Wm Cook
posted this
02 October 2023
I owe everyone a little background on this. I learned about cast bench rest competition in about February 2021. I bought my first rifle dedicated for Production Class Cast BR competition in about July 2021. I sat out the 2021 Nationals because I didn’t have my act together. September 2022 I had a knee replacement. So the 2023 Nationals was to be my first year competing in Cast Bullet BR competition.
Things came together nicely throughout May, June and July of this year. From May through the first week in August I shot 25 groups that agg’d .845. I neck turned another hundred pieces of Lapua brass and worked with Richard over at KC with a plan to preload rather than load at the range. I had a load I had confidence in and the only thing I was working with was the odd flier which I blamed on loose bullet nose to bore fit. But I was close. With the jacketed mentally I had I felt the spec's could produce accuracy well enough to be competitive. About the groups sizes mentioned. It was on my clubs 60 bench range that I've been shooting at for over 30 years and I was shooting in hand picked readable conditions. No buzzers going off, no bench rotations no timers.
As I was getting ready for the match I started to mess around with getting a better fill on the bullet to shake off the fliers. That’s when the wheels came off. From August 21st through September (using the same load I was working with in the spring/early summer) I shot 36 groups that agg’d 1.231 and I was dumbstruck, clueless and mystified. I mean, really, how could accuracy go so south so quickly. It was like a light switch was thrown. Like a scope issue or loose action screws but no that wasn’t it. I put a few groups together but they were random and there were a lot more patterns than groups shot in that time period. It looked like a pressure problem to me. Like it was powder related or too hard a jam between the bullet and the bore. I finally started to look at my casting records and I think I may have figured it out. I’ll try to make this as painlessly short as possible.
What I found was that the small groups I was shooting in the spring, early summer were coming from different casting lots with the alloy running ~680 (no PID) and the mold temperature (spot checked) was running around 370 to 400. If you need the numbers I can probably crunch something together. But the point I want to make is that those bullets were not filled out. They were not as “perfect” as the bullets I started to cast in early August. The spring/early summer casting session were not “perfect” but they fit the bore. As of today I believe I have the target temperatures for alloy and mold figured out where it will give me fully filled out bullets and are consistent in weight with a light frosty appearance. But these “perfect” bullets shoot bigger groups than the “imperfect” bullets because they no longer fit the bore.
A short explanation is needed here. The mold I’m talking about is an Egan style bore rider from Accurate. With the cooler alloy/mold I was dropping a bullet nose close to .3012 in diameter and could not feel the lands. With a hotter alloy of 710 and a hotter mold 430 I’m dropping bullets with a nose close to .3016 and just starting to feel the lands. But as the nose grew larger so did the taper of the Egan design. The taper of the Egan design (from the bullet base to the nose has a .005 taper between .300” to .460” from the bullet base (before check). With the fully filled out “perfect” bullet I was too tight into the freebore and it was shooting patterns off an on. Actually more on that off. I could feel my bullets starting to stick, not into the lands but as the front driving band made contacet with the freebore. In addition the extraction of a loaded round was problematic. The “imperfect” bullet I was casting in the spring may have been inperfect but it was small enough that I could kiss without sticking into the freebore.
Now here’s the funny part. I had a second 31-230E mold from Accurate made earlier this year and I asked Tom to make it with a bit bigger nose so I could feel the lands engage when chambering. With the .3019 nose I can feel it engage the lands. And it can be extracted from both the lands and the freebore because I ordered the bullet base at .309 not the .310 of the first mold. Thus the Egan taper design was small enough fit the freebore.
So at this point I think I can see some daylight. Using bullets from the backup mold the last three groups I shot on Friday were at .700, .545 and .637. I’m starting to think that consistent casting is important but it's trumped by bullet fit to bore. Before I wasn’t tracking groups I shot to the casting lot the bullets came from but I guess thats a good idea. That said I know those groups I shot in spring/early summer came from several casting events. I sure as heck hope this has a happy ending. Bill.
A “Measured Response” is as effective as tongue lashing a stuck door.