I was testing the rifle with Richard Graves and Rob,a friend of ours.
Fireforming loads using 215 gr H-50BMG under the 750 gr A-Max were producing an average velocity of 2455 fps and litereally one hole groups at 100 yards! The temp was around 90 degrees so we were only shooting two shot pairs for fireforming but every pair shot on paper at 100 yards had to be looked at at under 15 yards to tell if there were actually two bullet holes there. I was amazed to be honest.
We then started loading the formed 510 Allen Magnum cases. TO do so, since I do not have the dies made yet, I machined a neck sizing bushing that threaded into my press extention which I use to form 408 CT cases into 338-408 cases to fireform in my 338 Allen Mag.
The neck sizing insert worked perfectly and run outs were actually pretty good for such sizing techiques and tooling.
Seating bullets on the other had was about as crude as could be. The 50 BMG and 510 Allen Mag are so long that even in the RCBS Ammomaster press and my extension threaded into the top of the press, the bullets would reach proper seating depth well before the ram would reach the top of its stroke.
As such, I had to mark every bullets ogive at 0.505" diameter and then measure from the case head to the ring on the bullet ogive to get them to proper seating depth. I will admit this is about as inconsistant as it gets but thats all I had at the time to seat bullets with.
It took some time but most test loads were held to within 5 thou in seating length.
We started with the same 215 gr fireforming load but in these rounds we used Richards huge 835 gr ULD RBBT Soft point. Here are the velocity results for each load. These are two shots for each load:
215 gr............2375 fps
220 gr............2420 fps
225 gr............2540 fps
230 gr............2610 fps
235 gr............2668 fps
237 gr............2700 fps
Accuracy wise, I was a bit unimpressed with the starting loads, Richard was sick to his stomach!!! We were getting severe vertical stringing with the first two test loads. But knowing that Richards bullets tend to shoot much better the harder you push them, we were a bit down but no where near giving up.
At the 215 gr and 220 gr load level, we were getting 8 to 10" vertical stringing for the pairs of shots.
At 225 gr the spread dropped to around 4" and at 230 gr the pair again tightened to a 2" spread.
At 235 gr, the two shots cut each other in half, doing just as the A-Maxes were doing but with 150 fps more velocity and 85 gr more bullet weight.
I had two rounds left at the 237 gr level and decided to shoot then at the same target as the 235 gr loads. First shot again cut the two previous shots and the second landed 3/4" off the group but to be fair the barrel was getting VERY warm and I was getting a bit jumpy from the recoil which is not bad at all but mentally it wears on you being pushed that much while shooting.
Both Richards and my spirits rose dramatically after seeing that. Three shots well under 1/2" at 100 yards and the fourth opening things to 3/4" pr so out of a hot dirty barrel.
We were experiencing one issue that I wanted to correct and that was the shoulders which were retaining a rounded shoulder on the body/shoulder junction. The shoulder/neck junction, by design has a relatively large radius but I wanted the shoulder to sharpen up and even with top end pressures this was not happening.
We talked it over and we decided to anneal a case, just at the shoulder and see if this helped. I also used a pretty stout low consisting of 245 gr H-50BMG which was a full 8 grains over what we had topped out on the range test.
We just went out beside my shop and shot into the big cotton wood stump I use for a bullet stop, yes, it is 50 cal proven!! Big stump!!
At the shot everything was normal. Just a hint of a pop was needed to pop the bolt over the mechanical cam in the receiver but extraction was effortless. Best yet, the shoulder was perfectly formed, nice and sharp.
We did not shoot over the chrono with that load. More interested in the shoulder forming but from looking at the other data, I would estimate that load to produce in the 2800 to 2825 fps range with the 835 gr pill.
Thats 14,540 to 14,800 ft/lbs of energy.
Here is a standard 50 BMG on the left loaded with the 647 gr Ball. In the middle is the fully formed 510 Allen Mag with 835 gr ULD RBBT Soft Point and on the right is the "little" 7mm Allen Mag.
Close up of the shoulder angle. Annealing was the trick, worked perfectly.
From reading the data on Hodgdons new US869 compared to H-50BMG. IT is producing anywhere from 125 to 150 fps more velocity for any given bullet weight in the standard 50 BMG up to 800 gr so I will be testing it in the 510 Allen Mag here soon to see if that holds true for a bit more case capacity and heavier bullet as well. I can not imagine a 835 gr pill going 2900 fps but we will see what happens!!