I have no idea what the .224 Plumber case is based on, but if it is similar to a .22/6mm imp, then it would also be almost identical to a .224 Clark (22/257 Roberts improved) which I had for 12 years and fired about 1500 shots.

The water capacity of the 257 Roberts cases necked down to .22 were about 57.5 grains, and when fireformed increased to about 63.8 grains, which was almost identical to a 22/284 that I owned back in the 1970's.

Cases with this capacity are best suited to the use of bullets in the 70 - 80+ grain range, and will need a barrel with a 1 in 8 twist to stabilise them.

For the record I did chronograph the 55 grain bullet in my .224 Clark and achieved about 4280 fps, (same in 22/284)
about 4,000fps with 60 grain bullet, 3930fps with 69 grain bullet, 3830fps with 75 grain bullet, and 3740 fps with 80 grain bullet.

I had no trouble finding an accurate load with the heavier bullets (60+grains) in my .224 Clark, but in the very limited testing with the 55 grain bullet, I only found one load that would give reasonable accuracy.

Hope the above helps. Good luck, Brian