How to make your BB shooting replica more accurate

I like BB shooting replicas.  The use of BBs makes it possible to replicate the function of semi-auto firearms more accurately than is possible in replicas that shoot pellets.  However, I find it frustrating that so few BB replicas shoot well.  One of the first replicas I owned was a Tanfoglio Witness, and I loved its heavy weight and the way it felt just like a cartridge firing 1911.  But I found it irritating that, even at six yards, it scattered BBs over a 5” circle.  Given that current replicas are generally well made, surely they can be made to shoot with a little more accuracy than that?

The main problem here is consistency.  Pellet shooting replicas are good at sending pellets on a very similar trajectory every shot, leading to satisfactorily small groups.  BB shooters are affected by tiny imperfections in BBs and the barrel which leads variation in the trajectory of the BBs and larger groups.  But there are things you can do to make your BB shooting replica produce smaller groups.

How does it all work?

The first thing to consider is what happens when a pellet or BB travels down the barrel of an air or airsoft gun.  A .177” pellet (or a .177” lead ball) fits tightly into the barrel of an air gun and is squeezed against the rifling on the inner surface of the barrel.  When you fire the gun, gas pressure builds up behind the pellet until this is sufficient to overcome the friction holding the pellet against the sides of the barrel.  When the pellet starts to move forward, the rifling also causes it to spin.  When the pellet leaves the end of the barrel, it continues to spin, improving stability.  The friction caused by the pellet being squeezed against the sides of the barrel is the reason that pellets always leave the barrel with less speed than BBs.  The accuracy (or otherwise) of your pellet shooting airgun is largely dependent on how accurately the barrel was made in the first place and how much the rifling has eroded over time.  A build-up of deposits on the rifling can cause some minor degree of inconsistency in the flight of the pellet (though some lead build-up can actually improve accuracy), but generally the most important factor is how straight the barrel is in the first place.

Shooting a pellet in a rifled barrel

Now let’s look at what happens when you shoot a BB through a smoothbore barrel (I’ll talk about shooting BBs in rifled barrels in a moment).  And it doesn’t matter whether we’re talking about a 4.5mm steel BB or a 6 or 8mm plastic BB, the mechanics are the same.  The BB does not fit tightly inside the barrel.  A typical 6mm airsoft BB for example, is actually around 5.95mm external diameter while the barrel on most modern airsoft guns is anywhere from 6.04 – 6.08mm internal diameter.  So when the gas is pushing the BB down the barrel, some leaks past the BB and forms a thin layer of gas between the BB and the inside of the barrel.  Because of this, the BB doesn’t actually touch the sides of the barrel at all and this thin layer of gas actually helps to stabilize the BB and keep it travelling straight.

Shooting a BB in a smoothbore barrel

There are couple of things to think about here.  First, barrel length.  It takes time for the BB to stabilize on the layer of gas.  When it first enters the barrel, the BB tends to bounce off the inner sides of the barrel, especially if it hits a hop-up rubber on the way.  After it has travelled some distance, this bouncing is dampened down and the BB stabilizes in the centre of the barrel.  There is some argument about how long a barrel must be in order for the BB to stabilize fully, but most people seem to agree that anything less than around 70mm (a little under three inches) is unlikely to allow the BB to stabilize completely.  In general terms, the longer the barrel, the better stabilized the BB will be when it leaves the muzzle.

The second thing to consider is hop-up.  Most airsoft guns and some steel BB shooting guns have hop-up.  This is a rubber nub inside the barrel and close to the breech.  The nub is located on the top of the barrel and projects inside.  As a BB travels down the barrel, it strikes the rubber nub which causes it to spin backwards.  This backspin helps to overcome the force of gravity and allows the BB to maintain a flatter trajectory after it leaves the muzzle.  On many guns, the amount which the nub projects into the barrel (and therefore the amount of backspin) can be adjusted.  Most people will tell you that the effects of hop-up are not evident at ranges below 10m, but I haven’t found this to be entirely true.  Even when shooting at 6m, I have found that adjusting hop-up can affect the vertical point of impact of BBs by an inch or so.  However, hop-up initially de-stabilises the path of the BB through the barrel.  So, on a gun with hop-up, it may take more distance for the BB to stabilize.

Hop-up nub inside the barrel of 4.5mm ASG CZ75

OK, now let’s talk briefly about shooting steel 4.5mm BBs through a rifled barrel.  Some pellet shooting guns can also fire steel BBs.  Many manufacturers and some suppliers talk about .177” and 4.5mm as if they’re the same calibre.  They are not – a 4.5mm steel BB is notably smaller than a .177” pellet.  If you shoot a steel BB through a .177” rifled barrel, it is not large enough to engage with the rifling.  Instead, just as in a smoothbore barrel, it floats on a layer of gas in the centre of the barrel.  However, the flow of this layer of gas is much less stable than on a smoothbore barrel because of the rifling which causes it to swirl and tumble.  Also, as it initially enters the barrel and bounces off the sides, the hard steel BB can cause erosion and damage to rifling over time.  A BB will always leave the barrel travelling faster than a pellet because of the lack of friction, but in my experience, I have not come across any replica air pistol which shoots BBs accurately though a rifled barrel.  The higher speed at which BBs travel is unimportant and because of the lack of accuracy and the possibility of damaging rifling, I’d suggest that you shoot steel BBs only in guns which have smoothbore barrels and only shoot pellets or .177” lead balls in those which have rifled barrels.

Left, shooting eight .177” pellets from a replica with a rifled barrel (in this case, an Umarex H&K P30) at 25 feet, aim point is the centre of the black circle. Right, same replica, same range, same aim point but this time using eight steel 4.5mm BBs. As you can see, the steel BBs give notably less accuracy.

How to improve things

Right, so, now we know how it all works, how can we make our BB shooting guns more accurate?  If we’re talking about airsoft guns, the first thing many people think about is a tightbore barrel.  As the name suggests, these are aftermarket barrels which have a smaller internal diameter than the original.  That sounds good in theory, but I’m not totally convinced.  The critical thing that determines how straight your BB will travel is how well the BB stabilizes inside the barrel. Part of what determines this is the size of the layer of gas between the outside of the BB and the inside of the barrel.  Too big a layer is bad and can cause the BB to be unstable.  But, too small a gap is also bad and can prevent the BB from stabilizing fully.  If you do fit a tightbore barrel, you can expect to see your replica shooting with more power – less gas is lost round the BB and so more is available to propel it down the barrel.  However, I suspect that most accuracy gains which users report after fitting these parts come as much from improved tolerances in the manufacturing process used when making these aftermarket barrels compared to the processes used in creating the original barrel as from the tightness of their bore.  An expensive aftermarket barrel may be straighter than a more cheaply made original part (though there is no guarantee of this) but if the bore is too tight, it can actually make consistency worse.

If you don’t want to buy new bits, what else can you do?  Well, there are two things that affect the way the BB travels down the barrel. The first is the quality of the BB itself.  The layer of gas between the BB and the barrel is very thin – around 0.05mm.  That’s equivalent to about the width of two human hairs.  So, any tiny imperfection in the BB which is spinning after hitting the hop-up rubber can cause instability in the flow of gas and may cause the BB to move erratically in the barrel.  The closer to being perfectly spherical that your BBs are, the more consistently they will shoot.  If you can see seams or other moulding marks on your BBs, they are obviously not going to perform well. 

This is a pair of cheap and very nasty Chinese 6mm plastic BBs with clearly visible seams and moulding marks.  Very few 6mm BBs are this obviously crap, but no matter how good your replica, it’s never going to shoot consistently with poor quality BBs.

However, even if they look glossy and smooth, not all BBs are equal.  In general, you should avoid brightly coloured or transparent BBs (especially those which have visible bubbles of air inside them), any small packs of BBs which are supplied with an airsoft gun and any BBs which are not identified by weight.  Most BBs which are made in Japan are good as are the majority from Taiwan.  In my experience, Chinese BBs can be of comparatively poor quality and should generally be avoided.  Just because it says “Precision” or “High Quality” on the packaging is no guarantee that BBs are good.  Be prepared to try different brands and pay a little more for quality plastic BBs and when you can, choose those from manufacturers you recognize (Guarder and KWA, for example, produce very high quality 6mm BBs).

Hopefully, it’s also obvious that re-using plastic BBs isn’t a good idea. The plastic used to manufacture 6 and 8 mm BBS is fairly soft, so they tend to develop flat spots when they hit a target. This of course makes them unstable if you re-use them. While we’re talking about plastic BBs, it’s also worth thinking about weight. In general, the heavier the BB, the more stable it will be and so the smaller groupings you’ll see. You may have to experiment with different weights of BB to find one that works best for you, but the table below gives a general guide to the most appropriate weight BBs to use in your replica. The fps figures are based on the speed when shooting with standard 0.2g BBs.

Under 300 fps: 0.12g

300 – 350 fps: 0.2g

350-400fps: 0.25g

400-450fps: 0.28g

450 – 500fps: 0.36g

Over 500fps: 0.43g

I haven’t found the same variation in quality with 4.5mm BBs.  Most steel BBs from the big producers seem to be of very high and consistent quality with few blemishes or imperfections whether they say Blaster, Umarex, Crosman or ASG on the pack.  I tend to avoid steel BBs from unknown manufacturers – there are Chinese steel BBs around and though I haven’t tried them, I’d probably like to keep it that way.  I don’t use copper coated BBs because I find that they leave deposits on the inside of the barrel, though I know that many other shooters use them without problems.  I also don’t use lead balls in guns with smoothbore barrels intended for steel BBs either.  These lead balls are slightly larger than the 4.5mm steel BBs and they are often not perfectly spherical (or even if they start out that way, the soft lead can deform as they move through the feed system).  They also tend to leave deposits on the inside of the barrel. Lead balls are fine in rifled barrels, but generally not good in guns with smoothbore barrels.

The second thing that affects the way in which a BB travels down the barrel is the cleanliness of that barrel.  On a smoothbore barrel, any tiny speck of dust or other contamination on the inside surface can cause disruption to gas flow which will affect BB stability.  I have found that cleaning the inner barrel is the best way to quickly improve groupings and to reduce the number of flyers on any BB shooting gun.  Even a new replica will likely have traces of packing grease inside the barrel.

Cleaning the inner barrel is very simple.  Remove the barrel if possible, or at least dismantle the replica so that you can easily access both ends of the inner barrel.  If your replica has adjustable hop-up, turn it completely off (i.e. so that the rubber nub protrudes as little as possible into the barrel).  Make a simple pull-through using a piece of cord or string and a piece of clean, absorbent cloth. Do be careful what you use for a pull-through – many inner barrels are made of very light alloy and it’s frighteningly easy to cut the end of the barrel if you use a hard cord or wire pull-through. Soak the cloth in warm water which has a little washing-up liquid in it and pull through several times.  Finish off by doing the same again with a clean, dry piece of cloth.  That’s it!  Re-adjust the hop-up, re-fit the barrel and you will now have an inner barrel which is free of particles or deposits which are likely to affect the stability of the BB.

Cleaning the barrel from an ASG CZ75

Use top quality BBs and try shooting your replica after cleaning the barrel (and after re-adjusting the hop-up if fitted) and I think you’ll notice a marked improvement.  Groups should be noticeably smaller. Gas flow is critical on any BB shooter and gas flow through the barrel and around the BB is the single place where you can generate the most marked improvement.  Go on, give it a try.  And let me know if it works for you.

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Are all steel BBs the same?

Improving the accuracy of the Cybergun S&W M&P 9c


If you have read my review on the Cybergun S&W M&P 9c, you’ll know that I like it a lot. It’s a replica I shoot often and it gives consistent groups at six yards (the range at which I most often shoot). However, after a few hundred shots it shoots just over 2″ low using the recommended 0.2g BBs at that range. Now, I find that very irritating. I prefer a pistol that shoots high to one that shoots low, so I wondered if it might be possible to do something about it? The M&P 9c doesn’t have adjustable sights, I can’t easily modify the sights to improve things and I don’t want to fit a laser or some form of optical sight. So why not adjust the hop-up I hear you say? Well, in my experience with GBB pistols, hop-up makes little difference at six yards. It’s not completely ineffective, but I have found that adjusting hop-up will change the point of impact by only ½” or so at this range. Useful for fine-tuning, but not to correct an error of over 2″.

All the targets shown in this article were downloaded from the Umarex Boys Club forum (, though I filled in the centre circle in black, just to give a clear and consistent aim point for testing.

Establishing a baseline

There is no point in trying to improve the accuracy of your replica until it is giving consistent results. A combination of the pistol wearing in and you getting used to it is likely to change where your shots are striking over the first few hundred shots. I have now used my S&W M&P 9c enough to be confident that, with 0.2g BBs, it shoots just over two inches low and a hair to the right at six yards. For all testing, I used the pistol rested (to remove as far as possible errors due to my technique) and for each test I fired a string of ten shots with a fresh fill of gas. Although I show only one picture of each set of results below, I shot many, many more in the course of researching this article.


Ten shots, six yards, rested, 0.2g BBs. Group is 2″ and the centre of the group is just over 2″ below the point of aim (centre of the black circle)

BB weight

So, I have established the problem. But what’s the solution? The first thing to consider is BB weight. The recommended BB for this replica is 0.2g. In general, heavier BBs will lower the point of impact while lighter BBs will raise it. So, the first thing to try is 0.12g BBs to see if that will raise the point of impact.


Ten shots, six yards, rested, 0.12g BBs. Group is 3.75″ and the centre of the group is approximately 1½” below the point of aim (centre of the black circle)

Sure enough, using the lighter BBs has raised the centre of the group by around ½”, but the grouping is much worse. It’s clear that I won’t be using 0.12g BBs in this pistol. Just to check, I also try heavier 0.25g BBs.


Ten shots, six yards, rested, 0.25g BBs. Group is under 2″ but the centre of the group is almost 3″ below the point of aim (centre of the black circle)

As expected, the heavier BBs hit the target even lower, around 3″ from the point of aim. Grouping is good, but using different BB weights doesn’t seem to be the answer here.

So, what is the answer?

Next, time to have a look at the pistol and see if we can find anything that might be causing the problem. It doesn’t take long to find that the inner barrel is a very loose fit inside the outer barrel. With the pistol held level, the inner barrel is actually drooping slightly, which may be contributing to shooting low. The reason is easy to see.


The brass inner barrel is fitted with an O ring near the muzzle end. This is generally a good idea, because it helps to stabilise the inner barrel inside the outer barrel. Unfortunately, it isn’t working at all here.

t8With the inner barrel in place, the O ring is actually within the wider, threaded section of the outer barrel and isn’t making contact with the outer barrel at all. Hopefully the diagram below explains the problem (sizes and gaps are obviously exaggerated for clarity).


What can you do about it? A suppressor which screwed into the threaded part of the outer barrel might do the trick. If I had access to a lathe, I’d be tempted to cut a new O ring groove on the inner barrel about 15mm to the rear of the existing groove. This would then seat the O ring within the narrower (unthreaded) part of the outer barrel. However, I don’t have a lathe or a suppressor, so I need a simpler solution. The easiest is to add some packing to the bottom of the inside of the outer barrel, which will make the inner barrel sit straighter. After some experimentation, I used two layers of packing, each made up of a 3mm wide strip of duct tape approximately 40mm long, stuck inside the bottom of the outer barrel. Depending on what you use as packing you may need more or fewer layers. You’re aiming to have the inner barrel still able to move freely inside the outer barrel, but to be supported at the bottom. It’s a little fiddly to place the packing precisely, but it’s worth taking time to get it straight as if it’s off to one side, it will push the barrel off-centre.


With the packing in place, I tried shooting some more, concentrating on 0.2g BBs.


Ten shots, six yards, rested, 0.2g BBs, with barrel packing in place. Group is over 2″ but is centred much closer to the point of aim

OK, this is much better. It’s now time to start fine tuning by adjusting the hop-up. Moving one increment at a time, I tested until I was able to produce reasonable groups which are centred for elevation precisely on the point of aim.


Ten shots, six yards, rested, 0.2g BBs, with barrel packing in place and hop-up adjusted. Excluding the flyer on the right, the group is under 2″ and is centred for elevation on the point of aim


Well, that was easy! With a minimum of effort, I have been able to improve the accuracy of my Cybergun S&W M&P 9c.  With a three inch barrel it’s never going to be a tack-driver, but at least now I’m consistently producing groups right on the point of aim. The additional packing inside the outer barrel can’t be seen at all, and after around 100 shots is showing no signs of coming loose or affecting the performance of the pistol.

A poor fit between inner and outer barrels is a common issue on GBB pistols. Sometimes it doesn’t cause major problems, though it can contribute to inconsistent grouping. In the case of the S&W M&P 9c, it seems to have been the cause of the pistol shooting low and this simple fix it has increased my enjoyment of this replica.


You can buy the S&W M&P 9c at Pyramid Air here.

Related pages

Cybergun S&W M&P 9c review