Thread: So this happened at the range.

Check the freebore of your 303 they are notorious for being eroded.
With the muzzle pointing down drop a projectile in the breech and very lightly tap it with a rod or similar to make sure it's engage the rifling.
From the muzzle run a cleaning rod down till it touches the bullet Mark the rod at the muzzle with a sharpie felt pen.
Drive the bullet out and replace it with a dummy round with the bullet seated as you normally do.
Put the rod down the muzzle again to touch the bullet and mark the rod.
Measure the distanice between the two marks, that's your freebore.
I had a martini enfield that had 2 and a half inches of freebore couldn't shoot cast or boat tail jacketed bullets well enough to hit a flock of barns at 10 yards but flat base jacketed bullets managed 6 inches at 100.
With excessive freebore cast bullets chatter around and can't seal effectivly paper patching can help if you patch to about 4 or 8 thou over bore size that accomodate the eroded throat.