I have gotten close! Got to 5400fps, but not 5600 in the 300 Win Mag.
I was using homemade bullets with J4 1.00" jackets and loading them with a small ww core. Bullet weight was just over 75gr. Ogive from that die I think is 6.
I took my 300 Win Mag brass and necked it down to .22 caliber. I then opened it back up so only about .030 of neck was back to 30 caliber. This left kind of an hourglass shape right at the shoulder. I put in 52 grains of Uneeque followed by 21 grains of 295. The last little bit of powder had to be tamped in with an ice pick. I seated my bullet where the boattail just filled up that hourglass area and the main body of the bullet was .030 into the brass.
I lit these loads with a CCI BR 400 primer. Because the bullet was so light, there wasn't a lot of recoil. No more than my 45-70 with 610gr bullets.
Anyway, I asked a buddy of mine if I could borrow his little chrony so I could see how fast I was pushing these loads. Of course when he heard what I was loading, he had to come watch. He is a cop so he brought his patrol car. He looked at the loads and kind of laughed at me. He thought I was nuts. He would not even stay close to the bench when I fired the first shot. He was sitting in his car. I fired and the muzzle blast knocked his little chrony over and broke the glass window thingy. Couldn't confirm the speed so I just shot a few more shots at the target. First group was in there about .421" at a hundred yards. One of the best groups I ever shot. When I took the target down, I saw that the bullets had gone clean thru that old propane tank and cut that metal just like a wad cutter cuts paper.
I went back to the car and showed my buddy the target and the broken glass think on his chrony. He was mad about me breaking it, but was surprised when he saw my .241" group. He said just for fun that he had been tracking my bullets on his radar. Said per the computer they were coming out a few thousand miles an hour which computed right at 5400fps.
He was really suprised when he looked at my brass. Where that little hour glass shape was it was not almost perfectly straight. That round had so much pressure in it that the shoulder and neck opened up until it was almost a straight wall case. My buddy tried to see if a 45 bullet would fit. It wouldn't quite fit, but his back-up 40S&W round fit right in. We decided we would take all of my once fired 300 BS brass and use it in a 40x300 Encore barrel. He thought it would be a little over-kill on white tailed elk. I let him know I intended to use it on my next sasquatch hunt up in Canada. It's been a couple of years since I got one of them and my jerky supply is running low.
Well I best get off of here. I've got some errands to run and since the snow is gone, I can take my Ferrari in to have the studs taken off.
Have a good day!
Beal Sheeter