A Boring Topic

A Boring Topic

When you can and can’t shoot a .177 caliber lead BB 

By Dennis Adler

The caliber conundrum, when is a .177 caliber not 0.177 inches? When it is a steel BB (far left) which actually has a diameter of 0.173 inches (average) or 4.3mm, compared to a lead round pellet (center) which is just slightly larger at 4.5mm, or a wadcutter pellet (right). The difference in diameter is what keeps you from loading a lead ball into a .177 caliber, magazine-fed blowback action pistol designed for steel BBs. This is the same whether it is a stick magazine or a self-contained CO2 BB magazine; that .2mm difference is a lot with an air pistol.

The operative word in Airgun Experience is experience, and the way you get experience is by doing things and often doing them wrong. Failure is the best teacher, and hopefully it isn’t always costly, just educational. One of the early mistakes I made was trying to shoot .177 caliber lead BBs from a semi-auto pistol chambered for .177 caliber steel BBs. Lead BBs don’t fit. An air pistol that shoots steel BBs and is marked .177 caliber (4.5mm) does not actually shoot a .177 caliber diameter BB. Now, if it is a pellet-firing rifled barrel pistol it can, because the bore on a .177 caliber BB pistol and a 4.5mm pellet pistol are not exactly the same. A steel BB will drop right through the barrel on a smoothbore blowback action BB pistol. A 4.5mm pellet won’t even fit if you try to insert it, whereas with a pellet firing pistol you can actually push the pellet into the barrel.

Yes you can and no you can’t. A lead BB will not drop into a steel BB magazine as shown at left, it is larger than the loading port, good thing too, because it won’t work in the gun either. On the other hand, you can put a round lead pellet into a rotary magazine instead of a wadcutter or other pellet, though there is no reason to. At far right the design that gives two options; guns like the Umarex Beretta PX4 Storm or latest PPQ can load either pellets or BBs. Steel BBs are what most will shoot, but that can erode the rifling over time, so you can use a 4.5mm round lead ball if you want. Better for the life of the rifling and more accurate than a steel BB.

A properly sized lead pellet measures approximately 4.5mm in diameter and so does a .177 caliber lead round ball, like the popular Gamo Round 4.5mm (.177 caliber), but these are intended for use in rifles or pistols with rifled barrels. Just like a pellet, a 4.5mm round lead ball takes the rifling in a pistol or rifle and is about as accurate for a lead sphere (in relation to its caliber and velocity) as a round lead ball fired from a rifled barrel percussion pistol.

The 1851 Navy is an excellent example of a percussion pistol. Chambered in .36 caliber, the round lead bullet (ball) measured 0.375 inches, slightly larger than the bore. The ball was sized as it was pressed into the cylinder by the rammer (lever under the barrel which is hinged to pull down and push the rammer over the bullet and seat it into the cylinder). This is a copy of one of Wild Bill Hickok’s engraved 1851 Navy revolvers. The real gun is in the Autry Museum in Los Angeles. The copy was hand engraved for Colt’s by John J. Adams, Sr. of Adams & Adams.

I use the percussion pistol as an example because this is where round lead balls are near equals. Let’s take a .36 caliber percussion pistol; for example, a Colt 1851 Navy. The lead ball that is fired from that pistol has a diameter of 0.375 inches, slightly larger than the bore. If it were exactly the same size, it would not take the rifling and be about as accurate as a smoothbore pistol or musket. The slightly larger diameter lead ball is forced into the cylinder by the rammer and in so doing shaves off a little of the bullet, instant resizing. This makes a very tight fit and it requires the correct amount of black powder to develop enough energy to drive that round ball down the barrel, let the rifling stabilize its rotation, and head downrange at an effective velocity. Scale everything down to 0.177 inch diameter (4.5mm) and the same thing happens when you load a .177 caliber lead BB into a rifled barrel single shot rifle or pistol. The lead BB is just the right size to fit snugly. When the air charge drives it down the barrel the BB is similarly stabilized by the rifling which makes it more accurate than a steel BB down a smoothbore barrel. This is a very rudimentary explanation, but the point is, barrel caliber and BB size is not the same.

Airguns like the Umarex Beretta PX4 Storm that use reversible rotary stick magazines make it possible to shoot what you want, steel BBs, 4.5mm lead round balls or 4.5mm wadcutter pellets through a rifled steel barrel.

So why doesn’t that work in a CO2-powered blowback action BB pistol? Because smoothbore .177 caliber air pistols actually fire a steel round that has a diameter of only 0.172 to 0.173 inches, not 0.177 inches, or expressed in millimeters, 4.3mm, not 4.5mm. It is an incrementally small difference but in an airgun it is enough. Another important thing to know is that a .177 (4.5mm) round lead ball is a pellet, not a BB. But wait, you’re screaming there used to be lead BBs! And you’re right, but they were done away with for BB guns in the 1920s and replaced with steel BBs (pioneered by Daisy), which could be more accurately sized in manufacturing. And even there, steel BBs have small variances in diameter, as do lead pellets and the Gamo lead balls. There are also different grades of pellets, and more costly competition pellets are graded by lot numbers from one unbroken production run, for as much consistency of size as possible. Even with Meisterkugeln Professional Line pellets, you can go through a tin and find slight variations, some just a thousandth smaller, some larger, but not all the same. Many of you have run across this problem loading pellets into rotary magazines, some pellets are harder to insert, and some can actually fall through; I’ve had it happen, too.

Sig Sauer raised the bar for capacity and loading last year with their rotary pellet magazine design. It was introduced on the P320 ASP. Later this year an improved version of the P320, based on the military model and designated P320MHS ASP will introduce a first ever self-contained rotary pellet and CO2 magazine, blowback action and full functioning features, a groundbreaking design for CO2 rifled barrel semi-auto pistols.

There is yet another factor, magnetism. Most self-loading CO2 powered air pistols use a magnetic ring (rare earth such as Neodymium) to hold the steel BBs in place at the firing port; not so with pellet loading repeaters which have the pellets in individual chambers, (pellet-loading cartridges or rings like cast alloy or injection molded pellet magazines, or belt fed magazines like the new Sig Sauer models). The only way to load a .177 caliber round lead ball into a semi-auto would be using the same magazines that hold lead pellets. Models like the Umarex Beretta PX4 Storm even say you can shoot either. But that only works with rotary magazines; a 4.5mm lead ball won’t fit through the loading port on BB pistol magazines, so you never get any further than that.

In order to build a rifled barrel, semi-auto, CO2 powered, blowback action, self-contained round ball magazine model, you would need to make a dedicated lead round ball firing pistol and magazine with a feed design that did not rely on anything but the follower spring to hold the round ball is position for firing. Not exactly back to the drawing board but really too fine of a line to cross.

If you want to step back in time and draw a single action from your holster you can pretty much have what you want with rifled barrel pellet cartridge firing models like the Umarex Colt Peacemaker (top shown in deluxe hand engraved edition) or you can shoot BBs or pellets from the smoothbore Bear River Schofield (right shown in deluxe hand engraved edition) or the new Remington Model 1875, which actually comes with both BB and pellet firing cartridges.

Lastly, you’re probably wondering why then can you shoot a lead pellet out of a smoothbore revolver like the Bear River Schofield or the Remington Model 1875? Because the difference in size between a 4.5mm pellet and 0.172 to 0.173 inch diameter steel BB isn’t great enough to prevent a lead (or alloy) pellet fired from a cartridge from traveling down the smoothbore barrel. Just use pellet loading cartridges instead. The Remington even comes with both.

Things are about to change

Before this year is out, this entire discussion about greater accuracy and authenticity of handling with a rifled barrel semi-auto using a self-contained CO2 pellet magazine will become a moot point. Sig Sauer is about to reinvent the wheel with a CO2 magazine and rotary pellet mechanism in one, and if their solution works with the forthcoming P320MHS ASP, we will be entering a new era of blowback action CO2 pistol design. And if you still want to shoot 4.5mm round lead balls out of it, that’ll probably work, too.

6 thoughts on “A Boring Topic


  1. Very informative. At first I thought the new SigX5 pellet pistol was going to use the integral magazine /co2 system but it appears it will use the existing system with the320 current mag . The new 320 will use the integral system . It is also using 3D printed parts which should open the door for new airgun designs


  2. OK, let me make sure I understand this. Are you saying that I can safely use the Gamo 0.177 lead balls in any of my rifled barrel Umarex pellet pistols that use the 8 shot metallic magazine disks?

    Ever since the forth coming Umarex Cowboy Lever Action Rifle was announced, I’ve been wondering if it might possibly be able to shoot pellets, particularly the lead-free alloy pellets. Based on what you are telling us here, shooting pellets may not be possible because of the narrower smooth-bore steel BB barrel in the Cowboy Lever Action Rifle.


    • I am going to proof that theory this week with several pellet firing pistols using the round pellets instead of wadcutters. As has been demonstrated in previous articles you can shoot the pellet loading cartridges through smoothbore revolvers, and since the forthcoming lever action rifle uses the same BB loading cartridges as the pistols and you can also shoot the pellet cartridges through the smoothbore single actions, it stands to reason you should be able to do the same with the rifle. I will find that out as soon as a test gun arrives.


      • The cartridge firing lever rifle should herald in a tsunami of co2 cartridge rifles. I would suggest a Winchester 73, 82 and a Mares laig Josh Randall 92. Remington Rolling Block , Sharps as well



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