Well, finally got a premiminary barrel break in finished with the big 277 and today the wind decided to die off around noon which was a suprise!! Only bad thing was that it was very dark out with heavy clouds so the chrono decided it just was not going to work today.
Still, I figured I would be able to work up in powder charge and at least try to find the limits of the 277 Allen Magnum and check the accuracy potential of this rifle.
My fireforming load consisted of 98.0 gr AA8700 under the 169.5 gr ULD RBBT. This formed cases very well but was a bit on the warm side with some of the cases. Nothing really but in a fireforming load, a bit more pressure then I wanted, still, hard to complain with sharp shoulders!!
Cleaned the rifle up after the last of the barrel break/fireforming loads and loaded up some serious loads with the 169.5 gr ULD RBBT. Seeing the pressure produced by the 98.0 gr of AA8700 under the 169.5 gr ULD RBBT, I started with 99.0 gr WC872 which has proven much slower burning then my current lot of 8700.
I also seated the bullet out to just kiss the lands with an OAL of 3.850". In the 2.700" case length, this leaves alot of that 169.5 gr ULD RBBT outside the case. In fact there is only about 0.150" of bullet shank below the case mouth. THis is by design as this rifle was designed to shoot the soon to come 185 gr ULD RBBT but also with the capabilities to use the 169.5 gr ULD RBBTs.
Here is a pic of the new 277 Allen Mag. On the left is a 7mm Allen Mag loaded with the 200 gr ULD RBBT to fit in an H-S Precision DM system while still reaching the lands in the chamber. On the right are two 277 Allen Magnums seated to the lands in the Xtreme Sporter V-Block single shot.
Load testing started at 99.0 gr WC872 and the inital test loads went up to 103.0 gr. The first two groups were less then spectacular running right at 1" ctc.
Load 102.0 and 103.0 gr tightened up pretty well but still not where I was expecting for this rifle design.
I headed back to the bench to check primer pocket tightness. All were out of box tight still. For a reference, the 270 AM will allow around 104.0 gr WC872 under this bullet with 3400 fps velocity with top loads.
Next batch of test loads started at 104.0 gr and stopped at 107.0 gr with one grain increments. Again, as with all my inital testing I am looking for the limits of this chambering, not doing a ladder test, just want to find that point when the case and rifle tells me its time to stop.
This time, with the rifle tuned in a bit better on target I set the rifle up at 250 yards instead of 100. First group was nothing impressive again, around 2.5" to be honest.
Group 2 with 105.0 gr put the first two shots within 1/2" of each other and I was getting happier until shot three dropped out of the cluster to open things to just under 2.5". I was not very happy with the rifle or my groups.
I talked up and taped off the old bullet holes to allow the barrel to cool totally before proceeding with the next test loads. I had three rounds loaded with 106.0 gr and only one with 107.0 gr. I only had 10 formed cases to work with and again, I was more interested in finding the top end load then groups at this point, even though groups are always important.
I fired the first shot of the 106.0 gr test load. Landed where I expected it to. Shot two did not change the target any. I suspected I was getting to the point in velocity where these bullets may start coming apart due to velocity and rotational stresses.
Shot three landed just an inch away from the first shot, up and to the right but at what looked to be only an inch away. I can live with that at 250 yards I thought to myself. I figured since I only had one of the 107.0 gr loads I would throw it down range and call it a way. Again, no change on target...........
I packed everything up and drove up to the target. I was very happy to see that the two bullet holes I could see at 250 yards were actually two pairs of bullet holes. The first two shots were on top of each other as were the last two. The two pairs were 0.981" ctc. She seems to be starting to shoot.
Best thing is the primer pockets are still tight as tight can be. I have test loads staring at 108.0 gr up to 111.0 gr and will test weather permitting tomorrow. Hopefully with some chrono data as well.
Like most of the Allen Magnums, it seems we just needed some increase in pressure to get things to settle down. With 20 rounds down the pipe it was time to give the rifle a total cleaning which is where it currently is, soaking in solvent.
Will report on more results weather permitting tomorrow. My plan is to get the top end load figured out, and then select a somewhat less then max load and do some 800 and 1000 yard testing before the 277 AM heads north to its new home. We will see what happens. If I can make it work I will report how she does.
All for now!!