Seat the things for function and cut your loads if pressures are high.
I seat my 75 Hornady’s to a nominal 2.25”, which is magazine length for a AR15.
I conducted load development with various powders, N140 worked very well and I have a lot of that. So I am still loading my 75’s with that powder, though I think next I will use Varget. Maybe IMR 4895.
I am using 24.5 grains N140 with a 75 Hornady. In my Wilson barrel 25.0 was an overmax load, 24.7 a max, so I stayed with 24.5 grains. If you have a Kreiger barrel, cut the load by two grains.
It is difficult to interpret pressure signs in a AR15. Since it is a semi automatic mechanism you don’t get the warning signs of a stiff bolt lift. Since it is a push feed primers look flat at low pressures.
I found that lubricating my test cases with case lube, oil, whatever, I can see a transition between rounded primers and when the primers start to flatten. When cases stick to the chamber wall, which happens with clean cases and clean chambers, the primers back out early in the pressure curve. When the pressure finally gets high enough, cases stretch to the bolt face and stuff the primer back in the pocket. This primer stuffing flattens primers and gives the illusion of a max load. A lubricated case slides to the bolt face and it must be at the same speed as primer backout because you get rounded primers with lubricated cases where the same loads in dry cases give flattened primers.
Anyway, it works and that is my best guess why it works.
Primer reading is inexact and often inaccurate, but that and the chronograph are all you have with a semi auto.
Then you take your “best guess” safe load to the XTC match, and if you have blown or pierced primers, cut the load by a half a grain increments until it stops.
In my AR’s, I want to be at 2700 fps with a 75 Hornady. I have not had pressure problems as long as I stay within that range. I have buds who push 77’s a hundred fps faster, I don’t know why their guns stay together or why their primers are not blown.
My loads shot well until I shot out that Wilson barrel.
Code:
[SIZE="3"]M15A2 Armalite 1:8 Stainless Wilson Match Barrel
75 gr Hornady Match 24.5 grs N140 wtd LC 84, WSR, OAL 2.25"
6-Sep-99 T = 92 °F
Ave Vel = 2677
Std Dev = 8
ES = 30
Low = 2662
High = 2692
N = 10
primers slighty flattened
75 gr Hornady Match 24.7 grs N140 wtd LC 84, WSR, OAL 2.25"
6-Sep-99 T = 92 °F
Ave Vel = 2732
Std Dev = 25
ES = 86
Low = 2686
High = 2772
N = 8
Group Size
good
primers flattened
Max load[/SIZE]
