No, steel doesn't bend (compared to anything brass), and `fer absolute sure won't bend with a muzzle guide.
(But yeah, the OP needs a thicker 3/8 rod)
That said, for ball seating I use
THIS flex rod, cut to length (to where 6" shows after seating charge), with
THIS palm saver on one end and
THIS seating jag on the other.
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See
HERE for ball size, patch material/thickness and general "going-in" load data on the GPR/54cal.
Damp spit patch is fine (maybe even superior) for lube at this point.
Go to the fabric store with your calipers. Head to the pillow ticking section and check
for 100% cotton. Pick up a yard of 0.015" and another yard of 18-20 thousandths.
(One or the other of the ticking materials will work better.)
Cut a strip of [pillow-ticking] patch material that's five colored-stripes wide.
Using
DAMP saliva as lube, not Crisco or anything else for now.
Drive ball even with the muzzle using (short end) of the short starter; cut patch flush w/ sharp knife; then drive another 5" deep w/ the long end self-same short shorter.
Using two hands on the rod/about 6-8" above the muzzle and successive strokes, ball/patch combo should be "smoothly-hard" to seat to the powder firmly.
Wipe barrel 2X with a damp (not wet) patch between shots.