skizzums,
Here's the good, the bad and the ugly on cast boolits for the AR-15.
During the last great hoarding AR fmj bullets were obtainable for me. Thus to shoot I had to take up .223/556 casting. I already cast for 9mm, 38sp, .45acp, 44sp, and .308 for 30-06 and 30/30. I figured that with the reduced velocity obtained with cast vs. fjm's that I needed a heavy boolit- I chose the NOE 70 grain version with a .225" diameter. The boolits dropped at 0.2248-.2255" and at 72 grains with my alloy (Tin-0.43%, Antimony- 2.2%, the rest being lead). Air-cooled BHN hardness was 8, heat-treated and water dropped BHN was 15. I gas checked them with copper checks and sized with a Lee push through die at 0.225". I pan lubed with my own lube concoction which works well for my pistols and rifles- 1part Alox to 1 part candle wax to 2 parts 90wt. gear oil.
Next was determining a COAL that would fit the chamber, magazine and cause minimal boolit deformity with chambering. My max COAL that would fit my mags was 2.250". Unfortunately at this COAL there was moderate boolit nose deformity with chambering and I loaded progressively shorter rounds till I got to 2.184" which showed no nose deformity.
Note this is for a LMT-CQB AR with 1:7 twist.
Next was working up an accurate load. The only powder I could get was IMR-4064. It worked fine but metered slowly in my Dillon 650 or my Rockchucker. You have to make sure your inside-neck is cleaned of debris and lube or the 4064 would bridge across the neck and the backed up powder would spill onto the shell plate. I subsequently found some W-748 which metered well.
To prep the cleaned, resized and trimmed and chamfered brass (inside and outside neck) I used a Lyman M-dye to slightly open the case mouth to allow easier seating of the lead boolit. I did not crimp.
At velocities above 2000 ft/sec I started to see accuracy fall off. At velocities above 2200 ft/sec the nose of the boolit would twist off the base and two chunks of lead would fly downrange. Thus I kept to velocities of about 2000. Twenty grains of 4064 worked well and accuracy was approximately 2-3MOA using a Leupold 2-8 scope. There was no leading of the barrel, gas tube or bolt.
With the reduced velocity I did have problems with the action not-cycling. I installed a Wolf reduced power buffer spring and had no further problems. I put 975 cast lead rounds through my AR before FMJ bullets again became available at a reasonable price ($0.09 each). I have since stopped casting for my AR until the next hoarding.
So lead casting and reloading for the AR-15 is quite do-able but is a major labor intensive endeavor compared to loading FMJ's. Good luck.
best wishes- oldandslow