Many shooters mistakenly mislabel various phenomenon surrounding precision rifles. Often this ignorance at best leads to a misuse of a term, but at its worse, it can lead to improper technique or even the denial of occurrences. The “cold bore” phenomena is exactly such a phrase that instills much confusion thanks to hundreds if not thousand of the self reported anecdotal opinions of amateur shooters. To really diagnose we need to first define some terms.
"Cold Bore Shift" - Is a
consistent and
repeatable deviation between a point of impact and the desired point of aim, that occurs when the rifle has be fired in a state in which the temperature of the bore and the ambient environmental temperature is the same.
^This is kind of what it sounds like you have going on but to be honest you hardly have enough data points to correlate a trend. Being that it's an AR, they are also know to sometimes have "first round shifts" which manifest similarly but are not directly linked to temperature. Also note that not every rifle is going to have a cold bore, in fact most don't which is where the second term comes in...
"Cold Shooter Shift" - Is an
inconsistent and therefor
unrepeatable deviation between a point of impact and the desired point of aim, that occurs due to the shooters inability to engage the rifle in a consistent physical manner. This can manifest itself in variety of relations.
The easiest solution is to map out the rifle and determine what's actually occurring. This process should be then be repeated at least 5 to 10 times (the more precise the rifle generally the quicker process) in order to create a composite that denotes the potential average cold bore shift as to the average mean point of impacts from a warm bore. This averaged angular deviation can then be compensated for via dialing or holding in conjunction with ballistic solutions.
Step 1: With the a previously zeroed rifle (turrets set to “0/0”) that has been allowed to cool to ambient temperature (I normally make this my first rounds of the day), assume a good fundamentally sound prone position and fire one round at the left target.
Step 2: Shifting your natural point of aim to the right target, fire 4 rounds to determine a mean point of impact.
Step 3: Using the linear grid, measure the vertical and horizontal shift (if any) from the mean point of impact of the last 4 rounds to the 1st round. Then covert the linear measurement to an angular measurement.
Again key here is you're looking for consistent and repeatable. If you where to over lay a series of say 5- 10 targets and you notice a trend like this, then your rifle likely has a cold bore shift. If you find that's not the case then its likely you have something else going on.
