Start by reading the accurizing tips in the FAQs and Help sticky, that should go a long ways in getting you on the road to accuracy. Trying different ammo in the 17hmr will be necessary, mine shoots the Hornady Vmax very well, but doesn't do well at all with the CCI ammo, Rem and Federal aren't too bad. The .45-70 is one of the best shooting calibers H&R makes, I have 4 of them, they all shoot extremely well, I suspect yours will to with a little help! Also be aware that the transfer bar requires a full pull of the trigger for consistent firing pin strikes, anything less allows the t-bar to drop and the hammer won't hit the firing pin with full force causing erratic ignition.
Tim