Where has my velocity gone??
I don't know if this is the right forum for this or not, so feel free to move the thread. I am not new to rifles and shooting, but to be honest I have never kept a rifle long enough to turn a barrel into trash and I only started chonographing loads a few years ago. I have a couple of questions that are somewhat interesting to me, but they might be just business as normal to you folks.
First off, I just received a new barrel job from Pac-Nor in a 7mm Remington Magnum. The rifle is a Remington with a 26 inch tube, number 5 taper, 9" twist. I decided to try the Berger 168 VLD bullet for hunting. The chamber is on the short side so the bullet does stick down inside the case quite a bit. I worked up a load of 64 grains of Reloader 22 for a velocity of 2950 fps. The velocity is shy of what most manuals publish for various 168 grain bullets over that much powder, but I am getting way under one MOA and there is enough energy to serve my purposes so I am very pleased. I got similar velocity results with other powders as well. I am curious, do you suppose the velocity is a bit low for the pressures because the bullet takes up so much case capacity?
I then decided to try the load in my 7mm Sendero. I tried H-4831 SC, Retumbo, H1000 and Reloader 22. I got the best accuracy with Reloader 22 so I worked up to 64.7 grains seated .010 off the rifling. Before going any further, I can tell you I bought the rifle used and just from the feel of a patch going in the bore it's obvious the throat is well eroded. On paper I am getting sub MOA, but when I shot across the chronograph I came up with 2816 fps. Wow, plenty of thump in the shoulder, but nothing down range.
I ran the velocity and conditions through Exbal and came up with a drop of around 39.3 inches from 200 to 500 yards. I was at 200 and the target backer was about 42 inches high so I zeroed at 200 and shot a three shot group that measured about .6 center to center. The group was a tad high, but I accepted it as close enough because I just wanted a ball park confirmation that the chrono was working. Off to 500 yards. Three shots at 500 and I had a group with a vertical spread of around .75 inches and a horizontal spread of just under 2 inches. The group dropped 38 5/8 inches so I assume the chrono was OK. FWIW, I had some factory 150 grain Winchester Supreme Plus (the old moly stuff) in my range bag so I ran that across the chrono and it clocked at 3163 fps. That same ammo made 3223 in the Pac-Nor barrel.
As you can imagine I am scratching my head. I am assuming the throat is so eroded the gas is bleading off with the boat tail VLD. Is that a correct assumption? But I am wondering how the rifle can maintain it's accuracy with the throat so trashed out the velocity is in the dump? What gives? Time for a new barrel?