I work as a Ballistic Engineer and pro shooter. I spend a lot of time on ranges firing a variety of weapons. Not only do we conduct firings for new developments but also proof production runs for accuracy. This allows me to gather a lot of data which I simply could not afford to do if I were paying for rounds and range time. I am also responsible for the cleaning and maintenance of the firearms we use.
Something I worked out many years ago was that cleaning had a temporary negative impact on accuracy. Of course not cleaning at all also has some unpleasant results, accuracy being one and damage to the weapon being another. I began to leave the bores dirty for what I felt was a plateau in accuracy. Some rifles and calibers will shoot really nicely for a couple of hundred rounds between bore cleaning. Each rifle was different. Once cleaned I found it would take a good few rounds to settle back down – again dependant on rifle. I theorized that accuracy improves when the bore is lined with projectile material. The material covers imperfections in the Bore and provides consistent friction and possibly a consistent gas seal. Back then I had no way to prove it. The theory ties in to some extent to this article however I now think there are more variables than just projectile material.
So as stated I had been leaving the bores dirty between firings. I began to notice that the first few rounds after a nature change would be inaccurate and also shift point of impact (POI) considerably. Obviously different natures have different POI but I am talking about POI shifts occurring during shooting rounds of a different nature. I would sometimes shoot multiple natures through the rifle in one session. An example would be 25 standard OTM ball rounds, 25 AP, 25 monolithic barrier defeating. If the rifle had previously run ball then these would produce expected accuracy (around 0.5MOA) apart from the standard cold bore shot deviation. Moving on to the AP the first couple of groups would open up to around 1.2MOA. After a few rounds the accuracy would tighten right up again. The same would occur with the monolithic – first few poor followed by good accuracy. I would often see POI shift within the first few round as well. They would group poorly but then a shift would occur and they would begin to group in a different area. Some weapon/ammo combinations made for considerable shift while others were not as severe. When I say severe I have seen as much as the width of an A3 target at 75m! Obviously this would extrapolate to complete misses of target at increased range which I have experienced as well.
At first I thought these may be fliers or inconsistent ammunition so I began to look deeper. The ammunition has a very thorough quality inspection as it is made and when it is complete. There was no doubt looking back at quality records that the ammunition had very consistent charge weights and dimensions. This was further backed up with pressure and velocity readings which were also very consistent.. until you changed nature in the same proof barrel. A different nature produced a much larger extreme spread in velocity. There would be several rounds which were noticeably different to the expected velocity. This is where things get a little odd.. I have also seen that happen when changing propellant and even primers. Not to such a notable effect as changing projectile but it was there. There didn’t seem to be a hard and fast rule on number of rounds to settle the accuracy and POI. Some guns took a single round while others took 10 or so before they began to shine. I was able to repeat this with the guns on consecutive occasions. The solution would surely be to clean the bore thoroughly between nature or component changes. As mentioned at the beginning of the article the cleaning has an effect as well so the answer for me (with plenty of time and rounds) is simply to shoot conditioning rounds until accuracy and POI stabilize.
The inconsistent velocities after nature change suggest that there are changes to friction between projectile and bore. Once enough material has been deposited inside the bore then friction becomes more consistent and material deposits would reduce (at least that is my hypothesis). I believe propellants and primers contribute to the bore condition to a lesser degree. This is backed up with some studies on green primers (lead free). The green primers produced inconsistency in velocity and reduced velocity. The study suggested this was likely due to the lack of lead styphnate which appeared to aid consistent bore friction.
One could argue that the changes in velocity are just coincidence and attribute them to inconsistent powder weights. I have seen it happen with such regularity I do not buy that explanation. I cannot think of any other reason that a change in projectile material or powder/primer would produce temporary inaccuracy or POI shift.
Mitigating the effect
For most of us these things are not a serious problem. Most match shooters stick with the same ammunition. However imagine you shoot target and hunt with the same rifle. You might have run monolithics for some long range target and then go hunting the next day. Your first few shots are likely to be effected. Not good. Many would suggest you simply clean the bore between nature changes. Not a bad idea but cleaning produces a similar inaccuracy issue as discussed above. The only way to be sure is to get to know your rifle and begin recording the amount of rounds it takes to settle back down to its accuracy groove. Then just run that amount of rounds at the range before you take aim at a Trophy Buck.
If you are a Mil/LE user then this gets a little tricky. You cannot always run warm up rounds when changing nature. What you can do is study the effect and mitigate for the POI shift just as you would for a cold bore. Be aware of the possible inaccuracy after the nature change and factor that into your target acquisition.