A swooshing GC boolit?

Have a small lot of these 260grn flat point gc boolits, unknown mfg.

There sized at .432, gc at .431. Wanted to see how the .432 boolits would do in a marlin 1894s, normally use the lyman 429421 sized .431 and get 100 meter groups of 3 inches or better with this rifle.

Loaded up 10 rounds with 9gns of unique and tested at 50 meters. Must note, I have never used gc boolits before.

Target is a 2ft by 2ft square rifle target.
First 3 rounds were aweful, hit target on 3 different corners. 4th round pulled trigger normal bang but with a swooshing sound, like it was sucking air, have no idea as to where that round went never saw a hit. Checked bore just to make sure it was not a squib, good there.
2 more rounds fired barely hitting the target.
The 3rd one, bang and swooshing noise again, impact was 10-15 feet to the left of the target impacted into the bank. Done with those!!
Went back to my normal lyman loads and the rifle did fine.

The swooshing noise reminded me of a jet plane coming in for a landing as it was loud enough to hear with muffs on.
I've checked these boolits to see if the gc's were loose and I cannot get them off without damaging the boolit. Probably don't mean anything guessing the gc's on those two rounds came off or canted on the base upon firing that it caused an air pocket at the base causing the fliers and noise??
Just a guess? Just plain weird!
Attachment 233751

loose gas checks would be my guess for the swooshing sound .

Bullets too small and too hard. Sincerely doubt that a gas check separating from bullet in flight would cause such diverse impacts. The sound of the bullet going down range is indicative of a bullet tumbling in flight which is a product of too small bullet not gripping rifling.

sounds like your bullets are tumbling. not an unheard of thing with wfns. especially if your marlin is a micro grooved gun.

Are you sizing them different than your normal Lyman molds? If you are, might try sizing them the same size and try again!

All good advise here, I had similar sound when shooting GC cast in my 458WM when using H4198 and cci Mag primers accuracy was non existent, I was baffled till I got a squib load and found most of the powder singed into a block in the case, I changed to Remington Large standard Primers and accuracy was good and that sound was no longer. This may not be your problem but I felt I should mention it as I have never heard of such a thing before. Regards Stephen

I'm with Lloyd. I'll go with the tumbling theory./beagle