The barrel to frame gap, is USUALLY a "fine tuning" issue. The gun should fire, especially if some of the ammo worked on another frame.
But not always! A cartridge designed to headpace off of the shoulder, which someone correct me if I,m wrong such as the Herrett rounds this can be a big issue! A few .001 of an inch can make a huge difference. Then also ever reloading manual I've ever read stated never use loads work in one gun in another!
A pure wildcat, there are no factory loads available for the .30 Herrett. It is formed simply by setting the shoulder back on standard .30-30 brass, then trimming to an overall length of 1.60 inches. Even though forming the round is straightforward, however, there are critical aspects to the process. Where to put the shoulder is the most critical of these.
Even though it is a rimmed cartridge, the .30 Herrett must headspace on the shoulder. If the shoulder is set back too far, cases invariably suffer head separations, sometimes as soon as the second firing. Shoulders set too far forward will not allow the Contender action to lock up properly. This can result in poor accuracy, no ignition or inability to close the action at all. Decent accuracy and case life is obtained only when the cartridge fully and snugly fits the chamber. Typically, best accuracy comes with fully fire-formed cases and neck sizing.
The correct case-forming techniques are carefully spelled out in several places, including the Hornady Reloading Handbook, the free load data brochure put out by Thompson Center and elsewhere. That being the case, I won't go into extreme detail here, but the basic process is to very gradually adjust your sizing die downwards during the forming operation until the gun just barely closes on a sized case. Because no two chambers are alike, especially for wildcat calibers, you should never use ammo that was loaded for other .30 Herrett barrels. If you own more than one Herrett barrel, you should size, load and mark ammo specifically for each. Even a separate set of dies for each barrel wouldn't be out of line.