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.

Related Posts

Are all steel BBs the same?

Umarex Legends MP40 First Impressions

Pistol Place reader R-Gun Pete got an Umarex MP40 on Father’s Day. Lucky man – I got another ecard! But, what did he think of it?

The Umarex Legends MP40 was released in Canada in early June and my BB machine gun arrived just in time for Father’s Day. After opening the box and admiring this nice specimen for a little bit, I managed two complete cycles on the day I received it (4 CO2 cartridges and about 350 BBs).

Basically this will be a first impressions post.

This submachine gun is a realistic and weighty replica.

The magazine must be prepared with 2 CO2 cartridges and the instructions mention that it holds 52 BBs. In my case I decided to use only 50 to have a round number.

All the shooting has been done offhand, standing up and using the shoulder stock. First I shot on paper targets from two distances: 15 and 25 feet.

Not knowing the accuracy of the MP40, I started at 15 feet to make sure that I would not shoot outside my trap.

From the box, it was shooting a bit low and to the left. Flipping up the second rear sight took care of the elevation. For the windage, I discovered that placing the bullseye in the gap between the front blade and the left air space gap of the rear sight was giving me a point of impact where I wanted it.

The adjustment shots were made in semi-auto. The next step was to continue the same way for several more shots on a fresh target. Being confident that all the BBs will be kept contained in the trap I proceeded to have several full automatic bursts which were surprisingly bunched close together.

I moved out to 25 feet and continued shooting in semi-auto for the balance of the session. The system works well and will not waste CO2 because the trigger will just produce a click if there no BB present in the top of the mag (this means that the gun stops shooting on the last shot). When a fresh mag is re-inserted, the cocking knob is pulled to make the gun ready to shoot.

With a few full-auto bursts and mostly semi-auto shooting I was able to get close to 4 mags of 50 BBs for a total of approximately 200 without any problem. It should be noted that at the end the gas pressure was not enough to recock the striker, so for the last few shots I recocked the gun by hand for each shot until I felt that it was getting too slow. After removing the mag, I saw that only a couple of BBs were left unfired. When the cap was removed it could be felt that there was some pressure still in the reservoir.

I would say that realistically about 175 good blowback cycles could be achieved.

For my second session, I moved to my garage to engage pop cans. This time I had to split my session in two parts with a gap of several hours between. In the first part, I used a mix of semi and full auto for 2 mags and in the second part a few auto bursts and mostly semi-auto. It seems that the full-auto made a difference because this time I obtained only one full mag (#3) before starting to have some problems after only a few shots on the last (#4). The MP40 is brand new and should not have leaked CO2 during the time I was away, so it is probably the use of the full-auto that caused a lower number of shots (about 150 this time).

This would have to be confirmed by trial, but I suspect that using only full-auto will probably get 2 mags or less of usable shots.

This picture shows the paper targets covered in the previous text. The pop cans are not shown here but take my word that they were pretty much destroyed. The can lid shown in the upper right corner is from the full-auto test that will be explained a bit later in the post.

As for the problem I had at the end of my 2-part session, I noticed that there was no impact in my trap after a few shots on my last mag. After removing the mag and pushing a wooden rod through the barrel I found 3 BBs stacked together.

With the mag removed the breech is easily accessible to clear the barrel.

I suspect that there was not enough pressure to push the BB out of the barrel but there was enough pressure to operate the blowback to recock the gun. Since there was a BB present in the top of the mag, the trigger worked and a BB was then shot in the rear of the first with a repeat for the third BB.

Anyway when I unscrewed the cap there was barely any CO2 pressure left.

Some people might be wondering about the trigger pull. It is mentioned in some reviews as being heavy, and there is some truth to it but it is not overly bad. It is certainly not in the same category as a Colt 1911 or Tanfoglio Witness single action trigger feel but it is relatively easy to get used to it.

The other complaint is about the lack of a lock for the BB follower but this can easily be solved by using a small hook that helps to hold it in place during loading. I made mine from a small leftover piece of oak hardwood flooring.

To complete my post I decided to see how many mags the MP40 would run in full-auto. I was able to get 3 mags of 50 shots each without any problem. I pushed the limit by putting 10 BBs in the 4th mag and after a short burst it started to have BBs stuck in the barrel. After that test it seems that, shooting small bursts at a time, it would be possible to get around 150 shots. On the other hand, continuous fire might reduce that number.

In conclusion, for airgun collectors the MP40 is a nice addition that has an historical connection to a legendary model. Expectations should be realistic; this is not a competition target airgun but a very nice plinker.

R-Gun Pete

P.S. I had a piece of leather at home and I thought it would be a good idea to make a sling for my MP40. This is the result.

Making the Gletcher PM 1951 shoot to the point of aim

If you have read my original review of the Gletcher PM1951, you’ll know that, while it wasn’t a bad shooter, it consistently hit around 2” below the point of aim. In my recent COVID Backyard Shootout (you’ll find a link at the end of this article) it shot much better. Some people have asked what I did to it, so, here’s the answer…

I really hate replicas that shoot low. I don’t know why; I can just about tolerate something that shoots a bit high, but not low. On a replica such as this which has fixed front and rear sights, I can’t simply adjust the sights to take care of the problem. If this was a 6mm replica, I’d switch to using a lighter BB and/or adjust the hop-up. But on a 4.5mm replica, I don’t have either option. So, is there anything you can do about it if your 4.5mm replica shoots low?

Happily, the answer is yes. Sort of. Let me explain. If the point of impact is below the point of aim, you have two options: you can raise the point of impact by raising the barrel or you can lower the point of aim by modifying the sights. On the Gletcher PM1951, the barrel is rigidly fixed to the frame, just as it is on the Makarov pistol this replica is based on. That means that there isn’t a quick or simple way to change the angle of the barrel so the only option is to reduce the height of the front sight by filing it down. Luckily, that’s pretty easy.

Before you start anything like this, make sure that you first do plenty of reference shooting. You want to establish a known baseline before you start making any alterations. When you’re doing this, use the same steel BBs all the time to ensure consistency. I have shot a few hundred rounds through the PM and I am confident that the centre of typical groups is always around 2” below the point of aim. You can see a typical target below.

10 shots, Umarex Steel BBs, 6m, semi-rested. Point of aim is the centre of the black circle. The group is very respectable but its centre is approximately 2” below the point of aim. And yes, I know the target’s upside down…

So, we need to lower the front sight to raise the point of impact. On a replica where the sight is a non-removable part of the slide, the only option is to file the sight down. There are two things to consider here: the first is that a small change in the height of the front sight makes a large difference to the point of aim and the zinc alloy from which replicas are made is relatively soft so, don’t try to take too much off in one go. File off small amounts each time and then shoot to check how things are progressing.

The second thing is that, given the generally thin and fragile finishes applied to our replicas, it’s horribly easy to inadvertently put a large scrape or scratch on the finish of the top of the slide when you’re filing down the sight. Guess how I know that? That’s right, because I tried filing down the front sight of my Cybergun P226 X5 and I put a large scratch right along the top of the slide. So, you need to make a simple mask that will allow you to file the sight without risking damaging the slide.

On the PM, I removed the slide and then used a piece of card to make a mask which I taped in place. This left only the sight projecting and stopped me from inadvertently scraping the top of the slide. Then, it was just a case of filing a little at a time and testing by shooting.

Having said that you should take off a little at a time, I had to file the PM sight down more than I expected to get the point of aim and the point of impact to coincide. However, when I was done, I had a PM that shoots precisely where it’s aimed.

After filing, 10 shots, Umarex steel BBs, 6m, semi-rested. Aim point was again the centre of the black circle. The group isn’t as tight this time, probably because I shot fairly quickly, but at least it’s vertically centred on the black circle and six out of the ten shots are inside the inner circle. And yes, the target’s still upside down…

So, that wasn’t too difficult. In less than an hour I went from a replica that was hitting two inches below the point of aim at 6m to one that shoots pretty well where it’s aimed. The front sight has ended up smaller than I would have liked, but I’m willing to put up with that in return for a better shooting experience.

A quick dab with a black permanent marker pen and it’s good to go. Another job can be ticked off the list and I can start to really enjoy shooting this replica. You can see how well it did after this mod in my COVID Backyard Shootout.

Happy shooting

Related posts:

Gletcher PM 1951 review

COVID Backyard Shootout