[MENTION=85106]Morgan321[/MENTION] I posted this a few years back but you may find value in it...
Barrel temperature and its effects on 100y accuracy.
I receive the task from one in my Montana hunting party to develop a load for his 13 year old son to use hunting mule deer. The rifle is a Remington 700 ADL chambered in 223 with the light or sporter weight barrel profile (all factory and a 1 in 12 twist rifling). First thing I did was clean it so I would have place to start. While cleaning I discovered that this rifle has two barrel pads manufactured into the plastic molded stock located right at the end of the barrel channel in the 5 and 7 o’clock position. “So the barrel is not a free floater, thinks I, this ought to be interesting.”
We had decided to use the Barnes TSX 53 grain bullet so we would not have any fear of bullet blow ups if jr were to hit bone when taking his shot. I followed Barnes recommendations and had the bullets set to jump .05” into the lands. I worked up few test loads and went shooting.
What I found when shooting was interesting. This coming from a guy who’s rifles are all factory varmint contours or heavier. The first shot hit paper so I fired the first group before making any adjustments to the 3-9 power millet scope. I then made the scope adjustment and fired a second group. WOW an inch higher than the two inches I had adjusted for. I double checked the scope. Yup 1/4 moa indicated on the capped turret dial.
Anyway, I decided to move along and shoot the rest of my test loads. In which I found two loads worth exploring for the next trip; I am using IMR 8208 and just like I had thought, the better performing loads were right at the top end of Hodgdon’s loading data. Back to the loading room I go... and loaded my next set, this time choosing the two better loads. I loaded these the same but I seated the bullet .03” off the lands and a second set .07” off the lands. This time I brought some varmint loads to shoot that I had made for another rifle so I could see what was going on with this rifle/scope combo without expending expensive Barnes bullets.
When I got back to my range I fired a group of five of the varmint loads. From the resulting pattern I knew I did not have a scope problem but something else was afoot... shots 1-3 were all inside of a inch of each other in a slight horizontal climb and shots 4 and 5 were a full 1.5” higher also in a horizontal pattern. All shots were fired inside of a 90 seconds. “So” thinks I, “I bet I know what’s going on now.” More on that in a bit. I fired my test samples and was rewarded with a good grouping with the two loads seated .03” off the lands. Both were about 1” So I went back to the loading bench and loaded up some more of those this time just to zero the scope and get a crono reading. But before I did this I just had to satisfy my curiosity.
I disassembled the rifle and dropped the action into a BDL stock that had a Varmint barrel channel and heading back to my shooting area. The varmint barrel channel allowed the barrel to free float. Using the varmint ammo I fired a series of 5 shots over the same 90 seconds and before you think it... yes they all hit in a sporadic but circular pattern. So, I had been thinking that the barrel pads where causing the climb and I believe I proved this theory correct. But just not yet... My initial thought was as the barrel expanded from heat it increased the pressure on the pads and causing the impact to climb. Not to be proved wrong I put the original stock back on the action and torque it down to the setting I had used the first time when cleaning the rifle and waited for the new day to dawn.
Day two.
I headed back to my shooting area and started my new test. This time equipped with my super scientific barrel temperature monitoring device, a meat thermometer. Not the best I am sure and yes I will clean it off before using it on steaks. But it worked for what I wanted to do.
As a hunter we all know that the first shot is the most critical. A second may be required but the first one should be all that is needed. The air temp was 74 degrees when I started testing. I fired my first shot, stuck the thermometer down the muzzle and watched the temperature climb slowly four degrees and then start falling. I recorded the peak temp, and velocity (as I was shooting over the crono this time)and waited… and waited… and waited some more. After six minutes I was down to 76 degrees and I figured this was close enough. Fired round two and did the same as on the pervious shot. This went on for about 40 minutes for 6 shots or 2 groups of three. Guess what... Zero impact climb on target for each group.
So at 74 degrees it takes about 6 minutes with a 223 equipped with a 24” sporter barrel, .65” dia muzzle to get almost back to ambient air temperature. Hope you have enjoyed reading this as much as I have testing it.
