When one looks at and compares Monolithic bullets to Copper jacketed lead core bullets one of the biggest factors are the different materials Specific Gravity (SG). Typically lead has a SG of 11.34, copper has a SG of 8.9 and brass has a SG of 8.1. Jacketed lead core bullets have an SG of around 10.4 (this varies slightly according to jacket thickness. We can therefore see that on average solid copper bullets are 16% lighter than copper lead core bullets and brass bullets are around 20% lighter. What this means that in the exact same profile, length and diameter jacketed bullets will always be 16% to 20% heavier. (Most jacketed bullets have guilding metal jackets, not copper jackets. Guilding metal is copper with 5% to 10% of Zinc added to harden the copper.)
The next problem we need to look at is the manufacturing processes. Most monolithic bullets are turned on CNC lathes—the advantage of this method of manufacture is that set up is quick and relatively cheap which allows for smaller production runs. The process of turning has a further added benefit in that all components are always concentric which aids with the bullets accuracy.
Jacketed bullets are swaged. You have two components that are swaged individually and then combined and a final swaging operation will be swaging the combined jacket and core into the final shape. There are several problems that can occur during these processes, one of them being you are working with two different metals and linear coefficient of thermal expansion. Essentially this states that when an object is heated or cooled, its length and diameter changes by an amount proportional to the to the original length and diameter to the change in temperature. The factor for Copper is 16 and the factor for Lead is 29. A secondary problem is Springback, the geometric change that happens to a component at the end of the swaging process when the part has been released from the forces of the forming tool. The dimensional accuracy of a finished part is this affected, which makes it difficult to produce the part. As a result, the bullet manufacturer is faced with some real problems: Firstly, prediction of the final part geometry after springback and secondly, appropriate tools must be designed to compensate for these effects. The spring back between lead and copper are different and therefore makes it difficult to keep the two together at exacting sizes with no air gaps. In bullet manufacturing when boattails or rebated boattails are been swaged the possibility of springback is at its highest.
A further issue is in the drawing or extruding of the jacket is to keep the jacket walls concentric or having the same thickness all the way around the jacket, the J4 jackets are reputed to be the most accurate and are guaranteed to be within 0.0003”. J4 jackets are used by Berger bullets and a number of small precision bullet makers.
Let’s say during the swaging process tolerances are kept to a minimum with no air gaps between lead core and jacket and you fire a bullet at a long range target, due to friction the bullet through the air it heats up as it moves down range it now becomes possible due to the Linear coefficient of thermal expansion to start forming air gaps between jacket and core. We know that there is excessive heat generated in long range shooting as the bullet manufacturers had to go to a heat resistant plastic for the bullet tips as the older plastic tips were melting in flight on long shots.
What has this got to do with accuracy you might be asking at this stage? Way back in 1965 an American E.H. Harris started doing research on bullets and discovered that a bullet that has a 0.0001” (0.0025mm) concentricity problem would move the point of impact of the bullet 0.276” (7.01mm) when spun in a 1:8 twist barrel and 0.136” (3.505mm) when using a 1:16 twist barrel, so what we are seeing here is that any inconsistencies or imperfections in you bullet is going to be multiplied when shot in faster twist barrels. The same can be said for reloads where the bullets are not seated concentrically. The 0.0001” concentricity is one third of the tolerance that is allowable in the J4 jackets, I am sure you can see where this is going—there are inconsistencies in bullets you just need to keep them to a minimum. The large bullet manufacturers do an excellent job mass producing millions of bullets to extremely tight tolerances with very few problems.
Advantages of copper bullets:
Even when petals break of have excellent weight retention of between 70% and 90%.
Very rarely suffer from concentricity problems.
Easier and cheaper to change profiles in the manufacturing process
Healthier for humans and nature, contains no heavy metals i.e. lead. The technology surrounding monometal bullets will carry on in leaps and bounds and become far superior to Jacketed lead core bullets this will happen at such a rate that conventional bullets will not keep pace. In the past Benchrest shooting was the forefront of shooting technology and always lead the way as far as accuracy was concerned. If you take that rifle technology and combined that with cutting edge bullet technology that is what is required for Ultra Long Range Shooting as can be seen in the USA ultra high BC bullets of monometal design are starting to dominate these long range shoots. One just needs to look it is already happening see king of 2 mile.
Disadvantages of copper bullets:
Bullets of the same Length and profile will always be lighter than jacketed lead core bullets and will have lower BCs given the same length and profile.
Monolithic bullets generally need additional twist for the same weight bullets and this more problematic in the smaller calibres.
Copper is softer than Gilding Metal used in the manufacturing of bullet jackets and therefore shaves easier when seating bullets.
In the beginning monolithic bullets were playing catch up to the jacketed lead core bullets, but now we are currently in the transmission stage where the technology of monometal bullets is starting to overtake the jacketed lead core bullets. The swaging process in jacketed bullets is currently far quicker than the machining process and this drastically reducing costs. The downside to the swaging process is that it is nearly impossible to get perfectly concentric bullets. As technology improves the manufacturing process time will decrease with turning, until there is not much time difference in the manufacturing time between the two processes, pricing of monometal bullets will then reduce making them more economically viable and they will become more popular. It is possible to manufacture bullets that are more accurate and consistent by turning than by swaging and this will become more so as machine technology improves. (Currently machine manufacturers are claiming that that the latest ranges of CNC lathes when operated in airconditioned environments it is possible to maintain 0.001mm 1um.) Compare that to the current tolerances of the better-quality bullet manufacturers they cannot get close.
If one looks at the Ultra Long Range shooting scene in the USA it is dominated by the monometal bullets. The King of 2 Mile over the last 3 years the top bullets were CUTTING EDGE BULLETS and the BERGER experimental monometal bullets. I believe this the way technology and the industry is moving. There was a post this week where a gong was shot at over 6000 yards with a .416 Barret using a 550gr Cutting Edge Bullet of monometal design. These feats just go to show that monometal bullet technology is now pulling ahead of the technology used in jacketed lead core bullets.
A good tip for reloading:
You absolutely NEED to chamfer and debur the inside edge of the brass that will contact the bullets. I learned this lesson the hard away and saw very thin pieces of copper shaved off of the bullets sitting just above the top edge of the brass after the bullet had been seated. I didn’t find any huge accuracy problem generated by this scenario (yes I shot them just to see what the effect would be), but you don’t want to taunt the bullet gods.
Keep bullet neck tension between 0.001” and 0.002” to allow for sufficient neck tension as you have less bearing surface due to the driving bands.