peak. The peak of the power curve will be different on every rifle, even on 2 rifles of the same make, model and calibre. The only way to know for sure is to chrono the rifle from full to empty and record the power vs fill pressure. If you find the peak of the power curve is around 150bar then test different pellets around the 150bar mark to make sure you stay legal.
The maximum fill pressure quoted on rifles is just that, the maximum safe fill pressure, it is not the best pressure to fill it to. Often the power will be very low if you fill it to the maximum safe fill pressure. It is best to work out where your “sweet spot” starts and finishes and only fill to this pressure or else your first few shots will be low powered and off your zero. If you want to work out your sweet spot, Google how to do this, it is beyond the scope of this guide.
If you have a regulated rifle, in theory your power will be roughly the same from the maximum safe fill pressure, right down to where the regulator stops working (this will depend on the pressure regulator was set at). Sadly not all regulators are equal, some still have a “power curve” or the power gets either higher/lower as the cylinder/bottle pressure gets lower. Some rifles can be tuned to minimise the effects of a poor regulator, some can't. All you can do is chrono the rifle over a complete fill to see if it is legal throughout.. Some regulators work well between certain pressure ranges, which are sometimes different to the max fill pressure of the rifle e.g the maximum safe fill pressure may be 232bar but the regulator will be erratic above 200bar, in this case it is best to only fill to 200bar. Something else to watch out for with regulated rifles is what happens when the pressure gets to below it's pre-set pressure. If the regulator pressure is set too high the power can spike/rise when the regulator gets below it's pre-set pressure, which may take your rifle over the legal limit for a few shots. If this does happen it is best to get your regulator pressure adjusted to avoid it spiking.
Do I need 11.9ftlbs?
The simple answer is no! Most people would not be able to tell the difference between a rifle doing 10.5ftlbs and one doing 11.5ftlbs. The difference in point of impact (POI), even out to 45 yards, will be minimal. If you worry that your rifle is under powered because it is “only” doing 11ftlbs, input the details of your rifle into a free program like Hawke Chairgun http://www.hawkeoptics.co.uk/chairgun.html or one of the apps available on smart-phones. See what difference the extra half a foot pound will make to your POI and you may be surprised.
The same applies to hunters who claim their rifle is under powered because it is not killing things or the pellets are bouncing off. Assuming you are actually hunting legal airgun quarry and not animals that are too big to be shot with a sub 12ftlbs air rifle then this is more than likely down to shot placement than actual power. You only need around 3ftlbs of power to kill a rabbit, if you shoot it in the right place in the head.
Example: a .177 JSB Exact 8.44g, doing 8.7ftlbs at the muzzle will still be doing over 3ftlbs at 90+ yards, which is well past the range a responsible airgun hunter will shoot live quarry at.
Remember the legal limit for an unlicensed airgun in the UK is 12ftlbs. It is not goal or target, it is the maximum, 12ftlbs or higher is illegal and will invariably result in you getting a criminal record and having your rifle confiscated. The power of your rifle can fluctuate depending on the weather and other factors, it is worth setting your power well under the limit with the most efficient pellet you can find (even if it is not the actual pellet you use in the rifle), to avoid the risk of the power going over the limit.
I will add to this guide over time....