Brand new to the .243 AI so take this FWIW. Just got my LH M700 SA back from the smith and finally got it to the range this weekend. Barrel is a 24� Remington factory take-off I bought from the classified here a couple of years ago.
After fireforming 50 cases this past Sunday, I checked the capacity increase from cases of the same lot. Notes are at home, but the average gain in case capacity (weighing cass water filled to the base of the neck) was about 2 grains of water, or a whopping 3.8% capacity increase.
Using the 1 in 4 rule, that should allow about a 1% increase in velocity. Figuring around 3000-3050 with 100 grain bullets for the parent, that gives us 3030 to 3080 fps. Obviously not something to write home via Priority Mail about.
Factory ammo, Remington 100 gr. Core-Lokts, gave just over 3000 fps in the original chamber, about 3020 IIRC, and rounds from the exact same box of ammo gave 2949 fps in the improved chamber, a velocity loss to be expected.
But in fireforming, my velocities were a bit puzzling. I was using max book loads (for the parent case) of RL-22 and H-4831 under Sierra and Hornady 100 grain bullets, 44 and 45 grains each. 45.0 grains of H4831 with a Hornady 100 BTSP in my 23.75� barrel gave 3135 fps, it gave 3096 fps with the sierra bullet. That�s cooking. In previous trials with this rifle I�ve gotten 3050 fps pretty easy with the same bullets and loads of 42.5 grains of IMR-4350 and (the exact load escapes me here) IMR-4831.
Now, this is a little fudgy logic, but the .243 loads have been tamed due to it�s propensity for wider pressure swings in any given string. So, I�m using loads that the books claim to give 3000 fps from their 24� test barrels, in a larger chamber, and getting 100+ fps more. Pressure signs (yeah, I know) were totally missing. Primers had a good, well rounded radius, bolt lift was easy, every subjective indication said pressure was very civilized.
Not that I�m unhappy, just cannot explain the increased velocities from this enlarged chamber. Well, maybe � I had the bullets seated to touch the lands for fireforming, that would be the only difference over the normal loads where bullets are about .020� off the lands. That alone would be telling evidence of being careful when you seat any bullet hard up against the lands. Oh well, something for more experimentation.
Anyway, my whole reason for AI�ing the .243 was not for increased velocity anyway. I just wanted to tame it�s internal ballistic weirdness and it�s humongous appetite for eating throats. Prior to AI�ing, this brand new barrel had just over 600 rounds through it of moderate loads, and throat erosion had already advanced .070�.
Now that I�m getting my target velocities with book loads for the parent I know pressures just have to be mild (a goal I increasingly strive for as I get older) and I�m not so worried about those famous kabooms with 100 grain bullets the .243 apparently likes to spring on people as the throat gets all eaten up, I�m a happy camper.
