A Boring Topic
When you can and can’t shoot a .177 caliber lead BB
By Dennis Adler
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.
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.
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.
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.
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.
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.