Reloading Basics - Bullet Diameter
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- Last updated: 26/09/2021
While guns are generally described by calibre, like .243, .308 Winchester etc. there are both anomalies and variations of bullet diameters within a specific calibre. For example, both .30-06 and .308 are often both referred to as ’.30 calibre’ and in .357 magnum, bullets of .356”, .357” and .358” diameter are all readily available. So what does this mean for the reloader? And how do you ensure you are using bullets that are the best diameter for your specific gun?
For the purposes of matching bullet size to your gun, the groove diameter is the maximum distance between grooves in the rifling or the width of the space that the bullet must fill to seal the bore. The ‘bore diameter’ is the distance between the lands and is the smaller measurement. There are many guns generalised as ‘.30 calibre rifles’ like ones chambered in .303, 7.62, and .308 Winchester, all with a bore diameter of approximately .30 calibre but the groove diameters and the bullets they use are different. The old .303 Lee Enfield rifles generally use a .311” bullet while the .308 Winchester, as the name suggests, prefers a .308 diameter projectile.
Measuring the diameter of a barrel, a process known as ‘slugging the bore’, involves driving a welllubricated, soft lead bullet or ball into the lubricated barrel of a rifle, knocking it back out, then measuring it at its widest and narrowest points. There are demonstrations of this process available on the internet and with patience, it does yield reliable results.
Alternatively, if slugging is not your thing, you can simply take a ‘trial and error’ approach using bullets available for your gun, and calibre, in appropriate diameters.
For a bullet to function safely and accurately it must interact with the bore surface of the barrel correctly and it is important that it engages with the rifling properly, so that it can gain the spin necessary to stabilise it in flight. A lead bullet will expand to tightly fit and fill the barrel in a process known as obturation and this seals the barrel and prevents the hot gases and pressure behind it from escaping. If the gases do escape they can burn and melt the sides of the bullet, preventing the rifling from working properly. This can have a detrimental effect on accuracy. Heavier lead deposits in the barrel are often an indication that bullets are not obturating properly. There can also be a significant loss of pressure as the gases escape and bullet velocity will be reduced. Very hard bullets will not obturate as well as softer ones and hollow based bullets will obturate much better than solid based projectiles.
Jacketed bullets will not obturate to a significant degree and it is, therefore, more important to match the diameter of the bullet to the barrel exactly. If gases and pressure escape around a jacketed bullet then it is too small and not sealing the bore.
Oversized lead bullets will generally be squeezed down and resized in the barrel but if they are too far oversized, by that we mean more than 0.001” larger than the groove diameter, this can result in some lead being shaved off the bullet and left in the barrel. Jacketed bullets that are more than 0.001” larger than the bore diameter will not be so easily sized down in the barrel and will cause increased chamber pressure, which can be dangerous.
If you have successfully slugged your barrel and obtained an exact diameter, you can then purchase bullets to match that measurement as a starting point. If using lead bullets, then choosing a bullet that has a diameter that is 0.001” larger than the groove diameter is a good place to start as it will fill the bore and lands properly and give a good seal.
Sometimes bullets advertised as a particular size might actually vary in diameter due to manufacturing tolerances, for example, some branded .223 bullets measure .2240, and others .2247. This minor difference can affect the performance of your ammunition and if you are unhappy with your particular load it can be worth changing the make of the projectile you are using.
If your lead bullets seem oversized and you are getting a lot of leading in the barrel, it might be worth resizing them slightly. For example, .358” lead bullets can be pushed through a sizer and taken down to .357” or even .356”. Minor changes like this can often have a noticeable effect on accuracy so are well worth a try.
Jacketed bullets are not really suitable to be resized because when you squeeze the lead and jacket down, the copper jacket can ‘spring back’ to size and detach from the lead core. There are sizing dies in, for example, .308” and although intended for lead bullets, these can be used, with care, to regularise and check a batch of jacketed bullets. There are videos of how to do this online and they all show lubricating jacketed bullets before passing them through a sizer up to half a dozen times to get satisfactory results.
Although most .308 calibre projectiles are jacketed, there are lead bullets sized to .309 that can be used in this calibre in ‘light loads’, with great results. Being slightly oversized ensures they give a good seal in the barrel and as long as you do not try to drive them too fast, they will not strip lead in the barrel and are very accurate at sensible ranges.
Ensuring you are using the correct bullet is relatively straightforward, but if you do decide that you want to try a different diameter, you must stay within the limits for your particular calibre. Resizing what you have, or buying something just 0.001” larger, might have a big effect on accuracy but you must do so with caution, as always when reloading; Safety First.