Mystery group gremlin

I think I made a serious error in excessively firelapping my Ruger 77/357 because I kept feeling roughness in the bore with push through slugs (per Veral Smith's and Marshall Stanton's books). When I made a chamber cast afterwards I found that the rifling did not appear in the cast for almost 0.7" past the mouth of the cartridge case, so the throat is something like a giraffe neck, at least to me. The beginning of the throat measures 0.3615" and shrinks 0.0005" at 0.1" intervals till it gets to the rifling where it is a reasonable 0.3575". I was using up the last of the Beartooth lapping bullets (I think they're about 160 grains with no lapping compound) and got fairly decent groups (around 2" for five shots at 100 yards) using up to 4.9 grains of Titegroup and CCI SPM primers despite a heavy trigger and my newbie bench skills. Velocity was 1123 fps average.

So I thought I would be in good shape when I used unsized Lee 158 grain SWC GC made from water dropped COWW with 4.6 grains of Titegroup (starting over just slightly above the starting load because the bullet was different) after getting an initial three-shot cloverleaf sitting at 50 yards. The SWCs fed nicely from the rotary magazine and I felt like a god. Briefly.

So I cast a lot more, though I used a larger bucket so the quenching might have been different. It all went south from there when I tried to bench them (a pattern rather than a group). I was shooting without gas checks both times because of the fairly low velocity. So I quit the bench and shot five offhand at 25 yards and they went into a 3" circle in the black.

I don't have a hardness tester, so I don't know whether the first batch (I cast them by dipper from the kitchen stove) came out differently than the second batch when I used the Lee 4-20 bottom pour. The boolits come out of the Lee mold at 0.3575" diameter pretty consistently. I pan lubed all of the boolits with LLA mixed with some paraffin and paste wax and have gotten no leading. I will use BLL in the future, but this stuff seems OK for the moment, at least at this velocity.

I have been thinking of putting on gas checks and not sizing them, but the gas check measures 0.360" which may be nice for the chamber but bad for pressure? I can size them with a Lee push-through sizer. I can also anneal the boolits I have in case they're too hard, at least at this velocity.

I'm thinking that I may have too many variable variables, but I'm not sure which ones they are.

Any suggestions? Thanks!

I had a 94 Winchester .357 with a very similar throat and an extremely accurate shooter. I would recommend you do away with the water dropping, size .359 if possible and generally keep the bullets fairly soft at the speed you are shooting. The hard bullets will give you throat lead very quickly and destroy your accuracy with fast powders and lighter loads. In my 94 I was able to use harder bullets with good accuracy with 296 or H110 max loads. The ball powder tended to scour the throat and keep it clean
PS. that long throat may not be your doing. While poking around a reamer site I saw a drawing for what I believe was an "AMU chamber" which had a long throat. Maybe that's how Ruger chambers their rifles.

Stop with the water dropping , apply a gas check and size to .358 (or groove dia. +.001).
Bullets designed for gas checks should be checked....you want plain base, buy a PB mould.
I air cool , lead / COWW 50/50 mix , attach gas check, Lithium-Beeswax lube and size to .358 , with Accurate Arms #5 or Unique . In my 357 magnum loads these are accurate and do not lead the barrel.
Gary

The guys above are right. Let them age a bit as they might grow, no water dropping, and gas check them unsized. If you have a Lyman lubrisizer, use the gas check seater to seat the gas checks. Don't worry about the pressure with .360 gas checks. They are thin and will size down in the barrel just fine. If you have a 358 sizing die and want to use it, it will open up to 359 or 3595 no problem with a dowel or screwdriver wrapped in emory cloth/sandpaper and rolled back and forth on the thigh or any flat surface.