Thanks for the link.Cast bullets are rated by pressure to attain velocity. The BHN #22 Linotype or other high Antimony alloys with a gas checks and hard lube can with stand 15 " of barrel travel. It may attain a velocity greater than 2,000 fps before gas cutting occurs.
As the pressure over comes the bullet gases and heat escape around the bullet depositing "droplets" of hot alloy which adhere to the bore and gas ports. This is referred to as leading. The shorter barrels reduce the elapsed time of travel in the barrel. They may accept higher pressures before gas cutting occurs. Dropping Number II alloys BHN #7 in water is a temporary condition. The hardness begins to reduce significantly within 30 days. The bearing surface of the cast bullet and the barrels twist rate must be a consideration. This is a thumbnail report of cast bullets in the rifle. The CUP calculations are handy,
http://www.223reloads.com/home/cast-bullet-information/bhn-lead-harness-discussion
The CUP of your reloads is published in the reloading manuals. Take a typical .45 ACP load, using a 200-grain LSWC bullet – 5.0 grains of Bullseye. This load develops 900 FPS and is in common use among IPSC and IDPA gunners. The reloading manual shows that the pressure generated by this load is 20,000 CUP. So, the formula for optimal bullet hardness is
20,000 / 1279.8 = 15.62
Here is one you may be interested in.MBC. BHN calculation is not right with my real world experience. It always seems to hard on his method. I run cast lead out of a .308 bolt gun with good success. That is why I was asking about heavier bullet molds in .225. I would think I could get a 80 grain bullet at 1900-2000 fps to cycle the action of the AR. Might have my first attempt at making my own mold coming..... I joined castboolits forum after visiting their site for years. But I will keep this over here on THR aswell.
BB,
During the last great hording that ended about two years ago I was forced to learn how to shoot cast lead AR .224 rounds. I ended up putting 975 cast lead rounds through my LMT .223/556 with 1:7 twist with no leading of the barrel, gas tube or bolt. The only problem was getting any velocity over 2,000 f/sec as the bullets would break up into two pieces before exiting the barrel due to the rapid rotational forces. With the reduced velocity loads I did have to switch my buffer spring to a reduced power one from Wolf gunsprings to get reliable action cycling.
I used a NOE 70 grain mold which dropped 72 grain bullets and sized them to 0.225" and gas checked the bases and pan lubed (which is a real PIA). My alloy as tested by Rotometals was 97.18% lead, 2.22% antimony and 0.435% tin. MY BHN hardness after water dropping in 70 degree water was about 15 and would go up a bit if I used ice water.
I experimented with adding more tin and even copper to the alloy but could not get enough increased velocity above 2000 f/sec to justify the cost and time involved.
So the answer to the question of whether you can successfully shoot .224/556 out of your AR is yes but don't expect the same performance as an FMJ round.
good luck-oldandslow